includes/pager/IndexPager.php

   1 <?php
   2 /**
   3  * Efficient paging for SQL queries.
   4  *
   5  * This program is free software; you can redistribute it and/or modify
   6  * it under the terms of the GNU General Public License as published by
   7  * the Free Software Foundation; either version 2 of the License, or
   8  * (at your option) any later version.
   9  *
  10  * This program is distributed in the hope that it will be useful,
  11  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  12  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
  13  * GNU General Public License for more details.
  14  *
  15  * You should have received a copy of the GNU General Public License along
  16  * with this program; if not, write to the Free Software Foundation, Inc.,
  17  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
  18  * http://www.gnu.org/copyleft/gpl.html
  19  *
  20  * @file
  21  * @ingroup Pager
  22  */
  23
  24 use Wikimedia\Rdbms\ResultWrapper;
  25 use Wikimedia\Rdbms\IDatabase;
  26
  27 /**
  28  * IndexPager is an efficient pager which uses a (roughly unique) index in the
  29  * data set to implement paging, rather than a "LIMIT offset,limit" clause.
  30  * In MySQL, such a limit/offset clause requires counting through the
  31  * specified number of offset rows to find the desired data, which can be
  32  * expensive for large offsets.
  33  *
  34  * ReverseChronologicalPager is a child class of the abstract IndexPager, and
  35  * contains  some formatting and display code which is specific to the use of
  36  * timestamps as  indexes. Here is a synopsis of its operation:
  37  *
  38  *    * The query is specified by the offset, limit and direction (dir)
  39  *      parameters, in addition to any subclass-specific parameters.
  40  *    * The offset is the non-inclusive start of the DB query. A row with an
  41  *      index value equal to the offset will never be shown.
  42  *    * The query may either be done backwards, where the rows are returned by
  43  *      the database in the opposite order to which they are displayed to the
  44  *      user, or forwards. This is specified by the "dir" parameter, dir=prev
  45  *      means backwards, anything else means forwards. The offset value
  46  *      specifies the start of the database result set, which may be either
  47  *      the start or end of the displayed data set. This allows "previous"
  48  *      links to be implemented without knowledge of the index value at the
  49  *      start of the previous page.
  50  *    * An additional row beyond the user-specified limit is always requested.
  51  *      This allows us to tell whether we should display a "next" link in the
  52  *      case of forwards mode, or a "previous" link in the case of backwards
  53  *      mode. Determining whether to display the other link (the one for the
  54  *      page before the start of the database result set) can be done
  55  *      heuristically by examining the offset.
  56  *
  57  *    * An empty offset indicates that the offset condition should be omitted
  58  *      from the query. This naturally produces either the first page or the
  59  *      last page depending on the dir parameter.
  60  *
  61  *  Subclassing the pager to implement concrete functionality should be fairly
  62  *  simple, please see the examples in HistoryAction.php and
  63  *  SpecialBlockList.php. You just need to override formatRow(),
  64  *  getQueryInfo() and getIndexField(). Don't forget to call the parent
  65  *  constructor if you override it.
  66  *
  67  * @ingroup Pager
  68  */
  69 abstract class IndexPager extends ContextSource implements Pager {
  70         /**
  71          * Constants for the $mDefaultDirection field.
  72          *
  73          * These are boolean for historical reasons and should stay boolean for backwards-compatibility.
  74          */
  75         const DIR_ASCENDING = false;
  76         const DIR_DESCENDING = true;
  77
  78         public $mRequest;
  79         public $mLimitsShown = [ 20, 50, 100, 250, 500 ];
  80         public $mDefaultLimit = 50;
  81         public $mOffset, $mLimit;
  82         public $mQueryDone = false;
  83         public $mDb;
  84         public $mPastTheEndRow;
  85
  86         /**
  87          * The index to actually be used for ordering. This is a single column,
  88          * for one ordering, even if multiple orderings are supported.
  89          */
  90         protected $mIndexField;
  91         /**
  92          * An array of secondary columns to order by. These fields are not part of the offset.
  93          * This is a column list for one ordering, even if multiple orderings are supported.
  94          */
  95         protected $mExtraSortFields;
  96         /** For pages that support multiple types of ordering, which one to use.
  97          */
  98         protected $mOrderType;
  99         /**
 100          * $mDefaultDirection gives the direction to use when sorting results:
 101          * DIR_ASCENDING or DIR_DESCENDING.  If $mIsBackwards is set, we
 102          * start from the opposite end, but we still sort the page itself according
 103          * to $mDefaultDirection.  E.g., if $mDefaultDirection is false but we're
 104          * going backwards, we'll display the last page of results, but the last
 105          * result will be at the bottom, not the top.
 106          *
 107          * Like $mIndexField, $mDefaultDirection will be a single value even if the
 108          * class supports multiple default directions for different order types.
 109          */
 110         public $mDefaultDirection;
 111         public $mIsBackwards;
 112
 113         /** True if the current result set is the first one */
 114         public $mIsFirst;
 115         public $mIsLast;
 116
 117         protected $mLastShown, $mFirstShown, $mPastTheEndIndex, $mDefaultQuery, $mNavigationBar;
 118
 119         /**
 120          * Whether to include the offset in the query
 121          */
 122         protected $mIncludeOffset = false;
 123
 124         /**
 125          * Result object for the query. Warning: seek before use.
 126          *
 127          * @var ResultWrapper
 128          */
 129         public $mResult;
 130
 131         public function __construct( IContextSource $context = null ) {
 132                 if ( $context ) {
 133                         $this->setContext( $context );
 134                 }
 135
 136                 $this->mRequest = $this->getRequest();
 137
 138                 # NB: the offset is quoted, not validated. It is treated as an
 139                 # arbitrary string to support the widest variety of index types. Be
 140                 # careful outputting it into HTML!
 141                 $this->mOffset = $this->mRequest->getText( 'offset' );
 142
 143                 # Use consistent behavior for the limit options
 144                 $this->mDefaultLimit = $this->getUser()->getIntOption( 'rclimit' );
 145                 if ( !$this->mLimit ) {
 146                         // Don't override if a subclass calls $this->setLimit() in its constructor.
 147                         list( $this->mLimit, /* $offset */ ) = $this->mRequest->getLimitOffset();
 148                 }
 149
 150                 $this->mIsBackwards = ( $this->mRequest->getVal( 'dir' ) == 'prev' );
 151                 # Let the subclass set the DB here; otherwise use a replica DB for the current wiki
 152                 $this->mDb = $this->mDb ?: wfGetDB( DB_REPLICA );
 153
 154                 $index = $this->getIndexField(); // column to sort on
 155                 $extraSort = $this->getExtraSortFields(); // extra columns to sort on for query planning
 156                 $order = $this->mRequest->getVal( 'order' );
 157                 if ( is_array( $index ) && isset( $index[$order] ) ) {
 158                         $this->mOrderType = $order;
 159                         $this->mIndexField = $index[$order];
 160                         $this->mExtraSortFields = isset( $extraSort[$order] )
 161                                 ? (array)$extraSort[$order]
 162                                 : [];
 163                 } elseif ( is_array( $index ) ) {
 164                         # First element is the default
 165                         $this->mIndexField = reset( $index );
 166                         $this->mOrderType = key( $index );
 167                         $this->mExtraSortFields = isset( $extraSort[$this->mOrderType] )
 168                                 ? (array)$extraSort[$this->mOrderType]
 169                                 : [];
 170                 } else {
 171                         # $index is not an array
 172                         $this->mOrderType = null;
 173                         $this->mIndexField = $index;
 174                         $this->mExtraSortFields = (array)$extraSort;
 175                 }
 176
 177                 if ( !isset( $this->mDefaultDirection ) ) {
 178                         $dir = $this->getDefaultDirections();
 179                         $this->mDefaultDirection = is_array( $dir )
 180                                 ? $dir[$this->mOrderType]
 181                                 : $dir;
 182                 }
 183         }
 184
 185         /**
 186          * Get the Database object in use
 187          *
 188          * @return IDatabase
 189          */
 190         public function getDatabase() {
 191                 return $this->mDb;
 192         }
 193
 194         /**
 195          * Do the query, using information from the object context. This function
 196          * has been kept minimal to make it overridable if necessary, to allow for
 197          * result sets formed from multiple DB queries.
 198          */
 199         public function doQuery() {
 200                 # Use the child class name for profiling
 201                 $fname = __METHOD__ . ' (' . static::class . ')';
 202                 $section = Profiler::instance()->scopedProfileIn( $fname );
 203
 204                 // @todo This should probably compare to DIR_DESCENDING and DIR_ASCENDING constants
 205                 $descending = ( $this->mIsBackwards == $this->mDefaultDirection );
 206                 # Plus an extra row so that we can tell the "next" link should be shown
 207                 $queryLimit = $this->mLimit + 1;
 208
 209                 if ( $this->mOffset == '' ) {
 210                         $isFirst = true;
 211                 } else {
 212                         // If there's an offset, we may or may not be at the first entry.
 213                         // The only way to tell is to run the query in the opposite
 214                         // direction see if we get a row.
 215                         $oldIncludeOffset = $this->mIncludeOffset;
 216                         $this->mIncludeOffset = !$this->mIncludeOffset;
 217                         $isFirst = !$this->reallyDoQuery( $this->mOffset, 1, !$descending )->numRows();
 218                         $this->mIncludeOffset = $oldIncludeOffset;
 219                 }
 220
 221                 $this->mResult = $this->reallyDoQuery(
 222                         $this->mOffset,
 223                         $queryLimit,
 224                         $descending
 225                 );
 226
 227                 $this->extractResultInfo( $isFirst, $queryLimit, $this->mResult );
 228                 $this->mQueryDone = true;
 229
 230                 $this->preprocessResults( $this->mResult );
 231                 $this->mResult->rewind(); // Paranoia
 232         }
 233
 234         /**
 235          * @return ResultWrapper The result wrapper.
 236          */
 237         function getResult() {
 238                 return $this->mResult;
 239         }
 240
 241         /**
 242          * Set the offset from an other source than the request
 243          *
 244          * @param int|string $offset
 245          */
 246         function setOffset( $offset ) {
 247                 $this->mOffset = $offset;
 248         }
 249
 250         /**
 251          * Set the limit from an other source than the request
 252          *
 253          * Verifies limit is between 1 and 5000
 254          *
 255          * @param int|string $limit
 256          */
 257         function setLimit( $limit ) {
 258                 $limit = (int)$limit;
 259                 // WebRequest::getLimitOffset() puts a cap of 5000, so do same here.
 260                 if ( $limit > 5000 ) {
 261                         $limit = 5000;
 262                 }
 263                 if ( $limit > 0 ) {
 264                         $this->mLimit = $limit;
 265                 }
 266         }
 267
 268         /**
 269          * Get the current limit
 270          *
 271          * @return int
 272          */
 273         function getLimit() {
 274                 return $this->mLimit;
 275         }
 276
 277         /**
 278          * Set whether a row matching exactly the offset should be also included
 279          * in the result or not. By default this is not the case, but when the
 280          * offset is user-supplied this might be wanted.
 281          *
 282          * @param bool $include
 283          */
 284         public function setIncludeOffset( $include ) {
 285                 $this->mIncludeOffset = $include;
 286         }
 287
 288         /**
 289          * Extract some useful data from the result object for use by
 290          * the navigation bar, put it into $this
 291          *
 292          * @param bool $isFirst False if there are rows before those fetched (i.e.
 293          *     if a "previous" link would make sense)
 294          * @param int $limit Exact query limit
 295          * @param ResultWrapper $res
 296          */
 297         function extractResultInfo( $isFirst, $limit, ResultWrapper $res ) {
 298                 $numRows = $res->numRows();
 299                 if ( $numRows ) {
 300                         # Remove any table prefix from index field
 301                         $parts = explode( '.', $this->mIndexField );
 302                         $indexColumn = end( $parts );
 303
 304                         $row = $res->fetchRow();
 305                         $firstIndex = $row[$indexColumn];
 306
 307                         # Discard the extra result row if there is one
 308                         if ( $numRows > $this->mLimit && $numRows > 1 ) {
 309                                 $res->seek( $numRows - 1 );
 310                                 $this->mPastTheEndRow = $res->fetchObject();
 311                                 $this->mPastTheEndIndex = $this->mPastTheEndRow->$indexColumn;
 312                                 $res->seek( $numRows - 2 );
 313                                 $row = $res->fetchRow();
 314                                 $lastIndex = $row[$indexColumn];
 315                         } else {
 316                                 $this->mPastTheEndRow = null;
 317                                 # Setting indexes to an empty string means that they will be
 318                                 # omitted if they would otherwise appear in URLs. It just so
 319                                 # happens that this  is the right thing to do in the standard
 320                                 # UI, in all the relevant cases.
 321                                 $this->mPastTheEndIndex = '';
 322                                 $res->seek( $numRows - 1 );
 323                                 $row = $res->fetchRow();
 324                                 $lastIndex = $row[$indexColumn];
 325                         }
 326                 } else {
 327                         $firstIndex = '';
 328                         $lastIndex = '';
 329                         $this->mPastTheEndRow = null;
 330                         $this->mPastTheEndIndex = '';
 331                 }
 332
 333                 if ( $this->mIsBackwards ) {
 334                         $this->mIsFirst = ( $numRows < $limit );
 335                         $this->mIsLast = $isFirst;
 336                         $this->mLastShown = $firstIndex;
 337                         $this->mFirstShown = $lastIndex;
 338                 } else {
 339                         $this->mIsFirst = $isFirst;
 340                         $this->mIsLast = ( $numRows < $limit );
 341                         $this->mLastShown = $lastIndex;
 342                         $this->mFirstShown = $firstIndex;
 343                 }
 344         }
 345
 346         /**
 347          * Get some text to go in brackets in the "function name" part of the SQL comment
 348          *
 349          * @return string
 350          */
 351         function getSqlComment() {
 352                 return static::class;
 353         }
 354
 355         /**
 356          * Do a query with specified parameters, rather than using the object
 357          * context
 358          *
 359          * @param string $offset Index offset, inclusive
 360          * @param int $limit Exact query limit
 361          * @param bool $descending Query direction, false for ascending, true for descending
 362          * @return ResultWrapper
 363          */
 364         public function reallyDoQuery( $offset, $limit, $descending ) {
 365                 list( $tables, $fields, $conds, $fname, $options, $join_conds ) =
 366                         $this->buildQueryInfo( $offset, $limit, $descending );
 367
 368                 return $this->mDb->select( $tables, $fields, $conds, $fname, $options, $join_conds );
 369         }
 370
 371         /**
 372          * Build variables to use by the database wrapper.
 373          *
 374          * @param string $offset Index offset, inclusive
 375          * @param int $limit Exact query limit
 376          * @param bool $descending Query direction, false for ascending, true for descending
 377          * @return array
 378          */
 379         protected function buildQueryInfo( $offset, $limit, $descending ) {
 380                 $fname = __METHOD__ . ' (' . $this->getSqlComment() . ')';
 381                 $info = $this->getQueryInfo();
 382                 $tables = $info['tables'];
 383                 $fields = $info['fields'];
 384                 $conds = isset( $info['conds'] ) ? $info['conds'] : [];
 385                 $options = isset( $info['options'] ) ? $info['options'] : [];
 386                 $join_conds = isset( $info['join_conds'] ) ? $info['join_conds'] : [];
 387                 $sortColumns = array_merge( [ $this->mIndexField ], $this->mExtraSortFields );
 388                 if ( $descending ) {
 389                         $options['ORDER BY'] = $sortColumns;
 390                         $operator = $this->mIncludeOffset ? '>=' : '>';
 391                 } else {
 392                         $orderBy = [];
 393                         foreach ( $sortColumns as $col ) {
 394                                 $orderBy[] = $col . ' DESC';
 395                         }
 396                         $options['ORDER BY'] = $orderBy;
 397                         $operator = $this->mIncludeOffset ? '<=' : '<';
 398                 }
 399                 if ( $offset != '' ) {
 400                         $conds[] = $this->mIndexField . $operator . $this->mDb->addQuotes( $offset );
 401                 }
 402                 $options['LIMIT'] = intval( $limit );
 403                 return [ $tables, $fields, $conds, $fname, $options, $join_conds ];
 404         }
 405
 406         /**
 407          * Pre-process results; useful for performing batch existence checks, etc.
 408          *
 409          * @param ResultWrapper $result
 410          */
 411         protected function preprocessResults( $result ) {
 412         }
 413
 414         /**
 415          * Get the formatted result list. Calls getStartBody(), formatRow() and
 416          * getEndBody(), concatenates the results and returns them.
 417          *
 418          * @return string
 419          */
 420         public function getBody() {
 421                 if ( !$this->mQueryDone ) {
 422                         $this->doQuery();
 423                 }
 424
 425                 if ( $this->mResult->numRows() ) {
 426                         # Do any special query batches before display
 427                         $this->doBatchLookups();
 428                 }
 429
 430                 # Don't use any extra rows returned by the query
 431                 $numRows = min( $this->mResult->numRows(), $this->mLimit );
 432
 433                 $s = $this->getStartBody();
 434                 if ( $numRows ) {
 435                         if ( $this->mIsBackwards ) {
 436                                 for ( $i = $numRows - 1; $i >= 0; $i-- ) {
 437                                         $this->mResult->seek( $i );
 438                                         $row = $this->mResult->fetchObject();
 439                                         $s .= $this->formatRow( $row );
 440                                 }
 441                         } else {
 442                                 $this->mResult->seek( 0 );
 443                                 for ( $i = 0; $i < $numRows; $i++ ) {
 444                                         $row = $this->mResult->fetchObject();
 445                                         $s .= $this->formatRow( $row );
 446                                 }
 447                         }
 448                 } else {
 449                         $s .= $this->getEmptyBody();
 450                 }
 451                 $s .= $this->getEndBody();
 452                 return $s;
 453         }
 454
 455         /**
 456          * Make a self-link
 457          *
 458          * @param string $text Text displayed on the link
 459          * @param array $query Associative array of parameter to be in the query string
 460          * @param string $type Link type used to create additional attributes, like "rel", "class" or
 461          *  "title". Valid values (non-exhaustive list): 'first', 'last', 'prev', 'next', 'asc', 'desc'.
 462          * @return string HTML fragment
 463          */
 464         function makeLink( $text, array $query = null, $type = null ) {
 465                 if ( $query === null ) {
 466                         return $text;
 467                 }
 468
 469                 $attrs = [];
 470                 if ( in_array( $type, [ 'prev', 'next' ] ) ) {
 471                         $attrs['rel'] = $type;
 472                 }
 473
 474                 if ( in_array( $type, [ 'asc', 'desc' ] ) ) {
 475                         $attrs['title'] = wfMessage( $type == 'asc' ? 'sort-ascending' : 'sort-descending' )->text();
 476                 }
 477
 478                 if ( $type ) {
 479                         $attrs['class'] = "mw-{$type}link";
 480                 }
 481
 482                 return Linker::linkKnown(
 483                         $this->getTitle(),
 484                         $text,
 485                         $attrs,
 486                         $query + $this->getDefaultQuery()
 487                 );
 488         }
 489
 490         /**
 491          * Called from getBody(), before getStartBody() is called and
 492          * after doQuery() was called. This will be called only if there
 493          * are rows in the result set.
 494          *
 495          * @return void
 496          */
 497         protected function doBatchLookups() {
 498         }
 499
 500         /**
 501          * Hook into getBody(), allows text to be inserted at the start. This
 502          * will be called even if there are no rows in the result set.
 503          *
 504          * @return string
 505          */
 506         protected function getStartBody() {
 507                 return '';
 508         }
 509
 510         /**
 511          * Hook into getBody() for the end of the list
 512          *
 513          * @return string
 514          */
 515         protected function getEndBody() {
 516                 return '';
 517         }
 518
 519         /**
 520          * Hook into getBody(), for the bit between the start and the
 521          * end when there are no rows
 522          *
 523          * @return string
 524          */
 525         protected function getEmptyBody() {
 526                 return '';
 527         }
 528
 529         /**
 530          * Get an array of query parameters that should be put into self-links.
 531          * By default, all parameters passed in the URL are used, except for a
 532          * short blacklist.
 533          *
 534          * @return array Associative array
 535          */
 536         function getDefaultQuery() {
 537                 if ( !isset( $this->mDefaultQuery ) ) {
 538                         $this->mDefaultQuery = $this->getRequest()->getQueryValues();
 539                         unset( $this->mDefaultQuery['title'] );
 540                         unset( $this->mDefaultQuery['dir'] );
 541                         unset( $this->mDefaultQuery['offset'] );
 542                         unset( $this->mDefaultQuery['limit'] );
 543                         unset( $this->mDefaultQuery['order'] );
 544                         unset( $this->mDefaultQuery['month'] );
 545                         unset( $this->mDefaultQuery['year'] );
 546                 }
 547                 return $this->mDefaultQuery;
 548         }
 549
 550         /**
 551          * Get the number of rows in the result set
 552          *
 553          * @return int
 554          */
 555         function getNumRows() {
 556                 if ( !$this->mQueryDone ) {
 557                         $this->doQuery();
 558                 }
 559                 return $this->mResult->numRows();
 560         }
 561
 562         /**
 563          * Get a URL query array for the prev, next, first and last links.
 564          *
 565          * @return array
 566          */
 567         function getPagingQueries() {
 568                 if ( !$this->mQueryDone ) {
 569                         $this->doQuery();
 570                 }
 571
 572                 # Don't announce the limit everywhere if it's the default
 573                 $urlLimit = $this->mLimit == $this->mDefaultLimit ? null : $this->mLimit;
 574
 575                 if ( $this->mIsFirst ) {
 576                         $prev = false;
 577                         $first = false;
 578                 } else {
 579                         $prev = [
 580                                 'dir' => 'prev',
 581                                 'offset' => $this->mFirstShown,
 582                                 'limit' => $urlLimit
 583                         ];
 584                         $first = [ 'limit' => $urlLimit ];
 585                 }
 586                 if ( $this->mIsLast ) {
 587                         $next = false;
 588                         $last = false;
 589                 } else {
 590                         $next = [ 'offset' => $this->mLastShown, 'limit' => $urlLimit ];
 591                         $last = [ 'dir' => 'prev', 'limit' => $urlLimit ];
 592                 }
 593                 return [
 594                         'prev' => $prev,
 595                         'next' => $next,
 596                         'first' => $first,
 597                         'last' => $last
 598                 ];
 599         }
 600
 601         /**
 602          * Returns whether to show the "navigation bar"
 603          *
 604          * @return bool
 605          */
 606         function isNavigationBarShown() {
 607                 if ( !$this->mQueryDone ) {
 608                         $this->doQuery();
 609                 }
 610                 // Hide navigation by default if there is nothing to page
 611                 return !( $this->mIsFirst && $this->mIsLast );
 612         }
 613
 614         /**
 615          * Get paging links. If a link is disabled, the item from $disabledTexts
 616          * will be used. If there is no such item, the unlinked text from
 617          * $linkTexts will be used. Both $linkTexts and $disabledTexts are arrays
 618          * of HTML.
 619          *
 620          * @param array $linkTexts
 621          * @param array $disabledTexts
 622          * @return array
 623          */
 624         function getPagingLinks( $linkTexts, $disabledTexts = [] ) {
 625                 $queries = $this->getPagingQueries();
 626                 $links = [];
 627
 628                 foreach ( $queries as $type => $query ) {
 629                         if ( $query !== false ) {
 630                                 $links[$type] = $this->makeLink(
 631                                         $linkTexts[$type],
 632                                         $queries[$type],
 633                                         $type
 634                                 );
 635                         } elseif ( isset( $disabledTexts[$type] ) ) {
 636                                 $links[$type] = $disabledTexts[$type];
 637                         } else {
 638                                 $links[$type] = $linkTexts[$type];
 639                         }
 640                 }
 641
 642                 return $links;
 643         }
 644
 645         function getLimitLinks() {
 646                 $links = [];
 647                 if ( $this->mIsBackwards ) {
 648                         $offset = $this->mPastTheEndIndex;
 649                 } else {
 650                         $offset = $this->mOffset;
 651                 }
 652                 foreach ( $this->mLimitsShown as $limit ) {
 653                         $links[] = $this->makeLink(
 654                                 $this->getLanguage()->formatNum( $limit ),
 655                                 [ 'offset' => $offset, 'limit' => $limit ],
 656                                 'num'
 657                         );
 658                 }
 659                 return $links;
 660         }
 661
 662         /**
 663          * Abstract formatting function. This should return an HTML string
 664          * representing the result row $row. Rows will be concatenated and
 665          * returned by getBody()
 666          *
 667          * @param array|stdClass $row Database row
 668          * @return string
 669          */
 670         abstract function formatRow( $row );
 671
 672         /**
 673          * This function should be overridden to provide all parameters
 674          * needed for the main paged query. It returns an associative
 675          * array with the following elements:
 676          *    tables => Table(s) for passing to Database::select()
 677          *    fields => Field(s) for passing to Database::select(), may be *
 678          *    conds => WHERE conditions
 679          *    options => option array
 680          *    join_conds => JOIN conditions
 681          *
 682          * @return array
 683          */
 684         abstract function getQueryInfo();
 685
 686         /**
 687          * This function should be overridden to return the name of the index fi-
 688          * eld.  If the pager supports multiple orders, it may return an array of
 689          * 'querykey' => 'indexfield' pairs, so that a request with &count=querykey
 690          * will use indexfield to sort.  In this case, the first returned key is
 691          * the default.
 692          *
 693          * Needless to say, it's really not a good idea to use a non-unique index
 694          * for this!  That won't page right.
 695          *
 696          * @return string|array
 697          */
 698         abstract function getIndexField();
 699
 700         /**
 701          * This function should be overridden to return the names of secondary columns
 702          * to order by in addition to the column in getIndexField(). These fields will
 703          * not be used in the pager offset or in any links for users.
 704          *
 705          * If getIndexField() returns an array of 'querykey' => 'indexfield' pairs then
 706          * this must return a corresponding array of 'querykey' => [ fields... ] pairs
 707          * in order for a request with &count=querykey to use [ fields... ] to sort.
 708          *
 709          * This is useful for pagers that GROUP BY a unique column (say page_id)
 710          * and ORDER BY another (say page_len). Using GROUP BY and ORDER BY both on
 711          * page_len,page_id avoids temp tables (given a page_len index). This would
 712          * also work if page_id was non-unique but we had a page_len,page_id index.
 713          *
 714          * @return array
 715          */
 716         protected function getExtraSortFields() {
 717                 return [];
 718         }
 719
 720         /**
 721          * Return the default sorting direction: DIR_ASCENDING or DIR_DESCENDING.
 722          * You can also have an associative array of ordertype => dir,
 723          * if multiple order types are supported.  In this case getIndexField()
 724          * must return an array, and the keys of that must exactly match the keys
 725          * of this.
 726          *
 727          * For backward compatibility, this method's return value will be ignored
 728          * if $this->mDefaultDirection is already set when the constructor is
 729          * called, for instance if it's statically initialized.  In that case the
 730          * value of that variable (which must be a boolean) will be used.
 731          *
 732          * Note that despite its name, this does not return the value of the
 733          * $this->mDefaultDirection member variable.  That's the default for this
 734          * particular instantiation, which is a single value.  This is the set of
 735          * all defaults for the class.
 736          *
 737          * @return bool
 738          */
 739         protected function getDefaultDirections() {
 740                 return self::DIR_ASCENDING;
 741         }
 742 }