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