Enforce lagged-slave read-only mode on the DB layer

[lhc/web/wiklou.git] / includes / db / Database.php

<?php

/**
 * @defgroup Database Database
 *
 * This file deals with database interface functions
 * and query specifics/optimisations.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License along
 * with this program; if not, write to the Free Software Foundation, Inc.,
 * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 * http://www.gnu.org/copyleft/gpl.html
 *
 * @file
 * @ingroup Database
 */

/**
 * Database abstraction object
 * @ingroup Database
 */
abstract class DatabaseBase implements IDatabase {
        /** Number of times to re-try an operation in case of deadlock */
        const DEADLOCK_TRIES = 4;

        /** Minimum time to wait before retry, in microseconds */
        const DEADLOCK_DELAY_MIN = 500000;

        /** Maximum time to wait before retry */
        const DEADLOCK_DELAY_MAX = 1500000;

        protected $mLastQuery = '';
        protected $mDoneWrites = false;
        protected $mPHPError = false;

        protected $mServer, $mUser, $mPassword, $mDBname;

        /** @var resource Database connection */
        protected $mConn = null;
        protected $mOpened = false;

        /** @var callable[] */
        protected $mTrxIdleCallbacks = array();
        /** @var callable[] */
        protected $mTrxPreCommitCallbacks = array();

        protected $mTablePrefix;
        protected $mSchema;
        protected $mFlags;
        protected $mForeign;
        protected $mLBInfo = array();
        protected $mDefaultBigSelects = null;
        protected $mSchemaVars = false;
        /** @var array */
        protected $mSessionVars = array();

        protected $preparedArgs;

        protected $htmlErrors;

        protected $delimiter = ';';

        /**
         * Either 1 if a transaction is active or 0 otherwise.
         * The other Trx fields may not be meaningfull if this is 0.
         *
         * @var int
         */
        protected $mTrxLevel = 0;

        /**
         * Either a short hexidecimal string if a transaction is active or ""
         *
         * @var string
         * @see DatabaseBase::mTrxLevel
         */
        protected $mTrxShortId = '';

        /**
         * The UNIX time that the transaction started. Callers can assume that if
         * snapshot isolation is used, then the data is *at least* up to date to that
         * point (possibly more up-to-date since the first SELECT defines the snapshot).
         *
         * @var float|null
         * @see DatabaseBase::mTrxLevel
         */
        private $mTrxTimestamp = null;

        /**
         * Remembers the function name given for starting the most recent transaction via begin().
         * Used to provide additional context for error reporting.
         *
         * @var string
         * @see DatabaseBase::mTrxLevel
         */
        private $mTrxFname = null;

        /**
         * Record if possible write queries were done in the last transaction started
         *
         * @var bool
         * @see DatabaseBase::mTrxLevel
         */
        private $mTrxDoneWrites = false;

        /**
         * Record if the current transaction was started implicitly due to DBO_TRX being set.
         *
         * @var bool
         * @see DatabaseBase::mTrxLevel
         */
        private $mTrxAutomatic = false;

        /**
         * Array of levels of atomicity within transactions
         *
         * @var array
         */
        private $mTrxAtomicLevels = array();

        /**
         * Record if the current transaction was started implicitly by DatabaseBase::startAtomic
         *
         * @var bool
         */
        private $mTrxAutomaticAtomic = false;

        /**
         * Track the seconds spent in write queries for the current transaction
         *
         * @var float
         */
        private $mTrxWriteDuration = 0.0;

        /**
         * @since 1.21
         * @var resource File handle for upgrade
         */
        protected $fileHandle = null;

        /**
         * @since 1.22
         * @var string[] Process cache of VIEWs names in the database
         */
        protected $allViews = null;

        /** @var TransactionProfiler */
        protected $trxProfiler;

        /**
         * A string describing the current software version, and possibly
         * other details in a user-friendly way. Will be listed on Special:Version, etc.
         * Use getServerVersion() to get machine-friendly information.
         *
         * @return string Version information from the database server
         */
        public function getServerInfo() {
                return $this->getServerVersion();
        }

        /**
         * @return string Command delimiter used by this database engine
         */
        public function getDelimiter() {
                return $this->delimiter;
        }

        /**
         * Boolean, controls output of large amounts of debug information.
         * @param bool|null $debug
         *   - true to enable debugging
         *   - false to disable debugging
         *   - omitted or null to do nothing
         *
         * @return bool|null Previous value of the flag
         */
        public function debug( $debug = null ) {
                return wfSetBit( $this->mFlags, DBO_DEBUG, $debug );
        }

        /**
         * Turns buffering of SQL result sets on (true) or off (false). Default is
         * "on".
         *
         * Unbuffered queries are very troublesome in MySQL:
         *
         *   - If another query is executed while the first query is being read
         *     out, the first query is killed. This means you can't call normal
         *     MediaWiki functions while you are reading an unbuffered query result
         *     from a normal wfGetDB() connection.
         *
         *   - Unbuffered queries cause the MySQL server to use large amounts of
         *     memory and to hold broad locks which block other queries.
         *
         * If you want to limit client-side memory, it's almost always better to
         * split up queries into batches using a LIMIT clause than to switch off
         * buffering.
         *
         * @param null|bool $buffer
         * @return null|bool The previous value of the flag
         */
        public function bufferResults( $buffer = null ) {
                if ( is_null( $buffer ) ) {
                        return !(bool)( $this->mFlags & DBO_NOBUFFER );
                } else {
                        return !wfSetBit( $this->mFlags, DBO_NOBUFFER, !$buffer );
                }
        }

        /**
         * Turns on (false) or off (true) the automatic generation and sending
         * of a "we're sorry, but there has been a database error" page on
         * database errors. Default is on (false). When turned off, the
         * code should use lastErrno() and lastError() to handle the
         * situation as appropriate.
         *
         * Do not use this function outside of the Database classes.
         *
         * @param null|bool $ignoreErrors
         * @return bool The previous value of the flag.
         */
        protected function ignoreErrors( $ignoreErrors = null ) {
                return wfSetBit( $this->mFlags, DBO_IGNORE, $ignoreErrors );
        }

        /**
         * Gets the current transaction level.
         *
         * Historically, transactions were allowed to be "nested". This is no
         * longer supported, so this function really only returns a boolean.
         *
         * @return int The previous value
         */
        public function trxLevel() {
                return $this->mTrxLevel;
        }

        /**
         * Get the UNIX timestamp of the time that the transaction was established
         *
         * This can be used to reason about the staleness of SELECT data
         * in REPEATABLE-READ transaction isolation level.
         *
         * @return float|null Returns null if there is not active transaction
         * @since 1.25
         */
        public function trxTimestamp() {
                return $this->mTrxLevel ? $this->mTrxTimestamp : null;
        }

        /**
         * Get/set the table prefix.
         * @param string $prefix The table prefix to set, or omitted to leave it unchanged.
         * @return string The previous table prefix.
         */
        public function tablePrefix( $prefix = null ) {
                return wfSetVar( $this->mTablePrefix, $prefix );
        }

        /**
         * Get/set the db schema.
         * @param string $schema The database schema to set, or omitted to leave it unchanged.
         * @return string The previous db schema.
         */
        public function dbSchema( $schema = null ) {
                return wfSetVar( $this->mSchema, $schema );
        }

        /**
         * Set the filehandle to copy write statements to.
         *
         * @param resource $fh File handle
         */
        public function setFileHandle( $fh ) {
                $this->fileHandle = $fh;
        }

        /**
         * Get properties passed down from the server info array of the load
         * balancer.
         *
         * @param string $name The entry of the info array to get, or null to get the
         *   whole array
         *
         * @return array|mixed|null
         */
        public function getLBInfo( $name = null ) {
                if ( is_null( $name ) ) {
                        return $this->mLBInfo;
                } else {
                        if ( array_key_exists( $name, $this->mLBInfo ) ) {
                                return $this->mLBInfo[$name];
                        } else {
                                return null;
                        }
                }
        }

        /**
         * Set the LB info array, or a member of it. If called with one parameter,
         * the LB info array is set to that parameter. If it is called with two
         * parameters, the member with the given name is set to the given value.
         *
         * @param string $name
         * @param array $value
         */
        public function setLBInfo( $name, $value = null ) {
                if ( is_null( $value ) ) {
                        $this->mLBInfo = $name;
                } else {
                        $this->mLBInfo[$name] = $value;
                }
        }

        /**
         * @return TransactionProfiler
         */
        protected function getTransactionProfiler() {
                return $this->trxProfiler
                        ? $this->trxProfiler
                        : Profiler::instance()->getTransactionProfiler();
        }

        /**
         * Returns true if this database supports (and uses) cascading deletes
         *
         * @return bool
         */
        public function cascadingDeletes() {
                return false;
        }

        /**
         * Returns true if this database supports (and uses) triggers (e.g. on the page table)
         *
         * @return bool
         */
        public function cleanupTriggers() {
                return false;
        }

        /**
         * Returns true if this database is strict about what can be put into an IP field.
         * Specifically, it uses a NULL value instead of an empty string.
         *
         * @return bool
         */
        public function strictIPs() {
                return false;
        }

        /**
         * Returns true if this database uses timestamps rather than integers
         *
         * @return bool
         */
        public function realTimestamps() {
                return false;
        }

        /**
         * Returns true if this database does an implicit sort when doing GROUP BY
         *
         * @return bool
         */
        public function implicitGroupby() {
                return true;
        }

        /**
         * Returns true if this database does an implicit order by when the column has an index
         * For example: SELECT page_title FROM page LIMIT 1
         *
         * @return bool
         */
        public function implicitOrderby() {
                return true;
        }

        /**
         * Returns true if this database can do a native search on IP columns
         * e.g. this works as expected: .. WHERE rc_ip = '127.42.12.102/32';
         *
         * @return bool
         */
        public function searchableIPs() {
                return false;
        }

        /**
         * Returns true if this database can use functional indexes
         *
         * @return bool
         */
        public function functionalIndexes() {
                return false;
        }

        /**
         * Return the last query that went through DatabaseBase::query()
         * @return string
         */
        public function lastQuery() {
                return $this->mLastQuery;
        }

        /**
         * Returns true if the connection may have been used for write queries.
         * Should return true if unsure.
         *
         * @return bool
         */
        public function doneWrites() {
                return (bool)$this->mDoneWrites;
        }

        /**
         * Returns the last time the connection may have been used for write queries.
         * Should return a timestamp if unsure.
         *
         * @return int|float UNIX timestamp or false
         * @since 1.24
         */
        public function lastDoneWrites() {
                return $this->mDoneWrites ?: false;
        }

        /**
         * Returns true if there is a transaction open with possible write
         * queries or transaction pre-commit/idle callbacks waiting on it to finish.
         *
         * @return bool
         */
        public function writesOrCallbacksPending() {
                return $this->mTrxLevel && (
                        $this->mTrxDoneWrites || $this->mTrxIdleCallbacks || $this->mTrxPreCommitCallbacks
                );
        }

        /**
         * Get the time spend running write queries for this
         *
         * High times could be due to scanning, updates, locking, and such
         *
         * @return float|bool Returns false if not transaction is active
         * @since 1.26
         */
        public function pendingWriteQueryDuration() {
                return $this->mTrxLevel ? $this->mTrxWriteDuration : false;
        }

        /**
         * Is a connection to the database open?
         * @return bool
         */
        public function isOpen() {
                return $this->mOpened;
        }

        /**
         * Set a flag for this connection
         *
         * @param int $flag DBO_* constants from Defines.php:
         *   - DBO_DEBUG: output some debug info (same as debug())
         *   - DBO_NOBUFFER: don't buffer results (inverse of bufferResults())
         *   - DBO_TRX: automatically start transactions
         *   - DBO_DEFAULT: automatically sets DBO_TRX if not in command line mode
         *       and removes it in command line mode
         *   - DBO_PERSISTENT: use persistant database connection
         */
        public function setFlag( $flag ) {
                global $wgDebugDBTransactions;
                $this->mFlags |= $flag;
                if ( ( $flag & DBO_TRX ) && $wgDebugDBTransactions ) {
                        wfDebug( "Implicit transactions are now enabled.\n" );
                }
        }

        /**
         * Clear a flag for this connection
         *
         * @param int $flag DBO_* constants from Defines.php:
         *   - DBO_DEBUG: output some debug info (same as debug())
         *   - DBO_NOBUFFER: don't buffer results (inverse of bufferResults())
         *   - DBO_TRX: automatically start transactions
         *   - DBO_DEFAULT: automatically sets DBO_TRX if not in command line mode
         *       and removes it in command line mode
         *   - DBO_PERSISTENT: use persistant database connection
         */
        public function clearFlag( $flag ) {
                global $wgDebugDBTransactions;
                $this->mFlags &= ~$flag;
                if ( ( $flag & DBO_TRX ) && $wgDebugDBTransactions ) {
                        wfDebug( "Implicit transactions are now disabled.\n" );
                }
        }

        /**
         * Returns a boolean whether the flag $flag is set for this connection
         *
         * @param int $flag DBO_* constants from Defines.php:
         *   - DBO_DEBUG: output some debug info (same as debug())
         *   - DBO_NOBUFFER: don't buffer results (inverse of bufferResults())
         *   - DBO_TRX: automatically start transactions
         *   - DBO_PERSISTENT: use persistant database connection
         * @return bool
         */
        public function getFlag( $flag ) {
                return !!( $this->mFlags & $flag );
        }

        /**
         * General read-only accessor
         *
         * @param string $name
         * @return string
         */
        public function getProperty( $name ) {
                return $this->$name;
        }

        /**
         * @return string
         */
        public function getWikiID() {
                if ( $this->mTablePrefix ) {
                        return "{$this->mDBname}-{$this->mTablePrefix}";
                } else {
                        return $this->mDBname;
                }
        }

        /**
         * Return a path to the DBMS-specific SQL file if it exists,
         * otherwise default SQL file
         *
         * @param string $filename
         * @return string
         */
        private function getSqlFilePath( $filename ) {
                global $IP;
                $dbmsSpecificFilePath = "$IP/maintenance/" . $this->getType() . "/$filename";
                if ( file_exists( $dbmsSpecificFilePath ) ) {
                        return $dbmsSpecificFilePath;
                } else {
                        return "$IP/maintenance/$filename";
                }
        }

        /**
         * Return a path to the DBMS-specific schema file,
         * otherwise default to tables.sql
         *
         * @return string
         */
        public function getSchemaPath() {
                return $this->getSqlFilePath( 'tables.sql' );
        }

        /**
         * Return a path to the DBMS-specific update key file,
         * otherwise default to update-keys.sql
         *
         * @return string
         */
        public function getUpdateKeysPath() {
                return $this->getSqlFilePath( 'update-keys.sql' );
        }

        /**
         * Get information about an index into an object
         * @param string $table Table name
         * @param string $index Index name
         * @param string $fname Calling function name
         * @return mixed Database-specific index description class or false if the index does not exist
         */
        abstract function indexInfo( $table, $index, $fname = __METHOD__ );

        /**
         * Wrapper for addslashes()
         *
         * @param string $s String to be slashed.
         * @return string Slashed string.
         */
        abstract function strencode( $s );

        /**
         * Constructor.
         *
         * FIXME: It is possible to construct a Database object with no associated
         * connection object, by specifying no parameters to __construct(). This
         * feature is deprecated and should be removed.
         *
         * DatabaseBase subclasses should not be constructed directly in external
         * code. DatabaseBase::factory() should be used instead.
         *
         * @param array $params Parameters passed from DatabaseBase::factory()
         */
        function __construct( array $params ) {
                global $wgDBprefix, $wgDBmwschema, $wgCommandLineMode, $wgDebugDBTransactions;

                $server = $params['host'];
                $user = $params['user'];
                $password = $params['password'];
                $dbName = $params['dbname'];
                $flags = $params['flags'];
                $tablePrefix = $params['tablePrefix'];
                $schema = $params['schema'];
                $foreign = $params['foreign'];

                $this->mFlags = $flags;
                if ( $this->mFlags & DBO_DEFAULT ) {
                        if ( $wgCommandLineMode ) {
                                $this->mFlags &= ~DBO_TRX;
                                if ( $wgDebugDBTransactions ) {
                                        wfDebug( "Implicit transaction open disabled.\n" );
                                }
                        } else {
                                $this->mFlags |= DBO_TRX;
                                if ( $wgDebugDBTransactions ) {
                                        wfDebug( "Implicit transaction open enabled.\n" );
                                }
                        }
                }

                $this->mSessionVars = $params['variables'];

                /** Get the default table prefix*/
                if ( $tablePrefix === 'get from global' ) {
                        $this->mTablePrefix = $wgDBprefix;
                } else {
                        $this->mTablePrefix = $tablePrefix;
                }

                /** Get the database schema*/
                if ( $schema === 'get from global' ) {
                        $this->mSchema = $wgDBmwschema;
                } else {
                        $this->mSchema = $schema;
                }

                $this->mForeign = $foreign;

                if ( isset( $params['trxProfiler'] ) ) {
                        $this->trxProfiler = $params['trxProfiler']; // override
                }

                if ( $user ) {
                        $this->open( $server, $user, $password, $dbName );
                }
        }

        /**
         * Called by serialize. Throw an exception when DB connection is serialized.
         * This causes problems on some database engines because the connection is
         * not restored on unserialize.
         */
        public function __sleep() {
                throw new MWException( 'Database serialization may cause problems, since ' .
                        'the connection is not restored on wakeup.' );
        }

        /**
         * Given a DB type, construct the name of the appropriate child class of
         * DatabaseBase. This is designed to replace all of the manual stuff like:
         *    $class = 'Database' . ucfirst( strtolower( $dbType ) );
         * as well as validate against the canonical list of DB types we have
         *
         * This factory function is mostly useful for when you need to connect to a
         * database other than the MediaWiki default (such as for external auth,
         * an extension, et cetera). Do not use this to connect to the MediaWiki
         * database. Example uses in core:
         * @see LoadBalancer::reallyOpenConnection()
         * @see ForeignDBRepo::getMasterDB()
         * @see WebInstallerDBConnect::execute()
         *
         * @since 1.18
         *
         * @param string $dbType A possible DB type
         * @param array $p An array of options to pass to the constructor.
         *    Valid options are: host, user, password, dbname, flags, tablePrefix, schema, driver
         * @throws MWException If the database driver or extension cannot be found
         * @return DatabaseBase|null DatabaseBase subclass or null
         */
        final public static function factory( $dbType, $p = array() ) {
                $canonicalDBTypes = array(
                        'mysql' => array( 'mysqli', 'mysql' ),
                        'postgres' => array(),
                        'sqlite' => array(),
                        'oracle' => array(),
                        'mssql' => array(),
                );

                $driver = false;
                $dbType = strtolower( $dbType );
                if ( isset( $canonicalDBTypes[$dbType] ) && $canonicalDBTypes[$dbType] ) {
                        $possibleDrivers = $canonicalDBTypes[$dbType];
                        if ( !empty( $p['driver'] ) ) {
                                if ( in_array( $p['driver'], $possibleDrivers ) ) {
                                        $driver = $p['driver'];
                                } else {
                                        throw new MWException( __METHOD__ .
                                                " cannot construct Database with type '$dbType' and driver '{$p['driver']}'" );
                                }
                        } else {
                                foreach ( $possibleDrivers as $posDriver ) {
                                        if ( extension_loaded( $posDriver ) ) {
                                                $driver = $posDriver;
                                                break;
                                        }
                                }
                        }
                } else {
                        $driver = $dbType;
                }
                if ( $driver === false ) {
                        throw new MWException( __METHOD__ .
                                " no viable database extension found for type '$dbType'" );
                }

                // Determine schema defaults. Currently Microsoft SQL Server uses $wgDBmwschema,
                // and everything else doesn't use a schema (e.g. null)
                // Although postgres and oracle support schemas, we don't use them (yet)
                // to maintain backwards compatibility
                $defaultSchemas = array(
                        'mssql' => 'get from global',
                );

                $class = 'Database' . ucfirst( $driver );
                if ( class_exists( $class ) && is_subclass_of( $class, 'DatabaseBase' ) ) {
                        // Resolve some defaults for b/c
                        $p['host'] = isset( $p['host'] ) ? $p['host'] : false;
                        $p['user'] = isset( $p['user'] ) ? $p['user'] : false;
                        $p['password'] = isset( $p['password'] ) ? $p['password'] : false;
                        $p['dbname'] = isset( $p['dbname'] ) ? $p['dbname'] : false;
                        $p['flags'] = isset( $p['flags'] ) ? $p['flags'] : 0;
                        $p['variables'] = isset( $p['variables'] ) ? $p['variables'] : array();
                        $p['tablePrefix'] = isset( $p['tablePrefix'] ) ? $p['tablePrefix'] : 'get from global';
                        if ( !isset( $p['schema'] ) ) {
                                $p['schema'] = isset( $defaultSchemas[$dbType] ) ? $defaultSchemas[$dbType] : null;
                        }
                        $p['foreign'] = isset( $p['foreign'] ) ? $p['foreign'] : false;

                        return new $class( $p );
                } else {
                        return null;
                }
        }

        protected function installErrorHandler() {
                $this->mPHPError = false;
                $this->htmlErrors = ini_set( 'html_errors', '0' );
                set_error_handler( array( $this, 'connectionErrorHandler' ) );
        }

        /**
         * @return bool|string
         */
        protected function restoreErrorHandler() {
                restore_error_handler();
                if ( $this->htmlErrors !== false ) {
                        ini_set( 'html_errors', $this->htmlErrors );
                }
                if ( $this->mPHPError ) {
                        $error = preg_replace( '!\[<a.*</a>\]!', '', $this->mPHPError );
                        $error = preg_replace( '!^.*?:\s?(.*)$!', '$1', $error );

                        return $error;
                } else {
                        return false;
                }
        }

        /**
         * @param int $errno
         * @param string $errstr
         */
        public function connectionErrorHandler( $errno, $errstr ) {
                $this->mPHPError = $errstr;
        }

        /**
         * Create a log context to pass to wfLogDBError or other logging functions.
         *
         * @param array $extras Additional data to add to context
         * @return array
         */
        protected function getLogContext( array $extras = array() ) {
                return array_merge(
                        array(
                                'db_server' => $this->mServer,
                                'db_name' => $this->mDBname,
                                'db_user' => $this->mUser,
                        ),
                        $extras
                );
        }

        /**
         * Closes a database connection.
         * if it is open : commits any open transactions
         *
         * @throws MWException
         * @return bool Operation success. true if already closed.
         */
        public function close() {
                if ( count( $this->mTrxIdleCallbacks ) ) { // sanity
                        throw new MWException( "Transaction idle callbacks still pending." );
                }
                if ( $this->mConn ) {
                        if ( $this->trxLevel() ) {
                                if ( !$this->mTrxAutomatic ) {
                                        wfWarn( "Transaction still in progress (from {$this->mTrxFname}), " .
                                                " performing implicit commit before closing connection!" );
                                }

                                $this->commit( __METHOD__, 'flush' );
                        }

                        $closed = $this->closeConnection();
                        $this->mConn = false;
                } else {
                        $closed = true;
                }
                $this->mOpened = false;

                return $closed;
        }

        /**
         * Make sure isOpen() returns true as a sanity check
         *
         * @throws DBUnexpectedError
         */
        protected function assertOpen() {
                if ( !$this->isOpen() ) {
                        throw new DBUnexpectedError( $this, "DB connection was already closed." );
                }
        }

        /**
         * Closes underlying database connection
         * @since 1.20
         * @return bool Whether connection was closed successfully
         */
        abstract protected function closeConnection();

        /**
         * @param string $error Fallback error message, used if none is given by DB
         * @throws DBConnectionError
         */
        function reportConnectionError( $error = 'Unknown error' ) {
                $myError = $this->lastError();
                if ( $myError ) {
                        $error = $myError;
                }

                # New method
                throw new DBConnectionError( $this, $error );
        }

        /**
         * The DBMS-dependent part of query()
         *
         * @param string $sql SQL query.
         * @return ResultWrapper|bool Result object to feed to fetchObject,
         *   fetchRow, ...; or false on failure
         */
        abstract protected function doQuery( $sql );

        /**
         * Determine whether a query writes to the DB.
         * Should return true if unsure.
         *
         * @param string $sql
         * @return bool
         */
        protected function isWriteQuery( $sql ) {
                return !preg_match( '/^(?:SELECT|BEGIN|ROLLBACK|COMMIT|SET|SHOW|EXPLAIN|\(SELECT)\b/i', $sql );
        }

        /**
         * Determine whether a SQL statement is sensitive to isolation level.
         * A SQL statement is considered transactable if its result could vary
         * depending on the transaction isolation level. Operational commands
         * such as 'SET' and 'SHOW' are not considered to be transactable.
         *
         * @param string $sql
         * @return bool
         */
        protected function isTransactableQuery( $sql ) {
                $verb = substr( $sql, 0, strcspn( $sql, " \t\r\n" ) );
                return !in_array( $verb, array( 'BEGIN', 'COMMIT', 'ROLLBACK', 'SHOW', 'SET' ) );
        }

        /**
         * Run an SQL query and return the result. Normally throws a DBQueryError
         * on failure. If errors are ignored, returns false instead.
         *
         * In new code, the query wrappers select(), insert(), update(), delete(),
         * etc. should be used where possible, since they give much better DBMS
         * independence and automatically quote or validate user input in a variety
         * of contexts. This function is generally only useful for queries which are
         * explicitly DBMS-dependent and are unsupported by the query wrappers, such
         * as CREATE TABLE.
         *
         * However, the query wrappers themselves should call this function.
         *
         * @param string $sql SQL query
         * @param string $fname Name of the calling function, for profiling/SHOW PROCESSLIST
         *     comment (you can use __METHOD__ or add some extra info)
         * @param bool $tempIgnore Whether to avoid throwing an exception on errors...
         *     maybe best to catch the exception instead?
         * @throws MWException
         * @return bool|ResultWrapper True for a successful write query, ResultWrapper object
         *     for a successful read query, or false on failure if $tempIgnore set
         */
        public function query( $sql, $fname = __METHOD__, $tempIgnore = false ) {
                global $wgUser, $wgDebugDBTransactions, $wgDebugDumpSqlLength;

                $this->mLastQuery = $sql;

                $isWriteQuery = $this->isWriteQuery( $sql );
                if ( $isWriteQuery ) {
                        $reason = $this->getLBInfo( 'readOnlyReason' );
                        if ( is_string( $reason ) ) {
                                throw new DBReadOnlyError( $this, "Database is read-only: $reason" );
                        }
                        # Set a flag indicating that writes have been done
                        $this->mDoneWrites = microtime( true );
                }

                # Add a comment for easy SHOW PROCESSLIST interpretation
                if ( is_object( $wgUser ) && $wgUser->isItemLoaded( 'name' ) ) {
                        $userName = $wgUser->getName();
                        if ( mb_strlen( $userName ) > 15 ) {
                                $userName = mb_substr( $userName, 0, 15 ) . '...';
                        }
                        $userName = str_replace( '/', '', $userName );
                } else {
                        $userName = '';
                }

                // Add trace comment to the begin of the sql string, right after the operator.
                // Or, for one-word queries (like "BEGIN" or COMMIT") add it to the end (bug 42598)
                $commentedSql = preg_replace( '/\s|$/', " /* $fname $userName */ ", $sql, 1 );

                if ( !$this->mTrxLevel && $this->getFlag( DBO_TRX ) && $this->isTransactableQuery( $sql ) ) {
                        if ( $wgDebugDBTransactions ) {
                                wfDebug( "Implicit transaction start.\n" );
                        }
                        $this->begin( __METHOD__ . " ($fname)" );
                        $this->mTrxAutomatic = true;
                }

                # Keep track of whether the transaction has write queries pending
                if ( $this->mTrxLevel && !$this->mTrxDoneWrites && $isWriteQuery ) {
                        $this->mTrxDoneWrites = true;
                        $this->getTransactionProfiler()->transactionWritingIn(
                                $this->mServer, $this->mDBname, $this->mTrxShortId );
                }

                $isMaster = !is_null( $this->getLBInfo( 'master' ) );
                # generalizeSQL will probably cut down the query to reasonable
                # logging size most of the time. The substr is really just a sanity check.
                if ( $isMaster ) {
                        $queryProf = 'query-m: ' . substr( DatabaseBase::generalizeSQL( $sql ), 0, 255 );
                        $totalProf = 'DatabaseBase::query-master';
                } else {
                        $queryProf = 'query: ' . substr( DatabaseBase::generalizeSQL( $sql ), 0, 255 );
                        $totalProf = 'DatabaseBase::query';
                }
                # Include query transaction state
                $queryProf .= $this->mTrxShortId ? " [TRX#{$this->mTrxShortId}]" : "";

                $profiler = Profiler::instance();
                if ( !$profiler instanceof ProfilerStub ) {
                        $totalProfSection = $profiler->scopedProfileIn( $totalProf );
                        $queryProfSection = $profiler->scopedProfileIn( $queryProf );
                }

                if ( $this->debug() ) {
                        static $cnt = 0;

                        $cnt++;
                        $sqlx = $wgDebugDumpSqlLength ? substr( $commentedSql, 0, $wgDebugDumpSqlLength )
                                : $commentedSql;
                        $sqlx = strtr( $sqlx, "\t\n", '  ' );

                        $master = $isMaster ? 'master' : 'slave';
                        wfDebug( "Query {$this->mDBname} ($cnt) ($master): $sqlx\n" );
                }

                $queryId = MWDebug::query( $sql, $fname, $isMaster );

                # Avoid fatals if close() was called
                $this->assertOpen();

                # Do the query and handle errors
                $startTime = microtime( true );
                $ret = $this->doQuery( $commentedSql );
                $queryRuntime = microtime( true ) - $startTime;
                # Log the query time and feed it into the DB trx profiler
                $this->getTransactionProfiler()->recordQueryCompletion(
                        $queryProf, $startTime, $isWriteQuery, $this->affectedRows() );

                MWDebug::queryTime( $queryId );

                # Try reconnecting if the connection was lost
                if ( false === $ret && $this->wasErrorReissuable() ) {
                        # Transaction is gone, like it or not
                        $hadTrx = $this->mTrxLevel; // possible lost transaction
                        $this->mTrxLevel = 0;
                        $this->mTrxIdleCallbacks = array(); // bug 65263
                        $this->mTrxPreCommitCallbacks = array(); // bug 65263
                        wfDebug( "Connection lost, reconnecting...\n" );
                        # Stash the last error values since ping() might clear them
                        $lastError = $this->lastError();
                        $lastErrno = $this->lastErrno();
                        if ( $this->ping() ) {
                                wfDebug( "Reconnected\n" );
                                $server = $this->getServer();
                                $msg = __METHOD__ . ": lost connection to $server; reconnected";
                                wfDebugLog( 'DBPerformance', "$msg:\n" . wfBacktrace( true ) );

                                if ( $hadTrx ) {
                                        # Leave $ret as false and let an error be reported.
                                        # Callers may catch the exception and continue to use the DB.
                                        $this->reportQueryError( $lastError, $lastErrno, $sql, $fname, $tempIgnore );
                                } else {
                                        # Should be safe to silently retry (no trx and thus no callbacks)
                                        $startTime = microtime( true );
                                        $ret = $this->doQuery( $commentedSql );
                                        $queryRuntime = microtime( true ) - $startTime;
                                        # Log the query time and feed it into the DB trx profiler
                                        $this->getTransactionProfiler()->recordQueryCompletion(
                                                $queryProf, $startTime, $isWriteQuery, $this->affectedRows() );
                                }
                        } else {
                                wfDebug( "Failed\n" );
                        }
                }

                if ( false === $ret ) {
                        $this->reportQueryError(
                                $this->lastError(), $this->lastErrno(), $sql, $fname, $tempIgnore );
                }

                $res = $this->resultObject( $ret );

                // Destroy profile sections in the opposite order to their creation
                ScopedCallback::consume( $queryProfSection );
                ScopedCallback::consume( $totalProfSection );

                if ( $isWriteQuery && $this->mTrxLevel ) {
                        $this->mTrxWriteDuration += $queryRuntime;
                }

                return $res;
        }

        /**
         * Report a query error. Log the error, and if neither the object ignore
         * flag nor the $tempIgnore flag is set, throw a DBQueryError.
         *
         * @param string $error
         * @param int $errno
         * @param string $sql
         * @param string $fname
         * @param bool $tempIgnore
         * @throws DBQueryError
         */
        public function reportQueryError( $error, $errno, $sql, $fname, $tempIgnore = false ) {
                if ( $this->ignoreErrors() || $tempIgnore ) {
                        wfDebug( "SQL ERROR (ignored): $error\n" );
                } else {
                        $sql1line = mb_substr( str_replace( "\n", "\\n", $sql ), 0, 5 * 1024 );
                        wfLogDBError(
                                "{fname}\t{db_server}\t{errno}\t{error}\t{sql1line}",
                                $this->getLogContext( array(
                                        'method' => __METHOD__,
                                        'errno' => $errno,
                                        'error' => $error,
                                        'sql1line' => $sql1line,
                                        'fname' => $fname,
                                ) )
                        );
                        wfDebug( "SQL ERROR: " . $error . "\n" );
                        throw new DBQueryError( $this, $error, $errno, $sql, $fname );
                }
        }

        /**
         * Intended to be compatible with the PEAR::DB wrapper functions.
         * http://pear.php.net/manual/en/package.database.db.intro-execute.php
         *
         * ? = scalar value, quoted as necessary
         * ! = raw SQL bit (a function for instance)
         * & = filename; reads the file and inserts as a blob
         *     (we don't use this though...)
         *
         * @param string $sql
         * @param string $func
         *
         * @return array
         */
        protected function prepare( $sql, $func = 'DatabaseBase::prepare' ) {
                /* MySQL doesn't support prepared statements (yet), so just
                 * pack up the query for reference. We'll manually replace
                 * the bits later.
                 */
                return array( 'query' => $sql, 'func' => $func );
        }

        /**
         * Free a prepared query, generated by prepare().
         * @param string $prepared
         */
        protected function freePrepared( $prepared ) {
                /* No-op by default */
        }

        /**
         * Execute a prepared query with the various arguments
         * @param string $prepared The prepared sql
         * @param mixed $args Either an array here, or put scalars as varargs
         *
         * @return ResultWrapper
         */
        public function execute( $prepared, $args = null ) {
                if ( !is_array( $args ) ) {
                        # Pull the var args
                        $args = func_get_args();
                        array_shift( $args );
                }

                $sql = $this->fillPrepared( $prepared['query'], $args );

                return $this->query( $sql, $prepared['func'] );
        }

        /**
         * For faking prepared SQL statements on DBs that don't support it directly.
         *
         * @param string $preparedQuery A 'preparable' SQL statement
         * @param array $args Array of Arguments to fill it with
         * @return string Executable SQL
         */
        public function fillPrepared( $preparedQuery, $args ) {
                reset( $args );
                $this->preparedArgs =& $args;

                return preg_replace_callback( '/(\\\\[?!&]|[?!&])/',
                        array( &$this, 'fillPreparedArg' ), $preparedQuery );
        }

        /**
         * preg_callback func for fillPrepared()
         * The arguments should be in $this->preparedArgs and must not be touched
         * while we're doing this.
         *
         * @param array $matches
         * @throws DBUnexpectedError
         * @return string
         */
        protected function fillPreparedArg( $matches ) {
                switch ( $matches[1] ) {
                        case '\\?':
                                return '?';
                        case '\\!':
                                return '!';
                        case '\\&':
                                return '&';
                }

                list( /* $n */, $arg ) = each( $this->preparedArgs );

                switch ( $matches[1] ) {
                        case '?':
                                return $this->addQuotes( $arg );
                        case '!':
                                return $arg;
                        case '&':
                                # return $this->addQuotes( file_get_contents( $arg ) );
                                throw new DBUnexpectedError(
                                        $this,
                                        '& mode is not implemented. If it\'s really needed, uncomment the line above.'
                                );
                        default:
                                throw new DBUnexpectedError(
                                        $this,
                                        'Received invalid match. This should never happen!'
                                );
                }
        }

        /**
         * Free a result object returned by query() or select(). It's usually not
         * necessary to call this, just use unset() or let the variable holding
         * the result object go out of scope.
         *
         * @param mixed $res A SQL result
         */
        public function freeResult( $res ) {
        }

        /**
         * A SELECT wrapper which returns a single field from a single result row.
         *
         * Usually throws a DBQueryError on failure. If errors are explicitly
         * ignored, returns false on failure.
         *
         * If no result rows are returned from the query, false is returned.
         *
         * @param string|array $table Table name. See DatabaseBase::select() for details.
         * @param string $var The field name to select. This must be a valid SQL
         *   fragment: do not use unvalidated user input.
         * @param string|array $cond The condition array. See DatabaseBase::select() for details.
         * @param string $fname The function name of the caller.
         * @param string|array $options The query options. See DatabaseBase::select() for details.
         *
         * @return bool|mixed The value from the field, or false on failure.
         * @throws DBUnexpectedError
         */
        public function selectField(
                $table, $var, $cond = '', $fname = __METHOD__, $options = array()
        ) {
                if ( $var === '*' ) { // sanity
                        throw new DBUnexpectedError( $this, "Cannot use a * field: got '$var'" );
                }

                if ( !is_array( $options ) ) {
                        $options = array( $options );
                }

                $options['LIMIT'] = 1;

                $res = $this->select( $table, $var, $cond, $fname, $options );
                if ( $res === false || !$this->numRows( $res ) ) {
                        return false;
                }

                $row = $this->fetchRow( $res );

                if ( $row !== false ) {
                        return reset( $row );
                } else {
                        return false;
                }
        }

        /**
         * A SELECT wrapper which returns a list of single field values from result rows.
         *
         * Usually throws a DBQueryError on failure. If errors are explicitly
         * ignored, returns false on failure.
         *
         * If no result rows are returned from the query, false is returned.
         *
         * @param string|array $table Table name. See DatabaseBase::select() for details.
         * @param string $var The field name to select. This must be a valid SQL
         *   fragment: do not use unvalidated user input.
         * @param string|array $cond The condition array. See DatabaseBase::select() for details.
         * @param string $fname The function name of the caller.
         * @param string|array $options The query options. See DatabaseBase::select() for details.
         *
         * @return bool|array The values from the field, or false on failure
         * @throws DBUnexpectedError
         * @since 1.25
         */
        public function selectFieldValues(
                $table, $var, $cond = '', $fname = __METHOD__, $options = array()
        ) {
                if ( $var === '*' ) { // sanity
                        throw new DBUnexpectedError( $this, "Cannot use a * field: got '$var'" );
                }

                if ( !is_array( $options ) ) {
                        $options = array( $options );
                }

                $res = $this->select( $table, $var, $cond, $fname, $options );
                if ( $res === false ) {
                        return false;
                }

                $values = array();
                foreach ( $res as $row ) {
                        $values[] = $row->$var;
                }

                return $values;
        }

        /**
         * Returns an optional USE INDEX clause to go after the table, and a
         * string to go at the end of the query.
         *
         * @param array $options Associative array of options to be turned into
         *   an SQL query, valid keys are listed in the function.
         * @return array
         * @see DatabaseBase::select()
         */
        public function makeSelectOptions( $options ) {
                $preLimitTail = $postLimitTail = '';
                $startOpts = '';

                $noKeyOptions = array();

                foreach ( $options as $key => $option ) {
                        if ( is_numeric( $key ) ) {
                                $noKeyOptions[$option] = true;
                        }
                }

                $preLimitTail .= $this->makeGroupByWithHaving( $options );

                $preLimitTail .= $this->makeOrderBy( $options );

                // if (isset($options['LIMIT'])) {
                //      $tailOpts .= $this->limitResult('', $options['LIMIT'],
                //              isset($options['OFFSET']) ? $options['OFFSET']
                //              : false);
                // }

                if ( isset( $noKeyOptions['FOR UPDATE'] ) ) {
                        $postLimitTail .= ' FOR UPDATE';
                }

                if ( isset( $noKeyOptions['LOCK IN SHARE MODE'] ) ) {
                        $postLimitTail .= ' LOCK IN SHARE MODE';
                }

                if ( isset( $noKeyOptions['DISTINCT'] ) || isset( $noKeyOptions['DISTINCTROW'] ) ) {
                        $startOpts .= 'DISTINCT';
                }

                # Various MySQL extensions
                if ( isset( $noKeyOptions['STRAIGHT_JOIN'] ) ) {
                        $startOpts .= ' /*! STRAIGHT_JOIN */';
                }

                if ( isset( $noKeyOptions['HIGH_PRIORITY'] ) ) {
                        $startOpts .= ' HIGH_PRIORITY';
                }

                if ( isset( $noKeyOptions['SQL_BIG_RESULT'] ) ) {
                        $startOpts .= ' SQL_BIG_RESULT';
                }

                if ( isset( $noKeyOptions['SQL_BUFFER_RESULT'] ) ) {
                        $startOpts .= ' SQL_BUFFER_RESULT';
                }

                if ( isset( $noKeyOptions['SQL_SMALL_RESULT'] ) ) {
                        $startOpts .= ' SQL_SMALL_RESULT';
                }

                if ( isset( $noKeyOptions['SQL_CALC_FOUND_ROWS'] ) ) {
                        $startOpts .= ' SQL_CALC_FOUND_ROWS';
                }

                if ( isset( $noKeyOptions['SQL_CACHE'] ) ) {
                        $startOpts .= ' SQL_CACHE';
                }

                if ( isset( $noKeyOptions['SQL_NO_CACHE'] ) ) {
                        $startOpts .= ' SQL_NO_CACHE';
                }

                if ( isset( $options['USE INDEX'] ) && is_string( $options['USE INDEX'] ) ) {
                        $useIndex = $this->useIndexClause( $options['USE INDEX'] );
                } else {
                        $useIndex = '';
                }

                return array( $startOpts, $useIndex, $preLimitTail, $postLimitTail );
        }

        /**
         * Returns an optional GROUP BY with an optional HAVING
         *
         * @param array $options Associative array of options
         * @return string
         * @see DatabaseBase::select()
         * @since 1.21
         */
        public function makeGroupByWithHaving( $options ) {
                $sql = '';
                if ( isset( $options['GROUP BY'] ) ) {
                        $gb = is_array( $options['GROUP BY'] )
                                ? implode( ',', $options['GROUP BY'] )
                                : $options['GROUP BY'];
                        $sql .= ' GROUP BY ' . $gb;
                }
                if ( isset( $options['HAVING'] ) ) {
                        $having = is_array( $options['HAVING'] )
                                ? $this->makeList( $options['HAVING'], LIST_AND )
                                : $options['HAVING'];
                        $sql .= ' HAVING ' . $having;
                }

                return $sql;
        }

        /**
         * Returns an optional ORDER BY
         *
         * @param array $options Associative array of options
         * @return string
         * @see DatabaseBase::select()
         * @since 1.21
         */
        public function makeOrderBy( $options ) {
                if ( isset( $options['ORDER BY'] ) ) {
                        $ob = is_array( $options['ORDER BY'] )
                                ? implode( ',', $options['ORDER BY'] )
                                : $options['ORDER BY'];

                        return ' ORDER BY ' . $ob;
                }

                return '';
        }

        /**
         * Execute a SELECT query constructed using the various parameters provided.
         * See below for full details of the parameters.
         *
         * @param string|array $table Table name
         * @param string|array $vars Field names
         * @param string|array $conds Conditions
         * @param string $fname Caller function name
         * @param array $options Query options
         * @param array $join_conds Join conditions
         *
         *
         * @param string|array $table
         *
         * May be either an array of table names, or a single string holding a table
         * name. If an array is given, table aliases can be specified, for example:
         *
         *    array( 'a' => 'user' )
         *
         * This includes the user table in the query, with the alias "a" available
         * for use in field names (e.g. a.user_name).
         *
         * All of the table names given here are automatically run through
         * DatabaseBase::tableName(), which causes the table prefix (if any) to be
         * added, and various other table name mappings to be performed.
         *
         *
         * @param string|array $vars
         *
         * May be either a field name or an array of field names. The field names
         * can be complete fragments of SQL, for direct inclusion into the SELECT
         * query. If an array is given, field aliases can be specified, for example:
         *
         *   array( 'maxrev' => 'MAX(rev_id)' )
         *
         * This includes an expression with the alias "maxrev" in the query.
         *
         * If an expression is given, care must be taken to ensure that it is
         * DBMS-independent.
         *
         *
         * @param string|array $conds
         *
         * May be either a string containing a single condition, or an array of
         * conditions. If an array is given, the conditions constructed from each
         * element are combined with AND.
         *
         * Array elements may take one of two forms:
         *
         *   - Elements with a numeric key are interpreted as raw SQL fragments.
         *   - Elements with a string key are interpreted as equality conditions,
         *     where the key is the field name.
         *     - If the value of such an array element is a scalar (such as a
         *       string), it will be treated as data and thus quoted appropriately.
         *       If it is null, an IS NULL clause will be added.
         *     - If the value is an array, an IN (...) clause will be constructed
         *       from its non-null elements, and an IS NULL clause will be added
         *       if null is present, such that the field may match any of the
         *       elements in the array. The non-null elements will be quoted.
         *
         * Note that expressions are often DBMS-dependent in their syntax.
         * DBMS-independent wrappers are provided for constructing several types of
         * expression commonly used in condition queries. See:
         *    - DatabaseBase::buildLike()
         *    - DatabaseBase::conditional()
         *
         *
         * @param string|array $options
         *
         * Optional: Array of query options. Boolean options are specified by
         * including them in the array as a string value with a numeric key, for
         * example:
         *
         *    array( 'FOR UPDATE' )
         *
         * The supported options are:
         *
         *   - OFFSET: Skip this many rows at the start of the result set. OFFSET
         *     with LIMIT can theoretically be used for paging through a result set,
         *     but this is discouraged in MediaWiki for performance reasons.
         *
         *   - LIMIT: Integer: return at most this many rows. The rows are sorted
         *     and then the first rows are taken until the limit is reached. LIMIT
         *     is applied to a result set after OFFSET.
         *
         *   - FOR UPDATE: Boolean: lock the returned rows so that they can't be
         *     changed until the next COMMIT.
         *
         *   - DISTINCT: Boolean: return only unique result rows.
         *
         *   - GROUP BY: May be either an SQL fragment string naming a field or
         *     expression to group by, or an array of such SQL fragments.
         *
         *   - HAVING: May be either an string containing a HAVING clause or an array of
         *     conditions building the HAVING clause. If an array is given, the conditions
         *     constructed from each element are combined with AND.
         *
         *   - ORDER BY: May be either an SQL fragment giving a field name or
         *     expression to order by, or an array of such SQL fragments.
         *
         *   - USE INDEX: This may be either a string giving the index name to use
         *     for the query, or an array. If it is an associative array, each key
         *     gives the table name (or alias), each value gives the index name to
         *     use for that table. All strings are SQL fragments and so should be
         *     validated by the caller.
         *
         *   - EXPLAIN: In MySQL, this causes an EXPLAIN SELECT query to be run,
         *     instead of SELECT.
         *
         * And also the following boolean MySQL extensions, see the MySQL manual
         * for documentation:
         *
         *    - LOCK IN SHARE MODE
         *    - STRAIGHT_JOIN
         *    - HIGH_PRIORITY
         *    - SQL_BIG_RESULT
         *    - SQL_BUFFER_RESULT
         *    - SQL_SMALL_RESULT
         *    - SQL_CALC_FOUND_ROWS
         *    - SQL_CACHE
         *    - SQL_NO_CACHE
         *
         *
         * @param string|array $join_conds
         *
         * Optional associative array of table-specific join conditions. In the
         * most common case, this is unnecessary, since the join condition can be
         * in $conds. However, it is useful for doing a LEFT JOIN.
         *
         * The key of the array contains the table name or alias. The value is an
         * array with two elements, numbered 0 and 1. The first gives the type of
         * join, the second is an SQL fragment giving the join condition for that
         * table. For example:
         *
         *    array( 'page' => array( 'LEFT JOIN', 'page_latest=rev_id' ) )
         *
         * @return ResultWrapper|bool If the query returned no rows, a ResultWrapper
         *   with no rows in it will be returned. If there was a query error, a
         *   DBQueryError exception will be thrown, except if the "ignore errors"
         *   option was set, in which case false will be returned.
         */
        public function select( $table, $vars, $conds = '', $fname = __METHOD__,
                $options = array(), $join_conds = array() ) {
                $sql = $this->selectSQLText( $table, $vars, $conds, $fname, $options, $join_conds );

                return $this->query( $sql, $fname );
        }

        /**
         * The equivalent of DatabaseBase::select() except that the constructed SQL
         * is returned, instead of being immediately executed. This can be useful for
         * doing UNION queries, where the SQL text of each query is needed. In general,
         * however, callers outside of Database classes should just use select().
         *
         * @param string|array $table Table name
         * @param string|array $vars Field names
         * @param string|array $conds Conditions
         * @param string $fname Caller function name
         * @param string|array $options Query options
         * @param string|array $join_conds Join conditions
         *
         * @return string SQL query string.
         * @see DatabaseBase::select()
         */
        public function selectSQLText( $table, $vars, $conds = '', $fname = __METHOD__,
                $options = array(), $join_conds = array()
        ) {
                if ( is_array( $vars ) ) {
                        $vars = implode( ',', $this->fieldNamesWithAlias( $vars ) );
                }

                $options = (array)$options;
                $useIndexes = ( isset( $options['USE INDEX'] ) && is_array( $options['USE INDEX'] ) )
                        ? $options['USE INDEX']
                        : array();

                if ( is_array( $table ) ) {
                        $from = ' FROM ' .
                                $this->tableNamesWithUseIndexOrJOIN( $table, $useIndexes, $join_conds );
                } elseif ( $table != '' ) {
                        if ( $table[0] == ' ' ) {
                                $from = ' FROM ' . $table;
                        } else {
                                $from = ' FROM ' .
                                        $this->tableNamesWithUseIndexOrJOIN( array( $table ), $useIndexes, array() );
                        }
                } else {
                        $from = '';
                }

                list( $startOpts, $useIndex, $preLimitTail, $postLimitTail ) =
                        $this->makeSelectOptions( $options );

                if ( !empty( $conds ) ) {
                        if ( is_array( $conds ) ) {
                                $conds = $this->makeList( $conds, LIST_AND );
                        }
                        $sql = "SELECT $startOpts $vars $from $useIndex WHERE $conds $preLimitTail";
                } else {
                        $sql = "SELECT $startOpts $vars $from $useIndex $preLimitTail";
                }

                if ( isset( $options['LIMIT'] ) ) {
                        $sql = $this->limitResult( $sql, $options['LIMIT'],
                                isset( $options['OFFSET'] ) ? $options['OFFSET'] : false );
                }
                $sql = "$sql $postLimitTail";

                if ( isset( $options['EXPLAIN'] ) ) {
                        $sql = 'EXPLAIN ' . $sql;
                }

                return $sql;
        }

        /**
         * Single row SELECT wrapper. Equivalent to DatabaseBase::select(), except
         * that a single row object is returned. If the query returns no rows,
         * false is returned.
         *
         * @param string|array $table Table name
         * @param string|array $vars Field names
         * @param array $conds Conditions
         * @param string $fname Caller function name
         * @param string|array $options Query options
         * @param array|string $join_conds Join conditions
         *
         * @return stdClass|bool
         */
        public function selectRow( $table, $vars, $conds, $fname = __METHOD__,
                $options = array(), $join_conds = array()
        ) {
                $options = (array)$options;
                $options['LIMIT'] = 1;
                $res = $this->select( $table, $vars, $conds, $fname, $options, $join_conds );

                if ( $res === false ) {
                        return false;
                }

                if ( !$this->numRows( $res ) ) {
                        return false;
                }

                $obj = $this->fetchObject( $res );

                return $obj;
        }

        /**
         * Estimate the number of rows in dataset
         *
         * MySQL allows you to estimate the number of rows that would be returned
         * by a SELECT query, using EXPLAIN SELECT. The estimate is provided using
         * index cardinality statistics, and is notoriously inaccurate, especially
         * when large numbers of rows have recently been added or deleted.
         *
         * For DBMSs that don't support fast result size estimation, this function
         * will actually perform the SELECT COUNT(*).
         *
         * Takes the same arguments as DatabaseBase::select().
         *
         * @param string $table Table name
         * @param string $vars Unused
         * @param array|string $conds Filters on the table
         * @param string $fname Function name for profiling
         * @param array $options Options for select
         * @return int Row count
         */
        public function estimateRowCount(
                $table, $vars = '*', $conds = '', $fname = __METHOD__, $options = array()
        ) {
                $rows = 0;
                $res = $this->select( $table, array( 'rowcount' => 'COUNT(*)' ), $conds, $fname, $options );

                if ( $res ) {
                        $row = $this->fetchRow( $res );
                        $rows = ( isset( $row['rowcount'] ) ) ? (int)$row['rowcount'] : 0;
                }

                return $rows;
        }

        /**
         * Get the number of rows in dataset
         *
         * This is useful when trying to do COUNT(*) but with a LIMIT for performance.
         *
         * Takes the same arguments as DatabaseBase::select().
         *
         * @param string $table Table name
         * @param string $vars Unused
         * @param array|string $conds Filters on the table
         * @param string $fname Function name for profiling
         * @param array $options Options for select
         * @return int Row count
         * @since 1.24
         */
        public function selectRowCount(
                $table, $vars = '*', $conds = '', $fname = __METHOD__, $options = array()
        ) {
                $rows = 0;
                $sql = $this->selectSQLText( $table, '1', $conds, $fname, $options );
                $res = $this->query( "SELECT COUNT(*) AS rowcount FROM ($sql) tmp_count", $fname );

                if ( $res ) {
                        $row = $this->fetchRow( $res );
                        $rows = ( isset( $row['rowcount'] ) ) ? (int)$row['rowcount'] : 0;
                }

                return $rows;
        }

        /**
         * Removes most variables from an SQL query and replaces them with X or N for numbers.
         * It's only slightly flawed. Don't use for anything important.
         *
         * @param string $sql A SQL Query
         *
         * @return string
         */
        static function generalizeSQL( $sql ) {
                # This does the same as the regexp below would do, but in such a way
                # as to avoid crashing php on some large strings.
                # $sql = preg_replace( "/'([^\\\\']|\\\\.)*'|\"([^\\\\\"]|\\\\.)*\"/", "'X'", $sql );

                $sql = str_replace( "\\\\", '', $sql );
                $sql = str_replace( "\\'", '', $sql );
                $sql = str_replace( "\\\"", '', $sql );
                $sql = preg_replace( "/'.*'/s", "'X'", $sql );
                $sql = preg_replace( '/".*"/s', "'X'", $sql );

                # All newlines, tabs, etc replaced by single space
                $sql = preg_replace( '/\s+/', ' ', $sql );

                # All numbers => N,
                # except the ones surrounded by characters, e.g. l10n
                $sql = preg_replace( '/-?\d+(,-?\d+)+/s', 'N,...,N', $sql );
                $sql = preg_replace( '/(?<![a-zA-Z])-?\d+(?![a-zA-Z])/s', 'N', $sql );

                return $sql;
        }

        /**
         * Determines whether a field exists in a table
         *
         * @param string $table Table name
         * @param string $field Filed to check on that table
         * @param string $fname Calling function name (optional)
         * @return bool Whether $table has filed $field
         */
        public function fieldExists( $table, $field, $fname = __METHOD__ ) {
                $info = $this->fieldInfo( $table, $field );

                return (bool)$info;
        }

        /**
         * Determines whether an index exists
         * Usually throws a DBQueryError on failure
         * If errors are explicitly ignored, returns NULL on failure
         *
         * @param string $table
         * @param string $index
         * @param string $fname
         * @return bool|null
         */
        public function indexExists( $table, $index, $fname = __METHOD__ ) {
                if ( !$this->tableExists( $table ) ) {
                        return null;
                }

                $info = $this->indexInfo( $table, $index, $fname );
                if ( is_null( $info ) ) {
                        return null;
                } else {
                        return $info !== false;
                }
        }

        /**
         * Query whether a given table exists
         *
         * @param string $table
         * @param string $fname
         * @return bool
         */
        public function tableExists( $table, $fname = __METHOD__ ) {
                $table = $this->tableName( $table );
                $old = $this->ignoreErrors( true );
                $res = $this->query( "SELECT 1 FROM $table LIMIT 1", $fname );
                $this->ignoreErrors( $old );

                return (bool)$res;
        }

        /**
         * Determines if a given index is unique
         *
         * @param string $table
         * @param string $index
         *
         * @return bool
         */
        public function indexUnique( $table, $index ) {
                $indexInfo = $this->indexInfo( $table, $index );

                if ( !$indexInfo ) {
                        return null;
                }

                return !$indexInfo[0]->Non_unique;
        }

        /**
         * Helper for DatabaseBase::insert().
         *
         * @param array $options
         * @return string
         */
        protected function makeInsertOptions( $options ) {
                return implode( ' ', $options );
        }

        /**
         * INSERT wrapper, inserts an array into a table.
         *
         * $a may be either:
         *
         *   - A single associative array. The array keys are the field names, and
         *     the values are the values to insert. The values are treated as data
         *     and will be quoted appropriately. If NULL is inserted, this will be
         *     converted to a database NULL.
         *   - An array with numeric keys, holding a list of associative arrays.
         *     This causes a multi-row INSERT on DBMSs that support it. The keys in
         *     each subarray must be identical to each other, and in the same order.
         *
         * $options is an array of options, with boolean options encoded as values
         * with numeric keys, in the same style as $options in
         * DatabaseBase::select(). Supported options are:
         *
         *   - IGNORE: Boolean: if present, duplicate key errors are ignored, and
         *     any rows which cause duplicate key errors are not inserted. It's
         *     possible to determine how many rows were successfully inserted using
         *     DatabaseBase::affectedRows().
         *
         * @param string $table Table name. This will be passed through
         *   DatabaseBase::tableName().
         * @param array $a Array of rows to insert
         * @param string $fname Calling function name (use __METHOD__) for logs/profiling
         * @param array $options Array of options
         *
         * @throws DBQueryError Usually throws a DBQueryError on failure. If errors are explicitly ignored,
         * returns success.
         *
         * @return bool
         */
        public function insert( $table, $a, $fname = __METHOD__, $options = array() ) {
                # No rows to insert, easy just return now
                if ( !count( $a ) ) {
                        return true;
                }

                $table = $this->tableName( $table );

                if ( !is_array( $options ) ) {
                        $options = array( $options );
                }

                $fh = null;
                if ( isset( $options['fileHandle'] ) ) {
                        $fh = $options['fileHandle'];
                }
                $options = $this->makeInsertOptions( $options );

                if ( isset( $a[0] ) && is_array( $a[0] ) ) {
                        $multi = true;
                        $keys = array_keys( $a[0] );
                } else {
                        $multi = false;
                        $keys = array_keys( $a );
                }

                $sql = 'INSERT ' . $options .
                        " INTO $table (" . implode( ',', $keys ) . ') VALUES ';

                if ( $multi ) {
                        $first = true;
                        foreach ( $a as $row ) {
                                if ( $first ) {
                                        $first = false;
                                } else {
                                        $sql .= ',';
                                }
                                $sql .= '(' . $this->makeList( $row ) . ')';
                        }
                } else {
                        $sql .= '(' . $this->makeList( $a ) . ')';
                }

                if ( $fh !== null && false === fwrite( $fh, $sql ) ) {
                        return false;
                } elseif ( $fh !== null ) {
                        return true;
                }

                return (bool)$this->query( $sql, $fname );
        }

        /**
         * Make UPDATE options array for DatabaseBase::makeUpdateOptions
         *
         * @param array $options
         * @return array
         */
        protected function makeUpdateOptionsArray( $options ) {
                if ( !is_array( $options ) ) {
                        $options = array( $options );
                }

                $opts = array();

                if ( in_array( 'LOW_PRIORITY', $options ) ) {
                        $opts[] = $this->lowPriorityOption();
                }

                if ( in_array( 'IGNORE', $options ) ) {
                        $opts[] = 'IGNORE';
                }

                return $opts;
        }

        /**
         * Make UPDATE options for the DatabaseBase::update function
         *
         * @param array $options The options passed to DatabaseBase::update
         * @return string
         */
        protected function makeUpdateOptions( $options ) {
                $opts = $this->makeUpdateOptionsArray( $options );

                return implode( ' ', $opts );
        }

        /**
         * UPDATE wrapper. Takes a condition array and a SET array.
         *
         * @param string $table Name of the table to UPDATE. This will be passed through
         *   DatabaseBase::tableName().
         * @param array $values An array of values to SET. For each array element,
         *   the key gives the field name, and the value gives the data to set
         *   that field to. The data will be quoted by DatabaseBase::addQuotes().
         * @param array $conds An array of conditions (WHERE). See
         *   DatabaseBase::select() for the details of the format of condition
         *   arrays. Use '*' to update all rows.
         * @param string $fname The function name of the caller (from __METHOD__),
         *   for logging and profiling.
         * @param array $options An array of UPDATE options, can be:
         *   - IGNORE: Ignore unique key conflicts
         *   - LOW_PRIORITY: MySQL-specific, see MySQL manual.
         * @return bool
         */
        function update( $table, $values, $conds, $fname = __METHOD__, $options = array() ) {
                $table = $this->tableName( $table );
                $opts = $this->makeUpdateOptions( $options );
                $sql = "UPDATE $opts $table SET " . $this->makeList( $values, LIST_SET );

                if ( $conds !== array() && $conds !== '*' ) {
                        $sql .= " WHERE " . $this->makeList( $conds, LIST_AND );
                }

                return $this->query( $sql, $fname );
        }

        /**
         * Makes an encoded list of strings from an array
         *
         * @param array $a Containing the data
         * @param int $mode Constant
         *    - LIST_COMMA: Comma separated, no field names
         *    - LIST_AND:   ANDed WHERE clause (without the WHERE). See the
         *      documentation for $conds in DatabaseBase::select().
         *    - LIST_OR:    ORed WHERE clause (without the WHERE)
         *    - LIST_SET:   Comma separated with field names, like a SET clause
         *    - LIST_NAMES: Comma separated field names
         * @throws MWException|DBUnexpectedError
         * @return string
         */
        public function makeList( $a, $mode = LIST_COMMA ) {
                if ( !is_array( $a ) ) {
                        throw new DBUnexpectedError( $this, 'DatabaseBase::makeList called with incorrect parameters' );
                }

                $first = true;
                $list = '';

                foreach ( $a as $field => $value ) {
                        if ( !$first ) {
                                if ( $mode == LIST_AND ) {
                                        $list .= ' AND ';
                                } elseif ( $mode == LIST_OR ) {
                                        $list .= ' OR ';
                                } else {
                                        $list .= ',';
                                }
                        } else {
                                $first = false;
                        }

                        if ( ( $mode == LIST_AND || $mode == LIST_OR ) && is_numeric( $field ) ) {
                                $list .= "($value)";
                        } elseif ( ( $mode == LIST_SET ) && is_numeric( $field ) ) {
                                $list .= "$value";
                        } elseif ( ( $mode == LIST_AND || $mode == LIST_OR ) && is_array( $value ) ) {
                                // Remove null from array to be handled separately if found
                                $includeNull = false;
                                foreach ( array_keys( $value, null, true ) as $nullKey ) {
                                        $includeNull = true;
                                        unset( $value[$nullKey] );
                                }
                                if ( count( $value ) == 0 && !$includeNull ) {
                                        throw new MWException( __METHOD__ . ": empty input for field $field" );
                                } elseif ( count( $value ) == 0 ) {
                                        // only check if $field is null
                                        $list .= "$field IS NULL";
                                } else {
                                        // IN clause contains at least one valid element
                                        if ( $includeNull ) {
                                                // Group subconditions to ensure correct precedence
                                                $list .= '(';
                                        }
                                        if ( count( $value ) == 1 ) {
                                                // Special-case single values, as IN isn't terribly efficient
                                                // Don't necessarily assume the single key is 0; we don't
                                                // enforce linear numeric ordering on other arrays here.
                                                $value = array_values( $value );
                                                $list .= $field . " = " . $this->addQuotes( $value[0] );
                                        } else {
                                                $list .= $field . " IN (" . $this->makeList( $value ) . ") ";
                                        }
                                        // if null present in array, append IS NULL
                                        if ( $includeNull ) {
                                                $list .= " OR $field IS NULL)";
                                        }
                                }
                        } elseif ( $value === null ) {
                                if ( $mode == LIST_AND || $mode == LIST_OR ) {
                                        $list .= "$field IS ";
                                } elseif ( $mode == LIST_SET ) {
                                        $list .= "$field = ";
                                }
                                $list .= 'NULL';
                        } else {
                                if ( $mode == LIST_AND || $mode == LIST_OR || $mode == LIST_SET ) {
                                        $list .= "$field = ";
                                }
                                $list .= $mode == LIST_NAMES ? $value : $this->addQuotes( $value );
                        }
                }

                return $list;
        }

        /**
         * Build a partial where clause from a 2-d array such as used for LinkBatch.
         * The keys on each level may be either integers or strings.
         *
         * @param array $data Organized as 2-d
         *    array(baseKeyVal => array(subKeyVal => [ignored], ...), ...)
         * @param string $baseKey Field name to match the base-level keys to (eg 'pl_namespace')
         * @param string $subKey Field name to match the sub-level keys to (eg 'pl_title')
         * @return string|bool SQL fragment, or false if no items in array
         */
        public function makeWhereFrom2d( $data, $baseKey, $subKey ) {
                $conds = array();

                foreach ( $data as $base => $sub ) {
                        if ( count( $sub ) ) {
                                $conds[] = $this->makeList(
                                        array( $baseKey => $base, $subKey => array_keys( $sub ) ),
                                        LIST_AND );
                        }
                }

                if ( $conds ) {
                        return $this->makeList( $conds, LIST_OR );
                } else {
                        // Nothing to search for...
                        return false;
                }
        }

        /**
         * Return aggregated value alias
         *
         * @param array $valuedata
         * @param string $valuename
         *
         * @return string
         */
        public function aggregateValue( $valuedata, $valuename = 'value' ) {
                return $valuename;
        }

        /**
         * @param string $field
         * @return string
         */
        public function bitNot( $field ) {
                return "(~$field)";
        }

        /**
         * @param string $fieldLeft
         * @param string $fieldRight
         * @return string
         */
        public function bitAnd( $fieldLeft, $fieldRight ) {
                return "($fieldLeft & $fieldRight)";
        }

        /**
         * @param string $fieldLeft
         * @param string $fieldRight
         * @return string
         */
        public function bitOr( $fieldLeft, $fieldRight ) {
                return "($fieldLeft | $fieldRight)";
        }

        /**
         * Build a concatenation list to feed into a SQL query
         * @param array $stringList List of raw SQL expressions; caller is
         *   responsible for any quoting
         * @return string
         */
        public function buildConcat( $stringList ) {
                return 'CONCAT(' . implode( ',', $stringList ) . ')';
        }

        /**
         * Build a GROUP_CONCAT or equivalent statement for a query.
         *
         * This is useful for combining a field for several rows into a single string.
         * NULL values will not appear in the output, duplicated values will appear,
         * and the resulting delimiter-separated values have no defined sort order.
         * Code using the results may need to use the PHP unique() or sort() methods.
         *
         * @param string $delim Glue to bind the results together
         * @param string|array $table Table name
         * @param string $field Field name
         * @param string|array $conds Conditions
         * @param string|array $join_conds Join conditions
         * @return string SQL text
         * @since 1.23
         */
        public function buildGroupConcatField(
                $delim, $table, $field, $conds = '', $join_conds = array()
        ) {
                $fld = "GROUP_CONCAT($field SEPARATOR " . $this->addQuotes( $delim ) . ')';

                return '(' . $this->selectSQLText( $table, $fld, $conds, null, array(), $join_conds ) . ')';
        }

        /**
         * Change the current database
         *
         * @todo Explain what exactly will fail if this is not overridden.
         *
         * @param string $db
         *
         * @return bool Success or failure
         */
        public function selectDB( $db ) {
                # Stub. Shouldn't cause serious problems if it's not overridden, but
                # if your database engine supports a concept similar to MySQL's
                # databases you may as well.
                $this->mDBname = $db;

                return true;
        }

        /**
         * Get the current DB name
         * @return string
         */
        public function getDBname() {
                return $this->mDBname;
        }

        /**
         * Get the server hostname or IP address
         * @return string
         */
        public function getServer() {
                return $this->mServer;
        }

        /**
         * Format a table name ready for use in constructing an SQL query
         *
         * This does two important things: it quotes the table names to clean them up,
         * and it adds a table prefix if only given a table name with no quotes.
         *
         * All functions of this object which require a table name call this function
         * themselves. Pass the canonical name to such functions. This is only needed
         * when calling query() directly.
         *
         * @param string $name Database table name
         * @param string $format One of:
         *   quoted - Automatically pass the table name through addIdentifierQuotes()
         *            so that it can be used in a query.
         *   raw - Do not add identifier quotes to the table name
         * @return string Full database name
         */
        public function tableName( $name, $format = 'quoted' ) {
                global $wgSharedDB, $wgSharedPrefix, $wgSharedTables, $wgSharedSchema;
                # Skip the entire process when we have a string quoted on both ends.
                # Note that we check the end so that we will still quote any use of
                # use of `database`.table. But won't break things if someone wants
                # to query a database table with a dot in the name.
                if ( $this->isQuotedIdentifier( $name ) ) {
                        return $name;
                }

                # Lets test for any bits of text that should never show up in a table
                # name. Basically anything like JOIN or ON which are actually part of
                # SQL queries, but may end up inside of the table value to combine
                # sql. Such as how the API is doing.
                # Note that we use a whitespace test rather than a \b test to avoid
                # any remote case where a word like on may be inside of a table name
                # surrounded by symbols which may be considered word breaks.
                if ( preg_match( '/(^|\s)(DISTINCT|JOIN|ON|AS)(\s|$)/i', $name ) !== 0 ) {
                        return $name;
                }

                # Split database and table into proper variables.
                # We reverse the explode so that database.table and table both output
                # the correct table.
                $dbDetails = explode( '.', $name, 3 );
                if ( count( $dbDetails ) == 3 ) {
                        list( $database, $schema, $table ) = $dbDetails;
                        # We don't want any prefix added in this case
                        $prefix = '';
                } elseif ( count( $dbDetails ) == 2 ) {
                        list( $database, $table ) = $dbDetails;
                        # We don't want any prefix added in this case
                        # In dbs that support it, $database may actually be the schema
                        # but that doesn't affect any of the functionality here
                        $prefix = '';
                        $schema = null;
                } else {
                        list( $table ) = $dbDetails;
                        if ( $wgSharedDB !== null # We have a shared database
                                && $this->mForeign == false # We're not working on a foreign database
                                && !$this->isQuotedIdentifier( $table ) # Prevent shared tables listing '`table`'
                                && in_array( $table, $wgSharedTables ) # A shared table is selected
                        ) {
                                $database = $wgSharedDB;
                                $schema = $wgSharedSchema === null ? $this->mSchema : $wgSharedSchema;
                                $prefix = $wgSharedPrefix === null ? $this->mTablePrefix : $wgSharedPrefix;
                        } else {
                                $database = null;
                                $schema = $this->mSchema; # Default schema
                                $prefix = $this->mTablePrefix; # Default prefix
                        }
                }

                # Quote $table and apply the prefix if not quoted.
                # $tableName might be empty if this is called from Database::replaceVars()
                $tableName = "{$prefix}{$table}";
                if ( $format == 'quoted' && !$this->isQuotedIdentifier( $tableName ) && $tableName !== '' ) {
                        $tableName = $this->addIdentifierQuotes( $tableName );
                }

                # Quote $schema and merge it with the table name if needed
                if ( strlen( $schema ) ) {
                        if ( $format == 'quoted' && !$this->isQuotedIdentifier( $schema ) ) {
                                $schema = $this->addIdentifierQuotes( $schema );
                        }
                        $tableName = $schema . '.' . $tableName;
                }

                # Quote $database and merge it with the table name if needed
                if ( $database !== null ) {
                        if ( $format == 'quoted' && !$this->isQuotedIdentifier( $database ) ) {
                                $database = $this->addIdentifierQuotes( $database );
                        }
                        $tableName = $database . '.' . $tableName;
                }

                return $tableName;
        }

        /**
         * Fetch a number of table names into an array
         * This is handy when you need to construct SQL for joins
         *
         * Example:
         * extract( $dbr->tableNames( 'user', 'watchlist' ) );
         * $sql = "SELECT wl_namespace,wl_title FROM $watchlist,$user
         *         WHERE wl_user=user_id AND wl_user=$nameWithQuotes";
         *
         * @return array
         */
        public function tableNames() {
                $inArray = func_get_args();
                $retVal = array();

                foreach ( $inArray as $name ) {
                        $retVal[$name] = $this->tableName( $name );
                }

                return $retVal;
        }

        /**
         * Fetch a number of table names into an zero-indexed numerical array
         * This is handy when you need to construct SQL for joins
         *
         * Example:
         * list( $user, $watchlist ) = $dbr->tableNamesN( 'user', 'watchlist' );
         * $sql = "SELECT wl_namespace,wl_title FROM $watchlist,$user
         *         WHERE wl_user=user_id AND wl_user=$nameWithQuotes";
         *
         * @return array
         */
        public function tableNamesN() {
                $inArray = func_get_args();
                $retVal = array();

                foreach ( $inArray as $name ) {
                        $retVal[] = $this->tableName( $name );
                }

                return $retVal;
        }

        /**
         * Get an aliased table name
         * e.g. tableName AS newTableName
         *
         * @param string $name Table name, see tableName()
         * @param string|bool $alias Alias (optional)
         * @return string SQL name for aliased table. Will not alias a table to its own name
         */
        public function tableNameWithAlias( $name, $alias = false ) {
                if ( !$alias || $alias == $name ) {
                        return $this->tableName( $name );
                } else {
                        return $this->tableName( $name ) . ' ' . $this->addIdentifierQuotes( $alias );
                }
        }

        /**
         * Gets an array of aliased table names
         *
         * @param array $tables Array( [alias] => table )
         * @return string[] See tableNameWithAlias()
         */
        public function tableNamesWithAlias( $tables ) {
                $retval = array();
                foreach ( $tables as $alias => $table ) {
                        if ( is_numeric( $alias ) ) {
                                $alias = $table;
                        }
                        $retval[] = $this->tableNameWithAlias( $table, $alias );
                }

                return $retval;
        }

        /**
         * Get an aliased field name
         * e.g. fieldName AS newFieldName
         *
         * @param string $name Field name
         * @param string|bool $alias Alias (optional)
         * @return string SQL name for aliased field. Will not alias a field to its own name
         */
        public function fieldNameWithAlias( $name, $alias = false ) {
                if ( !$alias || (string)$alias === (string)$name ) {
                        return $name;
                } else {
                        return $name . ' AS ' . $alias; // PostgreSQL needs AS
                }
        }

        /**
         * Gets an array of aliased field names
         *
         * @param array $fields Array( [alias] => field )
         * @return string[] See fieldNameWithAlias()
         */
        public function fieldNamesWithAlias( $fields ) {
                $retval = array();
                foreach ( $fields as $alias => $field ) {
                        if ( is_numeric( $alias ) ) {
                                $alias = $field;
                        }
                        $retval[] = $this->fieldNameWithAlias( $field, $alias );
                }

                return $retval;
        }

        /**
         * Get the aliased table name clause for a FROM clause
         * which might have a JOIN and/or USE INDEX clause
         *
         * @param array $tables ( [alias] => table )
         * @param array $use_index Same as for select()
         * @param array $join_conds Same as for select()
         * @return string
         */
        protected function tableNamesWithUseIndexOrJOIN(
                $tables, $use_index = array(), $join_conds = array()
        ) {
                $ret = array();
                $retJOIN = array();
                $use_index = (array)$use_index;
                $join_conds = (array)$join_conds;

                foreach ( $tables as $alias => $table ) {
                        if ( !is_string( $alias ) ) {
                                // No alias? Set it equal to the table name
                                $alias = $table;
                        }
                        // Is there a JOIN clause for this table?
                        if ( isset( $join_conds[$alias] ) ) {
                                list( $joinType, $conds ) = $join_conds[$alias];
                                $tableClause = $joinType;
                                $tableClause .= ' ' . $this->tableNameWithAlias( $table, $alias );
                                if ( isset( $use_index[$alias] ) ) { // has USE INDEX?
                                        $use = $this->useIndexClause( implode( ',', (array)$use_index[$alias] ) );
                                        if ( $use != '' ) {
                                                $tableClause .= ' ' . $use;
                                        }
                                }
                                $on = $this->makeList( (array)$conds, LIST_AND );
                                if ( $on != '' ) {
                                        $tableClause .= ' ON (' . $on . ')';
                                }

                                $retJOIN[] = $tableClause;
                        } elseif ( isset( $use_index[$alias] ) ) {
                                // Is there an INDEX clause for this table?
                                $tableClause = $this->tableNameWithAlias( $table, $alias );
                                $tableClause .= ' ' . $this->useIndexClause(
                                        implode( ',', (array)$use_index[$alias] )
                                );

                                $ret[] = $tableClause;
                        } else {
                                $tableClause = $this->tableNameWithAlias( $table, $alias );

                                $ret[] = $tableClause;
                        }
                }

                // We can't separate explicit JOIN clauses with ',', use ' ' for those
                $implicitJoins = !empty( $ret ) ? implode( ',', $ret ) : "";
                $explicitJoins = !empty( $retJOIN ) ? implode( ' ', $retJOIN ) : "";

                // Compile our final table clause
                return implode( ' ', array( $implicitJoins, $explicitJoins ) );
        }

        /**
         * Get the name of an index in a given table.
         *
         * @protected Don't use outside of DatabaseBase and childs
         * @param string $index
         * @return string
         */
        public function indexName( $index ) {
                // @FIXME: Make this protected once we move away from PHP 5.3
                // Needs to be public because of usage in closure (in DatabaseBase::replaceVars)

                // Backwards-compatibility hack
                $renamed = array(
                        'ar_usertext_timestamp' => 'usertext_timestamp',
                        'un_user_id' => 'user_id',
                        'un_user_ip' => 'user_ip',
                );

                if ( isset( $renamed[$index] ) ) {
                        return $renamed[$index];
                } else {
                        return $index;
                }
        }

        /**
         * Adds quotes and backslashes.
         *
         * @param string|Blob $s
         * @return string
         */
        public function addQuotes( $s ) {
                if ( $s instanceof Blob ) {
                        $s = $s->fetch();
                }
                if ( $s === null ) {
                        return 'NULL';
                } else {
                        # This will also quote numeric values. This should be harmless,
                        # and protects against weird problems that occur when they really
                        # _are_ strings such as article titles and string->number->string
                        # conversion is not 1:1.
                        return "'" . $this->strencode( $s ) . "'";
                }
        }

        /**
         * Quotes an identifier using `backticks` or "double quotes" depending on the database type.
         * MySQL uses `backticks` while basically everything else uses double quotes.
         * Since MySQL is the odd one out here the double quotes are our generic
         * and we implement backticks in DatabaseMysql.
         *
         * @param string $s
         * @return string
         */
        public function addIdentifierQuotes( $s ) {
                return '"' . str_replace( '"', '""', $s ) . '"';
        }

        /**
         * Returns if the given identifier looks quoted or not according to
         * the database convention for quoting identifiers .
         *
         * @param string $name
         * @return bool
         */
        public function isQuotedIdentifier( $name ) {
                return $name[0] == '"' && substr( $name, -1, 1 ) == '"';
        }

        /**
         * @param string $s
         * @return string
         */
        protected function escapeLikeInternal( $s ) {
                return addcslashes( $s, '\%_' );
        }

        /**
         * LIKE statement wrapper, receives a variable-length argument list with
         * parts of pattern to match containing either string literals that will be
         * escaped or tokens returned by anyChar() or anyString(). Alternatively,
         * the function could be provided with an array of aforementioned
         * parameters.
         *
         * Example: $dbr->buildLike( 'My_page_title/', $dbr->anyString() ) returns
         * a LIKE clause that searches for subpages of 'My page title'.
         * Alternatively:
         *   $pattern = array( 'My_page_title/', $dbr->anyString() );
         *   $query .= $dbr->buildLike( $pattern );
         *
         * @since 1.16
         * @return string Fully built LIKE statement
         */
        public function buildLike() {
                $params = func_get_args();

                if ( count( $params ) > 0 && is_array( $params[0] ) ) {
                        $params = $params[0];
                }

                $s = '';

                foreach ( $params as $value ) {
                        if ( $value instanceof LikeMatch ) {
                                $s .= $value->toString();
                        } else {
                                $s .= $this->escapeLikeInternal( $value );
                        }
                }

                return " LIKE {$this->addQuotes( $s )} ";
        }

        /**
         * Returns a token for buildLike() that denotes a '_' to be used in a LIKE query
         *
         * @return LikeMatch
         */
        public function anyChar() {
                return new LikeMatch( '_' );
        }

        /**
         * Returns a token for buildLike() that denotes a '%' to be used in a LIKE query
         *
         * @return LikeMatch
         */
        public function anyString() {
                return new LikeMatch( '%' );
        }

        /**
         * Returns an appropriately quoted sequence value for inserting a new row.
         * MySQL has autoincrement fields, so this is just NULL. But the PostgreSQL
         * subclass will return an integer, and save the value for insertId()
         *
         * Any implementation of this function should *not* involve reusing
         * sequence numbers created for rolled-back transactions.
         * See http://bugs.mysql.com/bug.php?id=30767 for details.
         * @param string $seqName
         * @return null|int
         */
        public function nextSequenceValue( $seqName ) {
                return null;
        }

        /**
         * USE INDEX clause. Unlikely to be useful for anything but MySQL. This
         * is only needed because a) MySQL must be as efficient as possible due to
         * its use on Wikipedia, and b) MySQL 4.0 is kind of dumb sometimes about
         * which index to pick. Anyway, other databases might have different
         * indexes on a given table. So don't bother overriding this unless you're
         * MySQL.
         * @param string $index
         * @return string
         */
        public function useIndexClause( $index ) {
                return '';
        }

        /**
         * REPLACE query wrapper.
         *
         * REPLACE is a very handy MySQL extension, which functions like an INSERT
         * except that when there is a duplicate key error, the old row is deleted
         * and the new row is inserted in its place.
         *
         * We simulate this with standard SQL with a DELETE followed by INSERT. To
         * perform the delete, we need to know what the unique indexes are so that
         * we know how to find the conflicting rows.
         *
         * It may be more efficient to leave off unique indexes which are unlikely
         * to collide. However if you do this, you run the risk of encountering
         * errors which wouldn't have occurred in MySQL.
         *
         * @param string $table The table to replace the row(s) in.
         * @param array $uniqueIndexes Is an array of indexes. Each element may be either
         *    a field name or an array of field names
         * @param array $rows Can be either a single row to insert, or multiple rows,
         *    in the same format as for DatabaseBase::insert()
         * @param string $fname Calling function name (use __METHOD__) for logs/profiling
         */
        public function replace( $table, $uniqueIndexes, $rows, $fname = __METHOD__ ) {
                $quotedTable = $this->tableName( $table );

                if ( count( $rows ) == 0 ) {
                        return;
                }

                # Single row case
                if ( !is_array( reset( $rows ) ) ) {
                        $rows = array( $rows );
                }

                // @FXIME: this is not atomic, but a trx would break affectedRows()
                foreach ( $rows as $row ) {
                        # Delete rows which collide
                        if ( $uniqueIndexes ) {
                                $sql = "DELETE FROM $quotedTable WHERE ";
                                $first = true;
                                foreach ( $uniqueIndexes as $index ) {
                                        if ( $first ) {
                                                $first = false;
                                                $sql .= '( ';
                                        } else {
                                                $sql .= ' ) OR ( ';
                                        }
                                        if ( is_array( $index ) ) {
                                                $first2 = true;
                                                foreach ( $index as $col ) {
                                                        if ( $first2 ) {
                                                                $first2 = false;
                                                        } else {
                                                                $sql .= ' AND ';
                                                        }
                                                        $sql .= $col . '=' . $this->addQuotes( $row[$col] );
                                                }
                                        } else {
                                                $sql .= $index . '=' . $this->addQuotes( $row[$index] );
                                        }
                                }
                                $sql .= ' )';
                                $this->query( $sql, $fname );
                        }

                        # Now insert the row
                        $this->insert( $table, $row, $fname );
                }
        }

        /**
         * REPLACE query wrapper for MySQL and SQLite, which have a native REPLACE
         * statement.
         *
         * @param string $table Table name
         * @param array|string $rows Row(s) to insert
         * @param string $fname Caller function name
         *
         * @return ResultWrapper
         */
        protected function nativeReplace( $table, $rows, $fname ) {
                $table = $this->tableName( $table );

                # Single row case
                if ( !is_array( reset( $rows ) ) ) {
                        $rows = array( $rows );
                }

                $sql = "REPLACE INTO $table (" . implode( ',', array_keys( $rows[0] ) ) . ') VALUES ';
                $first = true;

                foreach ( $rows as $row ) {
                        if ( $first ) {
                                $first = false;
                        } else {
                                $sql .= ',';
                        }

                        $sql .= '(' . $this->makeList( $row ) . ')';
                }

                return $this->query( $sql, $fname );
        }

        /**
         * INSERT ON DUPLICATE KEY UPDATE wrapper, upserts an array into a table.
         *
         * This updates any conflicting rows (according to the unique indexes) using
         * the provided SET clause and inserts any remaining (non-conflicted) rows.
         *
         * $rows may be either:
         *   - A single associative array. The array keys are the field names, and
         *     the values are the values to insert. The values are treated as data
         *     and will be quoted appropriately. If NULL is inserted, this will be
         *     converted to a database NULL.
         *   - An array with numeric keys, holding a list of associative arrays.
         *     This causes a multi-row INSERT on DBMSs that support it. The keys in
         *     each subarray must be identical to each other, and in the same order.
         *
         * It may be more efficient to leave off unique indexes which are unlikely
         * to collide. However if you do this, you run the risk of encountering
         * errors which wouldn't have occurred in MySQL.
         *
         * Usually throws a DBQueryError on failure. If errors are explicitly ignored,
         * returns success.
         *
         * @since 1.22
         *
         * @param string $table Table name. This will be passed through DatabaseBase::tableName().
         * @param array $rows A single row or list of rows to insert
         * @param array $uniqueIndexes List of single field names or field name tuples
         * @param array $set An array of values to SET. For each array element, the
         *   key gives the field name, and the value gives the data to set that
         *   field to. The data will be quoted by DatabaseBase::addQuotes().
         * @param string $fname Calling function name (use __METHOD__) for logs/profiling
         * @throws Exception
         * @return bool
         */
        public function upsert( $table, array $rows, array $uniqueIndexes, array $set,
                $fname = __METHOD__
        ) {
                if ( !count( $rows ) ) {
                        return true; // nothing to do
                }

                if ( !is_array( reset( $rows ) ) ) {
                        $rows = array( $rows );
                }

                if ( count( $uniqueIndexes ) ) {
                        $clauses = array(); // list WHERE clauses that each identify a single row
                        foreach ( $rows as $row ) {
                                foreach ( $uniqueIndexes as $index ) {
                                        $index = is_array( $index ) ? $index : array( $index ); // columns
                                        $rowKey = array(); // unique key to this row
                                        foreach ( $index as $column ) {
                                                $rowKey[$column] = $row[$column];
                                        }
                                        $clauses[] = $this->makeList( $rowKey, LIST_AND );
                                }
                        }
                        $where = array( $this->makeList( $clauses, LIST_OR ) );
                } else {
                        $where = false;
                }

                $useTrx = !$this->mTrxLevel;
                if ( $useTrx ) {
                        $this->begin( $fname );
                }
                try {
                        # Update any existing conflicting row(s)
                        if ( $where !== false ) {
                                $ok = $this->update( $table, $set, $where, $fname );
                        } else {
                                $ok = true;
                        }
                        # Now insert any non-conflicting row(s)
                        $ok = $this->insert( $table, $rows, $fname, array( 'IGNORE' ) ) && $ok;
                } catch ( Exception $e ) {
                        if ( $useTrx ) {
                                $this->rollback( $fname );
                        }
                        throw $e;
                }
                if ( $useTrx ) {
                        $this->commit( $fname );
                }

                return $ok;
        }

        /**
         * DELETE where the condition is a join.
         *
         * MySQL overrides this to use a multi-table DELETE syntax, in other databases
         * we use sub-selects
         *
         * For safety, an empty $conds will not delete everything. If you want to
         * delete all rows where the join condition matches, set $conds='*'.
         *
         * DO NOT put the join condition in $conds.
         *
         * @param string $delTable The table to delete from.
         * @param string $joinTable The other table.
         * @param string $delVar The variable to join on, in the first table.
         * @param string $joinVar The variable to join on, in the second table.
         * @param array $conds Condition array of field names mapped to variables,
         *   ANDed together in the WHERE clause
         * @param string $fname Calling function name (use __METHOD__) for logs/profiling
         * @throws DBUnexpectedError
         */
        public function deleteJoin( $delTable, $joinTable, $delVar, $joinVar, $conds,
                $fname = __METHOD__
        ) {
                if ( !$conds ) {
                        throw new DBUnexpectedError( $this,
                                'DatabaseBase::deleteJoin() called with empty $conds' );
                }

                $delTable = $this->tableName( $delTable );
                $joinTable = $this->tableName( $joinTable );
                $sql = "DELETE FROM $delTable WHERE $delVar IN (SELECT $joinVar FROM $joinTable ";
                if ( $conds != '*' ) {
                        $sql .= 'WHERE ' . $this->makeList( $conds, LIST_AND );
                }
                $sql .= ')';

                $this->query( $sql, $fname );
        }

        /**
         * Returns the size of a text field, or -1 for "unlimited"
         *
         * @param string $table
         * @param string $field
         * @return int
         */
        public function textFieldSize( $table, $field ) {
                $table = $this->tableName( $table );
                $sql = "SHOW COLUMNS FROM $table LIKE \"$field\";";
                $res = $this->query( $sql, 'DatabaseBase::textFieldSize' );
                $row = $this->fetchObject( $res );

                $m = array();

                if ( preg_match( '/\((.*)\)/', $row->Type, $m ) ) {
                        $size = $m[1];
                } else {
                        $size = -1;
                }

                return $size;
        }

        /**
         * A string to insert into queries to show that they're low-priority, like
         * MySQL's LOW_PRIORITY. If no such feature exists, return an empty
         * string and nothing bad should happen.
         *
         * @return string Returns the text of the low priority option if it is
         *   supported, or a blank string otherwise
         */
        public function lowPriorityOption() {
                return '';
        }

        /**
         * DELETE query wrapper.
         *
         * @param array $table Table name
         * @param string|array $conds Array of conditions. See $conds in DatabaseBase::select()
         *   for the format. Use $conds == "*" to delete all rows
         * @param string $fname Name of the calling function
         * @throws DBUnexpectedError
         * @return bool|ResultWrapper
         */
        public function delete( $table, $conds, $fname = __METHOD__ ) {
                if ( !$conds ) {
                        throw new DBUnexpectedError( $this, 'DatabaseBase::delete() called with no conditions' );
                }

                $table = $this->tableName( $table );
                $sql = "DELETE FROM $table";

                if ( $conds != '*' ) {
                        if ( is_array( $conds ) ) {
                                $conds = $this->makeList( $conds, LIST_AND );
                        }
                        $sql .= ' WHERE ' . $conds;
                }

                return $this->query( $sql, $fname );
        }

        /**
         * INSERT SELECT wrapper. Takes data from a SELECT query and inserts it
         * into another table.
         *
         * @param string $destTable The table name to insert into
         * @param string|array $srcTable May be either a table name, or an array of table names
         *    to include in a join.
         *
         * @param array $varMap Must be an associative array of the form
         *    array( 'dest1' => 'source1', ...). Source items may be literals
         *    rather than field names, but strings should be quoted with
         *    DatabaseBase::addQuotes()
         *
         * @param array $conds Condition array. See $conds in DatabaseBase::select() for
         *    the details of the format of condition arrays. May be "*" to copy the
         *    whole table.
         *
         * @param string $fname The function name of the caller, from __METHOD__
         *
         * @param array $insertOptions Options for the INSERT part of the query, see
         *    DatabaseBase::insert() for details.
         * @param array $selectOptions Options for the SELECT part of the query, see
         *    DatabaseBase::select() for details.
         *
         * @return ResultWrapper
         */
        public function insertSelect( $destTable, $srcTable, $varMap, $conds,
                $fname = __METHOD__,
                $insertOptions = array(), $selectOptions = array()
        ) {
                $destTable = $this->tableName( $destTable );

                if ( !is_array( $insertOptions ) ) {
                        $insertOptions = array( $insertOptions );
                }

                $insertOptions = $this->makeInsertOptions( $insertOptions );

                if ( !is_array( $selectOptions ) ) {
                        $selectOptions = array( $selectOptions );
                }

                list( $startOpts, $useIndex, $tailOpts ) = $this->makeSelectOptions( $selectOptions );

                if ( is_array( $srcTable ) ) {
                        $srcTable = implode( ',', array_map( array( &$this, 'tableName' ), $srcTable ) );
                } else {
                        $srcTable = $this->tableName( $srcTable );
                }

                $sql = "INSERT $insertOptions INTO $destTable (" . implode( ',', array_keys( $varMap ) ) . ')' .
                        " SELECT $startOpts " . implode( ',', $varMap ) .
                        " FROM $srcTable $useIndex ";

                if ( $conds != '*' ) {
                        if ( is_array( $conds ) ) {
                                $conds = $this->makeList( $conds, LIST_AND );
                        }
                        $sql .= " WHERE $conds";
                }

                $sql .= " $tailOpts";

                return $this->query( $sql, $fname );
        }

        /**
         * Construct a LIMIT query with optional offset. This is used for query
         * pages. The SQL should be adjusted so that only the first $limit rows
         * are returned. If $offset is provided as well, then the first $offset
         * rows should be discarded, and the next $limit rows should be returned.
         * If the result of the query is not ordered, then the rows to be returned
         * are theoretically arbitrary.
         *
         * $sql is expected to be a SELECT, if that makes a difference.
         *
         * The version provided by default works in MySQL and SQLite. It will very
         * likely need to be overridden for most other DBMSes.
         *
         * @param string $sql SQL query we will append the limit too
         * @param int $limit The SQL limit
         * @param int|bool $offset The SQL offset (default false)
         * @throws DBUnexpectedError
         * @return string
         */
        public function limitResult( $sql, $limit, $offset = false ) {
                if ( !is_numeric( $limit ) ) {
                        throw new DBUnexpectedError( $this, "Invalid non-numeric limit passed to limitResult()\n" );
                }

                return "$sql LIMIT "
                        . ( ( is_numeric( $offset ) && $offset != 0 ) ? "{$offset}," : "" )
                        . "{$limit} ";
        }

        /**
         * Returns true if current database backend supports ORDER BY or LIMIT for separate subqueries
         * within the UNION construct.
         * @return bool
         */
        public function unionSupportsOrderAndLimit() {
                return true; // True for almost every DB supported
        }

        /**
         * Construct a UNION query
         * This is used for providing overload point for other DB abstractions
         * not compatible with the MySQL syntax.
         * @param array $sqls SQL statements to combine
         * @param bool $all Use UNION ALL
         * @return string SQL fragment
         */
        public function unionQueries( $sqls, $all ) {
                $glue = $all ? ') UNION ALL (' : ') UNION (';

                return '(' . implode( $glue, $sqls ) . ')';
        }

        /**
         * Returns an SQL expression for a simple conditional. This doesn't need
         * to be overridden unless CASE isn't supported in your DBMS.
         *
         * @param string|array $cond SQL expression which will result in a boolean value
         * @param string $trueVal SQL expression to return if true
         * @param string $falseVal SQL expression to return if false
         * @return string SQL fragment
         */
        public function conditional( $cond, $trueVal, $falseVal ) {
                if ( is_array( $cond ) ) {
                        $cond = $this->makeList( $cond, LIST_AND );
                }

                return " (CASE WHEN $cond THEN $trueVal ELSE $falseVal END) ";
        }

        /**
         * Returns a comand for str_replace function in SQL query.
         * Uses REPLACE() in MySQL
         *
         * @param string $orig Column to modify
         * @param string $old Column to seek
         * @param string $new Column to replace with
         *
         * @return string
         */
        public function strreplace( $orig, $old, $new ) {
                return "REPLACE({$orig}, {$old}, {$new})";
        }

        /**
         * Determines how long the server has been up
         * STUB
         *
         * @return int
         */
        public function getServerUptime() {
                return 0;
        }

        /**
         * Determines if the last failure was due to a deadlock
         * STUB
         *
         * @return bool
         */
        public function wasDeadlock() {
                return false;
        }

        /**
         * Determines if the last failure was due to a lock timeout
         * STUB
         *
         * @return bool
         */
        public function wasLockTimeout() {
                return false;
        }

        /**
         * Determines if the last query error was something that should be dealt
         * with by pinging the connection and reissuing the query.
         * STUB
         *
         * @return bool
         */
        public function wasErrorReissuable() {
                return false;
        }

        /**
         * Determines if the last failure was due to the database being read-only.
         * STUB
         *
         * @return bool
         */
        public function wasReadOnlyError() {
                return false;
        }

        /**
         * Determines if the given query error was a connection drop
         * STUB
         *
         * @param integer|string $errno
         * @return bool
         */
        public function wasConnectionError( $errno ) {
                return false;
        }

        /**
         * Perform a deadlock-prone transaction.
         *
         * This function invokes a callback function to perform a set of write
         * queries. If a deadlock occurs during the processing, the transaction
         * will be rolled back and the callback function will be called again.
         *
         * Usage:
         *   $dbw->deadlockLoop( callback, ... );
         *
         * Extra arguments are passed through to the specified callback function.
         *
         * Returns whatever the callback function returned on its successful,
         * iteration, or false on error, for example if the retry limit was
         * reached.
         *
         * @return mixed
         * @throws DBQueryError
         */
        public function deadlockLoop() {
                $args = func_get_args();
                $function = array_shift( $args );
                $tries = self::DEADLOCK_TRIES;

                $this->begin( __METHOD__ );

                $retVal = null;
                $e = null;
                do {
                        try {
                                $retVal = call_user_func_array( $function, $args );
                                break;
                        } catch ( DBQueryError $e ) {
                                if ( $this->wasDeadlock() ) {
                                        // Retry after a randomized delay
                                        usleep( mt_rand( self::DEADLOCK_DELAY_MIN, self::DEADLOCK_DELAY_MAX ) );
                                } else {
                                        // Throw the error back up
                                        throw $e;
                                }
                        }
                } while ( --$tries > 0 );

                if ( $tries <= 0 ) {
                        // Too many deadlocks; give up
                        $this->rollback( __METHOD__ );
                        throw $e;
                } else {
                        $this->commit( __METHOD__ );

                        return $retVal;
                }
        }

        /**
         * Wait for the slave to catch up to a given master position.
         *
         * @param DBMasterPos $pos
         * @param int $timeout The maximum number of seconds to wait for
         *   synchronisation
         * @return int Zero if the slave was past that position already,
         *   greater than zero if we waited for some period of time, less than
         *   zero if we timed out.
         */
        public function masterPosWait( DBMasterPos $pos, $timeout ) {
                # Real waits are implemented in the subclass.
                return 0;
        }

        /**
         * Get the replication position of this slave
         *
         * @return DBMasterPos|bool False if this is not a slave.
         */
        public function getSlavePos() {
                # Stub
                return false;
        }

        /**
         * Get the position of this master
         *
         * @return DBMasterPos|bool False if this is not a master
         */
        public function getMasterPos() {
                # Stub
                return false;
        }

        /**
         * Run an anonymous function as soon as there is no transaction pending.
         * If there is a transaction and it is rolled back, then the callback is cancelled.
         * Queries in the function will run in AUTO-COMMIT mode unless there are begin() calls.
         * Callbacks must commit any transactions that they begin.
         *
         * This is useful for updates to different systems or when separate transactions are needed.
         * For example, one might want to enqueue jobs into a system outside the database, but only
         * after the database is updated so that the jobs will see the data when they actually run.
         * It can also be used for updates that easily cause deadlocks if locks are held too long.
         *
         * @param callable $callback
         * @since 1.20
         */
        final public function onTransactionIdle( $callback ) {
                $this->mTrxIdleCallbacks[] = array( $callback, wfGetCaller() );
                if ( !$this->mTrxLevel ) {
                        $this->runOnTransactionIdleCallbacks();
                }
        }

        /**
         * Run an anonymous function before the current transaction commits or now if there is none.
         * If there is a transaction and it is rolled back, then the callback is cancelled.
         * Callbacks must not start nor commit any transactions.
         *
         * This is useful for updates that easily cause deadlocks if locks are held too long
         * but where atomicity is strongly desired for these updates and some related updates.
         *
         * @param callable $callback
         * @since 1.22
         */
        final public function onTransactionPreCommitOrIdle( $callback ) {
                if ( $this->mTrxLevel ) {
                        $this->mTrxPreCommitCallbacks[] = array( $callback, wfGetCaller() );
                } else {
                        $this->onTransactionIdle( $callback ); // this will trigger immediately
                }
        }

        /**
         * Actually any "on transaction idle" callbacks.
         *
         * @since 1.20
         */
        protected function runOnTransactionIdleCallbacks() {
                $autoTrx = $this->getFlag( DBO_TRX ); // automatic begin() enabled?

                $e = $ePrior = null; // last exception
                do { // callbacks may add callbacks :)
                        $callbacks = $this->mTrxIdleCallbacks;
                        $this->mTrxIdleCallbacks = array(); // recursion guard
                        foreach ( $callbacks as $callback ) {
                                try {
                                        list( $phpCallback ) = $callback;
                                        $this->clearFlag( DBO_TRX ); // make each query its own transaction
                                        call_user_func( $phpCallback );
                                        $this->setFlag( $autoTrx ? DBO_TRX : 0 ); // restore automatic begin()
                                } catch ( Exception $e ) {
                                        if ( $ePrior ) {
                                                MWExceptionHandler::logException( $ePrior );
                                        }
                                        $ePrior = $e;
                                }
                        }
                } while ( count( $this->mTrxIdleCallbacks ) );

                if ( $e instanceof Exception ) {
                        throw $e; // re-throw any last exception
                }
        }

        /**
         * Actually any "on transaction pre-commit" callbacks.
         *
         * @since 1.22
         */
        protected function runOnTransactionPreCommitCallbacks() {
                $e = $ePrior = null; // last exception
                do { // callbacks may add callbacks :)
                        $callbacks = $this->mTrxPreCommitCallbacks;
                        $this->mTrxPreCommitCallbacks = array(); // recursion guard
                        foreach ( $callbacks as $callback ) {
                                try {
                                        list( $phpCallback ) = $callback;
                                        call_user_func( $phpCallback );
                                } catch ( Exception $e ) {
                                        if ( $ePrior ) {
                                                MWExceptionHandler::logException( $ePrior );
                                        }
                                        $ePrior = $e;
                                }
                        }
                } while ( count( $this->mTrxPreCommitCallbacks ) );

                if ( $e instanceof Exception ) {
                        throw $e; // re-throw any last exception
                }
        }

        /**
         * Begin an atomic section of statements
         *
         * If a transaction has been started already, just keep track of the given
         * section name to make sure the transaction is not committed pre-maturely.
         * This function can be used in layers (with sub-sections), so use a stack
         * to keep track of the different atomic sections. If there is no transaction,
         * start one implicitly.
         *
         * The goal of this function is to create an atomic section of SQL queries
         * without having to start a new transaction if it already exists.
         *
         * Atomic sections are more strict than transactions. With transactions,
         * attempting to begin a new transaction when one is already running results
         * in MediaWiki issuing a brief warning and doing an implicit commit. All
         * atomic levels *must* be explicitly closed using DatabaseBase::endAtomic(),
         * and any database transactions cannot be began or committed until all atomic
         * levels are closed. There is no such thing as implicitly opening or closing
         * an atomic section.
         *
         * @since 1.23
         * @param string $fname
         * @throws DBError
         */
        final public function startAtomic( $fname = __METHOD__ ) {
                if ( !$this->mTrxLevel ) {
                        $this->begin( $fname );
                        $this->mTrxAutomatic = true;
                        // If DBO_TRX is set, a series of startAtomic/endAtomic pairs will result
                        // in all changes being in one transaction to keep requests transactional.
                        if ( !$this->getFlag( DBO_TRX ) ) {
                                $this->mTrxAutomaticAtomic = true;
                        }
                }

                $this->mTrxAtomicLevels[] = $fname;
        }

        /**
         * Ends an atomic section of SQL statements
         *
         * Ends the next section of atomic SQL statements and commits the transaction
         * if necessary.
         *
         * @since 1.23
         * @see DatabaseBase::startAtomic
         * @param string $fname
         * @throws DBError
         */
        final public function endAtomic( $fname = __METHOD__ ) {
                if ( !$this->mTrxLevel ) {
                        throw new DBUnexpectedError( $this, 'No atomic transaction is open.' );
                }
                if ( !$this->mTrxAtomicLevels ||
                        array_pop( $this->mTrxAtomicLevels ) !== $fname
                ) {
                        throw new DBUnexpectedError( $this, 'Invalid atomic section ended.' );
                }

                if ( !$this->mTrxAtomicLevels && $this->mTrxAutomaticAtomic ) {
                        $this->commit( $fname, 'flush' );
                }
        }

        /**
         * Begin a transaction. If a transaction is already in progress,
         * that transaction will be committed before the new transaction is started.
         *
         * Note that when the DBO_TRX flag is set (which is usually the case for web
         * requests, but not for maintenance scripts), any previous database query
         * will have started a transaction automatically.
         *
         * Nesting of transactions is not supported. Attempts to nest transactions
         * will cause a warning, unless the current transaction was started
         * automatically because of the DBO_TRX flag.
         *
         * @param string $fname
         * @throws DBError
         */
        final public function begin( $fname = __METHOD__ ) {
                global $wgDebugDBTransactions;

                if ( $this->mTrxLevel ) { // implicit commit
                        if ( $this->mTrxAtomicLevels ) {
                                // If the current transaction was an automatic atomic one, then we definitely have
                                // a problem. Same if there is any unclosed atomic level.
                                throw new DBUnexpectedError( $this,
                                        "Attempted to start explicit transaction when atomic levels are still open."
                                );
                        } elseif ( !$this->mTrxAutomatic ) {
                                // We want to warn about inadvertently nested begin/commit pairs, but not about
                                // auto-committing implicit transactions that were started by query() via DBO_TRX
                                $msg = "$fname: Transaction already in progress (from {$this->mTrxFname}), " .
                                        " performing implicit commit!";
                                wfWarn( $msg );
                                wfLogDBError( $msg,
                                        $this->getLogContext( array(
                                                'method' => __METHOD__,
                                                'fname' => $fname,
                                        ) )
                                );
                        } else {
                                // if the transaction was automatic and has done write operations,
                                // log it if $wgDebugDBTransactions is enabled.
                                if ( $this->mTrxDoneWrites && $wgDebugDBTransactions ) {
                                        wfDebug( "$fname: Automatic transaction with writes in progress" .
                                                " (from {$this->mTrxFname}), performing implicit commit!\n"
                                        );
                                }
                        }

                        $this->runOnTransactionPreCommitCallbacks();
                        $writeTime = $this->pendingWriteQueryDuration();
                        $this->doCommit( $fname );
                        if ( $this->mTrxDoneWrites ) {
                                $this->mDoneWrites = microtime( true );
                                $this->getTransactionProfiler()->transactionWritingOut(
                                        $this->mServer, $this->mDBname, $this->mTrxShortId, $writeTime );
                        }
                        $this->runOnTransactionIdleCallbacks();
                }

                # Avoid fatals if close() was called
                $this->assertOpen();

                $this->doBegin( $fname );
                $this->mTrxTimestamp = microtime( true );
                $this->mTrxFname = $fname;
                $this->mTrxDoneWrites = false;
                $this->mTrxAutomatic = false;
                $this->mTrxAutomaticAtomic = false;
                $this->mTrxAtomicLevels = array();
                $this->mTrxIdleCallbacks = array();
                $this->mTrxPreCommitCallbacks = array();
                $this->mTrxShortId = wfRandomString( 12 );
                $this->mTrxWriteDuration = 0.0;
        }

        /**
         * Issues the BEGIN command to the database server.
         *
         * @see DatabaseBase::begin()
         * @param string $fname
         */
        protected function doBegin( $fname ) {
                $this->query( 'BEGIN', $fname );
                $this->mTrxLevel = 1;
        }

        /**
         * Commits a transaction previously started using begin().
         * If no transaction is in progress, a warning is issued.
         *
         * Nesting of transactions is not supported.
         *
         * @param string $fname
         * @param string $flush Flush flag, set to 'flush' to disable warnings about
         *   explicitly committing implicit transactions, or calling commit when no
         *   transaction is in progress. This will silently break any ongoing
         *   explicit transaction. Only set the flush flag if you are sure that it
         *   is safe to ignore these warnings in your context.
         * @throws DBUnexpectedError
         */
        final public function commit( $fname = __METHOD__, $flush = '' ) {
                if ( $this->mTrxLevel && $this->mTrxAtomicLevels ) {
                        // There are still atomic sections open. This cannot be ignored
                        throw new DBUnexpectedError(
                                $this,
                                "Attempted to commit transaction while atomic sections are still open"
                        );
                }

                if ( $flush === 'flush' ) {
                        if ( !$this->mTrxLevel ) {
                                return; // nothing to do
                        } elseif ( !$this->mTrxAutomatic ) {
                                wfWarn( "$fname: Flushing an explicit transaction, getting out of sync!" );
                        }
                } else {
                        if ( !$this->mTrxLevel ) {
                                wfWarn( "$fname: No transaction to commit, something got out of sync!" );
                                return; // nothing to do
                        } elseif ( $this->mTrxAutomatic ) {
                                wfWarn( "$fname: Explicit commit of implicit transaction. Something may be out of sync!" );
                        }
                }

                # Avoid fatals if close() was called
                $this->assertOpen();

                $this->runOnTransactionPreCommitCallbacks();
                $writeTime = $this->pendingWriteQueryDuration();
                $this->doCommit( $fname );
                if ( $this->mTrxDoneWrites ) {
                        $this->mDoneWrites = microtime( true );
                        $this->getTransactionProfiler()->transactionWritingOut(
                                $this->mServer, $this->mDBname, $this->mTrxShortId, $writeTime );
                }
                $this->runOnTransactionIdleCallbacks();
        }

        /**
         * Issues the COMMIT command to the database server.
         *
         * @see DatabaseBase::commit()
         * @param string $fname
         */
        protected function doCommit( $fname ) {
                if ( $this->mTrxLevel ) {
                        $this->query( 'COMMIT', $fname );
                        $this->mTrxLevel = 0;
                }
        }

        /**
         * Rollback a transaction previously started using begin().
         * If no transaction is in progress, a warning is issued.
         *
         * No-op on non-transactional databases.
         *
         * @param string $fname
         * @param string $flush Flush flag, set to 'flush' to disable warnings about
         *   calling rollback when no transaction is in progress. This will silently
         *   break any ongoing explicit transaction. Only set the flush flag if you
         *   are sure that it is safe to ignore these warnings in your context.
         * @throws DBUnexpectedError
         * @since 1.23 Added $flush parameter
         */
        final public function rollback( $fname = __METHOD__, $flush = '' ) {
                if ( $flush !== 'flush' ) {
                        if ( !$this->mTrxLevel ) {
                                wfWarn( "$fname: No transaction to rollback, something got out of sync!" );
                                return; // nothing to do
                        } elseif ( $this->mTrxAutomatic ) {
                                wfWarn( "$fname: Explicit rollback of implicit transaction. Something may be out of sync!" );
                        }
                } else {
                        if ( !$this->mTrxLevel ) {
                                return; // nothing to do
                        } elseif ( !$this->mTrxAutomatic ) {
                                wfWarn( "$fname: Flushing an explicit transaction, getting out of sync!" );
                        }
                }

                # Avoid fatals if close() was called
                $this->assertOpen();

                $this->doRollback( $fname );
                $this->mTrxIdleCallbacks = array(); // cancel
                $this->mTrxPreCommitCallbacks = array(); // cancel
                $this->mTrxAtomicLevels = array();
                if ( $this->mTrxDoneWrites ) {
                        $this->getTransactionProfiler()->transactionWritingOut(
                                $this->mServer, $this->mDBname, $this->mTrxShortId );
                }
        }

        /**
         * Issues the ROLLBACK command to the database server.
         *
         * @see DatabaseBase::rollback()
         * @param string $fname
         */
        protected function doRollback( $fname ) {
                if ( $this->mTrxLevel ) {
                        $this->query( 'ROLLBACK', $fname, true );
                        $this->mTrxLevel = 0;
                }
        }

        /**
         * Creates a new table with structure copied from existing table
         * Note that unlike most database abstraction functions, this function does not
         * automatically append database prefix, because it works at a lower
         * abstraction level.
         * The table names passed to this function shall not be quoted (this
         * function calls addIdentifierQuotes when needed).
         *
         * @param string $oldName Name of table whose structure should be copied
         * @param string $newName Name of table to be created
         * @param bool $temporary Whether the new table should be temporary
         * @param string $fname Calling function name
         * @throws MWException
         * @return bool True if operation was successful
         */
        public function duplicateTableStructure( $oldName, $newName, $temporary = false,
                $fname = __METHOD__
        ) {
                throw new MWException(
                        'DatabaseBase::duplicateTableStructure is not implemented in descendant class' );
        }

        /**
         * List all tables on the database
         *
         * @param string $prefix Only show tables with this prefix, e.g. mw_
         * @param string $fname Calling function name
         * @throws MWException
         * @return array
         */
        function listTables( $prefix = null, $fname = __METHOD__ ) {
                throw new MWException( 'DatabaseBase::listTables is not implemented in descendant class' );
        }

        /**
         * Reset the views process cache set by listViews()
         * @since 1.22
         */
        final public function clearViewsCache() {
                $this->allViews = null;
        }

        /**
         * Lists all the VIEWs in the database
         *
         * For caching purposes the list of all views should be stored in
         * $this->allViews. The process cache can be cleared with clearViewsCache()
         *
         * @param string $prefix Only show VIEWs with this prefix, eg. unit_test_
         * @param string $fname Name of calling function
         * @throws MWException
         * @return array
         * @since 1.22
         */
        public function listViews( $prefix = null, $fname = __METHOD__ ) {
                throw new MWException( 'DatabaseBase::listViews is not implemented in descendant class' );
        }

        /**
         * Differentiates between a TABLE and a VIEW
         *
         * @param string $name Name of the database-structure to test.
         * @throws MWException
         * @return bool
         * @since 1.22
         */
        public function isView( $name ) {
                throw new MWException( 'DatabaseBase::isView is not implemented in descendant class' );
        }

        /**
         * Convert a timestamp in one of the formats accepted by wfTimestamp()
         * to the format used for inserting into timestamp fields in this DBMS.
         *
         * The result is unquoted, and needs to be passed through addQuotes()
         * before it can be included in raw SQL.
         *
         * @param string|int $ts
         *
         * @return string
         */
        public function timestamp( $ts = 0 ) {
                return wfTimestamp( TS_MW, $ts );
        }

        /**
         * Convert a timestamp in one of the formats accepted by wfTimestamp()
         * to the format used for inserting into timestamp fields in this DBMS. If
         * NULL is input, it is passed through, allowing NULL values to be inserted
         * into timestamp fields.
         *
         * The result is unquoted, and needs to be passed through addQuotes()
         * before it can be included in raw SQL.
         *
         * @param string|int $ts
         *
         * @return string
         */
        public function timestampOrNull( $ts = null ) {
                if ( is_null( $ts ) ) {
                        return null;
                } else {
                        return $this->timestamp( $ts );
                }
        }

        /**
         * Take the result from a query, and wrap it in a ResultWrapper if
         * necessary. Boolean values are passed through as is, to indicate success
         * of write queries or failure.
         *
         * Once upon a time, DatabaseBase::query() returned a bare MySQL result
         * resource, and it was necessary to call this function to convert it to
         * a wrapper. Nowadays, raw database objects are never exposed to external
         * callers, so this is unnecessary in external code.
         *
         * @param bool|ResultWrapper|resource|object $result
         * @return bool|ResultWrapper
         */
        protected function resultObject( $result ) {
                if ( !$result ) {
                        return false;
                } elseif ( $result instanceof ResultWrapper ) {
                        return $result;
                } elseif ( $result === true ) {
                        // Successful write query
                        return $result;
                } else {
                        return new ResultWrapper( $this, $result );
                }
        }

        /**
         * Ping the server and try to reconnect if it there is no connection
         *
         * @return bool Success or failure
         */
        public function ping() {
                # Stub. Not essential to override.
                return true;
        }

        /**
         * Get slave lag. Currently supported only by MySQL.
         *
         * Note that this function will generate a fatal error on many
         * installations. Most callers should use LoadBalancer::safeGetLag()
         * instead.
         *
         * @return int Database replication lag in seconds
         */
        public function getLag() {
                return 0;
        }

        /**
         * Return the maximum number of items allowed in a list, or 0 for unlimited.
         *
         * @return int
         */
        function maxListLen() {
                return 0;
        }

        /**
         * Some DBMSs have a special format for inserting into blob fields, they
         * don't allow simple quoted strings to be inserted. To insert into such
         * a field, pass the data through this function before passing it to
         * DatabaseBase::insert().
         *
         * @param string $b
         * @return string
         */
        public function encodeBlob( $b ) {
                return $b;
        }

        /**
         * Some DBMSs return a special placeholder object representing blob fields
         * in result objects. Pass the object through this function to return the
         * original string.
         *
         * @param string|Blob $b
         * @return string
         */
        public function decodeBlob( $b ) {
                if ( $b instanceof Blob ) {
                        $b = $b->fetch();
                }
                return $b;
        }

        /**
         * Override database's default behavior. $options include:
         *     'connTimeout' : Set the connection timeout value in seconds.
         *                     May be useful for very long batch queries such as
         *                     full-wiki dumps, where a single query reads out over
         *                     hours or days.
         *
         * @param array $options
         * @return void
         */
        public function setSessionOptions( array $options ) {
        }

        /**
         * Read and execute SQL commands from a file.
         *
         * Returns true on success, error string or exception on failure (depending
         * on object's error ignore settings).
         *
         * @param string $filename File name to open
         * @param bool|callable $lineCallback Optional function called before reading each line
         * @param bool|callable $resultCallback Optional function called for each MySQL result
         * @param bool|string $fname Calling function name or false if name should be
         *   generated dynamically using $filename
         * @param bool|callable $inputCallback Optional function called for each
         *   complete line sent
         * @throws Exception|MWException
         * @return bool|string
         */
        public function sourceFile(
                $filename, $lineCallback = false, $resultCallback = false, $fname = false, $inputCallback = false
        ) {
                MediaWiki\suppressWarnings();
                $fp = fopen( $filename, 'r' );
                MediaWiki\restoreWarnings();

                if ( false === $fp ) {
                        throw new MWException( "Could not open \"{$filename}\".\n" );
                }

                if ( !$fname ) {
                        $fname = __METHOD__ . "( $filename )";
                }

                try {
                        $error = $this->sourceStream( $fp, $lineCallback, $resultCallback, $fname, $inputCallback );
                } catch ( Exception $e ) {
                        fclose( $fp );
                        throw $e;
                }

                fclose( $fp );

                return $error;
        }

        /**
         * Get the full path of a patch file. Originally based on archive()
         * from updaters.inc. Keep in mind this always returns a patch, as
         * it fails back to MySQL if no DB-specific patch can be found
         *
         * @param string $patch The name of the patch, like patch-something.sql
         * @return string Full path to patch file
         */
        public function patchPath( $patch ) {
                global $IP;

                $dbType = $this->getType();
                if ( file_exists( "$IP/maintenance/$dbType/archives/$patch" ) ) {
                        return "$IP/maintenance/$dbType/archives/$patch";
                } else {
                        return "$IP/maintenance/archives/$patch";
                }
        }

        /**
         * Set variables to be used in sourceFile/sourceStream, in preference to the
         * ones in $GLOBALS. If an array is set here, $GLOBALS will not be used at
         * all. If it's set to false, $GLOBALS will be used.
         *
         * @param bool|array $vars Mapping variable name to value.
         */
        public function setSchemaVars( $vars ) {
                $this->mSchemaVars = $vars;
        }

        /**
         * Read and execute commands from an open file handle.
         *
         * Returns true on success, error string or exception on failure (depending
         * on object's error ignore settings).
         *
         * @param resource $fp File handle
         * @param bool|callable $lineCallback Optional function called before reading each query
         * @param bool|callable $resultCallback Optional function called for each MySQL result
         * @param string $fname Calling function name
         * @param bool|callable $inputCallback Optional function called for each complete query sent
         * @return bool|string
         */
        public function sourceStream( $fp, $lineCallback = false, $resultCallback = false,
                $fname = __METHOD__, $inputCallback = false
        ) {
                $cmd = '';

                while ( !feof( $fp ) ) {
                        if ( $lineCallback ) {
                                call_user_func( $lineCallback );
                        }

                        $line = trim( fgets( $fp ) );

                        if ( $line == '' ) {
                                continue;
                        }

                        if ( '-' == $line[0] && '-' == $line[1] ) {
                                continue;
                        }

                        if ( $cmd != '' ) {
                                $cmd .= ' ';
                        }

                        $done = $this->streamStatementEnd( $cmd, $line );

                        $cmd .= "$line\n";

                        if ( $done || feof( $fp ) ) {
                                $cmd = $this->replaceVars( $cmd );

                                if ( ( $inputCallback && call_user_func( $inputCallback, $cmd ) ) || !$inputCallback ) {
                                        $res = $this->query( $cmd, $fname );

                                        if ( $resultCallback ) {
                                                call_user_func( $resultCallback, $res, $this );
                                        }

                                        if ( false === $res ) {
                                                $err = $this->lastError();

                                                return "Query \"{$cmd}\" failed with error code \"$err\".\n";
                                        }
                                }
                                $cmd = '';
                        }
                }

                return true;
        }

        /**
         * Called by sourceStream() to check if we've reached a statement end
         *
         * @param string $sql SQL assembled so far
         * @param string $newLine New line about to be added to $sql
         * @return bool Whether $newLine contains end of the statement
         */
        public function streamStatementEnd( &$sql, &$newLine ) {
                if ( $this->delimiter ) {
                        $prev = $newLine;
                        $newLine = preg_replace( '/' . preg_quote( $this->delimiter, '/' ) . '$/', '', $newLine );
                        if ( $newLine != $prev ) {
                                return true;
                        }
                }

                return false;
        }

        /**
         * Database independent variable replacement. Replaces a set of variables
         * in an SQL statement with their contents as given by $this->getSchemaVars().
         *
         * Supports '{$var}' `{$var}` and / *$var* / (without the spaces) style variables.
         *
         * - '{$var}' should be used for text and is passed through the database's
         *   addQuotes method.
         * - `{$var}` should be used for identifiers (e.g. table and database names).
         *   It is passed through the database's addIdentifierQuotes method which
         *   can be overridden if the database uses something other than backticks.
         * - / *_* / or / *$wgDBprefix* / passes the name that follows through the
         *   database's tableName method.
         * - / *i* / passes the name that follows through the database's indexName method.
         * - In all other cases, / *$var* / is left unencoded. Except for table options,
         *   its use should be avoided. In 1.24 and older, string encoding was applied.
         *
         * @param string $ins SQL statement to replace variables in
         * @return string The new SQL statement with variables replaced
         */
        protected function replaceVars( $ins ) {
                $that = $this;
                $vars = $this->getSchemaVars();
                return preg_replace_callback(
                        '!
                                /\* (\$wgDBprefix|[_i]) \*/ (\w*) | # 1-2. tableName, indexName
                                \'\{\$ (\w+) }\'                  | # 3. addQuotes
                                `\{\$ (\w+) }`                    | # 4. addIdentifierQuotes
                                /\*\$ (\w+) \*/                     # 5. leave unencoded
                        !x',
                        function ( $m ) use ( $that, $vars ) {
                                // Note: Because of <https://bugs.php.net/bug.php?id=51881>,
                                // check for both nonexistent keys *and* the empty string.
                                if ( isset( $m[1] ) && $m[1] !== '' ) {
                                        if ( $m[1] === 'i' ) {
                                                return $that->indexName( $m[2] );
                                        } else {
                                                return $that->tableName( $m[2] );
                                        }
                                } elseif ( isset( $m[3] ) && $m[3] !== '' && array_key_exists( $m[3], $vars ) ) {
                                        return $that->addQuotes( $vars[$m[3]] );
                                } elseif ( isset( $m[4] ) && $m[4] !== '' && array_key_exists( $m[4], $vars ) ) {
                                        return $that->addIdentifierQuotes( $vars[$m[4]] );
                                } elseif ( isset( $m[5] ) && $m[5] !== '' && array_key_exists( $m[5], $vars ) ) {
                                        return $vars[$m[5]];
                                } else {
                                        return $m[0];
                                }
                        },
                        $ins
                );
        }

        /**
         * Get schema variables. If none have been set via setSchemaVars(), then
         * use some defaults from the current object.
         *
         * @return array
         */
        protected function getSchemaVars() {
                if ( $this->mSchemaVars ) {
                        return $this->mSchemaVars;
                } else {
                        return $this->getDefaultSchemaVars();
                }
        }

        /**
         * Get schema variables to use if none have been set via setSchemaVars().
         *
         * Override this in derived classes to provide variables for tables.sql
         * and SQL patch files.
         *
         * @return array
         */
        protected function getDefaultSchemaVars() {
                return array();
        }

        /**
         * Check to see if a named lock is available (non-blocking)
         *
         * @param string $lockName Name of lock to poll
         * @param string $method Name of method calling us
         * @return bool
         * @since 1.20
         */
        public function lockIsFree( $lockName, $method ) {
                return true;
        }

        /**
         * Acquire a named lock
         *
         * Named locks are not related to transactions
         *
         * @param string $lockName Name of lock to aquire
         * @param string $method Name of method calling us
         * @param int $timeout
         * @return bool
         */
        public function lock( $lockName, $method, $timeout = 5 ) {
                return true;
        }

        /**
         * Release a lock
         *
         * Named locks are not related to transactions
         *
         * @param string $lockName Name of lock to release
         * @param string $method Name of method calling us
         *
         * @return int Returns 1 if the lock was released, 0 if the lock was not established
         * by this thread (in which case the lock is not released), and NULL if the named
         * lock did not exist
         */
        public function unlock( $lockName, $method ) {
                return true;
        }

        /**
         * Check to see if a named lock used by lock() use blocking queues
         *
         * @return bool
         * @since 1.26
         */
        public function namedLocksEnqueue() {
                return false;
        }

        /**
         * Lock specific tables
         *
         * @param array $read Array of tables to lock for read access
         * @param array $write Array of tables to lock for write access
         * @param string $method Name of caller
         * @param bool $lowPriority Whether to indicate writes to be LOW PRIORITY
         * @return bool
         */
        public function lockTables( $read, $write, $method, $lowPriority = true ) {
                return true;
        }

        /**
         * Unlock specific tables
         *
         * @param string $method The caller
         * @return bool
         */
        public function unlockTables( $method ) {
                return true;
        }

        /**
         * Delete a table
         * @param string $tableName
         * @param string $fName
         * @return bool|ResultWrapper
         * @since 1.18
         */
        public function dropTable( $tableName, $fName = __METHOD__ ) {
                if ( !$this->tableExists( $tableName, $fName ) ) {
                        return false;
                }
                $sql = "DROP TABLE " . $this->tableName( $tableName );
                if ( $this->cascadingDeletes() ) {
                        $sql .= " CASCADE";
                }

                return $this->query( $sql, $fName );
        }

        /**
         * Get search engine class. All subclasses of this need to implement this
         * if they wish to use searching.
         *
         * @return string
         */
        public function getSearchEngine() {
                return 'SearchEngineDummy';
        }

        /**
         * Find out when 'infinity' is. Most DBMSes support this. This is a special
         * keyword for timestamps in PostgreSQL, and works with CHAR(14) as well
         * because "i" sorts after all numbers.
         *
         * @return string
         */
        public function getInfinity() {
                return 'infinity';
        }

        /**
         * Encode an expiry time into the DBMS dependent format
         *
         * @param string $expiry Timestamp for expiry, or the 'infinity' string
         * @return string
         */
        public function encodeExpiry( $expiry ) {
                return ( $expiry == '' || $expiry == 'infinity' || $expiry == $this->getInfinity() )
                        ? $this->getInfinity()
                        : $this->timestamp( $expiry );
        }

        /**
         * Decode an expiry time into a DBMS independent format
         *
         * @param string $expiry DB timestamp field value for expiry
         * @param int $format TS_* constant, defaults to TS_MW
         * @return string
         */
        public function decodeExpiry( $expiry, $format = TS_MW ) {
                return ( $expiry == '' || $expiry == 'infinity' || $expiry == $this->getInfinity() )
                        ? 'infinity'
                        : wfTimestamp( $format, $expiry );
        }

        /**
         * Allow or deny "big selects" for this session only. This is done by setting
         * the sql_big_selects session variable.
         *
         * This is a MySQL-specific feature.
         *
         * @param bool|string $value True for allow, false for deny, or "default" to
         *   restore the initial value
         */
        public function setBigSelects( $value = true ) {
                // no-op
        }

        /**
         * @since 1.19
         * @return string
         */
        public function __toString() {
                return (string)$this->mConn;
        }

        /**
         * Run a few simple sanity checks
         */
        public function __destruct() {
                if ( $this->mTrxLevel && $this->mTrxDoneWrites ) {
                        trigger_error( "Uncommitted DB writes (transaction from {$this->mTrxFname})." );
                }
                if ( count( $this->mTrxIdleCallbacks ) || count( $this->mTrxPreCommitCallbacks ) ) {
                        $callers = array();
                        foreach ( $this->mTrxIdleCallbacks as $callbackInfo ) {
                                $callers[] = $callbackInfo[1];
                        }
                        $callers = implode( ', ', $callers );
                        trigger_error( "DB transaction callbacks still pending (from $callers)." );
                }
        }
}