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\IResultWrapper;
  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         /** @var IDatabase */
  84         public $mDb;
  85         public $mPastTheEndRow;
  86
  87         /**
  88          * The index to actually be used for ordering. This is a single column,
  89          * for one ordering, even if multiple orderings are supported.
  90          */
  91         protected $mIndexField;
  92         /**
  93          * An array of secondary columns to order by. These fields are not part of the offset.
  94          * This is a column list for one ordering, even if multiple orderings are supported.
  95          */
  96         protected $mExtraSortFields;
  97         /** For pages that support multiple types of ordering, which one to use.
  98          */
  99         protected $mOrderType;
 100         /**
 101          * $mDefaultDirection gives the direction to use when sorting results:
 102          * DIR_ASCENDING or DIR_DESCENDING.  If $mIsBackwards is set, we
 103          * start from the opposite end, but we still sort the page itself according
 104          * to $mDefaultDirection.  E.g., if $mDefaultDirection is false but we're
 105          * going backwards, we'll display the last page of results, but the last
 106          * result will be at the bottom, not the top.
 107          *
 108          * Like $mIndexField, $mDefaultDirection will be a single value even if the
 109          * class supports multiple default directions for different order types.
 110          */
 111         public $mDefaultDirection;
 112         public $mIsBackwards;
 113
 114         /** True if the current result set is the first one */
 115         public $mIsFirst;
 116         public $mIsLast;
 117
 118         protected $mLastShown, $mFirstShown, $mPastTheEndIndex, $mDefaultQuery, $mNavigationBar;
 119
 120         /**
 121          * Whether to include the offset in the query
 122          */
 123         protected $mIncludeOffset = false;
 124
 125         /**
 126          * Result object for the query. Warning: seek before use.
 127          *
 128          * @var IResultWrapper
 129          */
 130         public $mResult;
 131
 132         public function __construct( IContextSource $context = null ) {
 133                 if ( $context ) {
 134                         $this->setContext( $context );
 135                 }
 136
 137                 $this->mRequest = $this->getRequest();
 138
 139                 # NB: the offset is quoted, not validated. It is treated as an
 140                 # arbitrary string to support the widest variety of index types. Be
 141                 # careful outputting it into HTML!
 142                 $this->mOffset = $this->mRequest->getText( 'offset' );
 143
 144                 # Use consistent behavior for the limit options
 145                 $this->mDefaultLimit = $this->getUser()->getIntOption( 'rclimit' );
 146                 if ( !$this->mLimit ) {
 147                         // Don't override if a subclass calls $this->setLimit() in its constructor.
 148                         list( $this->mLimit, /* $offset */ ) = $this->mRequest->getLimitOffset();
 149                 }
 150
 151                 $this->mIsBackwards = ( $this->mRequest->getVal( 'dir' ) == 'prev' );
 152                 # Let the subclass set the DB here; otherwise use a replica DB for the current wiki
 153                 $this->mDb = $this->mDb ?: wfGetDB( DB_REPLICA );
 154
 155                 $index = $this->getIndexField(); // column to sort on
 156                 $extraSort = $this->getExtraSortFields(); // extra columns to sort on for query planning
 157                 $order = $this->mRequest->getVal( 'order' );
 158                 if ( is_array( $index ) && isset( $index[$order] ) ) {
 159                         $this->mOrderType = $order;
 160                         $this->mIndexField = $index[$order];
 161                         $this->mExtraSortFields = isset( $extraSort[$order] )
 162                                 ? (array)$extraSort[$order]
 163                                 : [];
 164                 } elseif ( is_array( $index ) ) {
 165                         # First element is the default
 166                         $this->mIndexField = reset( $index );
 167                         $this->mOrderType = key( $index );
 168                         $this->mExtraSortFields = isset( $extraSort[$this->mOrderType] )
 169                                 ? (array)$extraSort[$this->mOrderType]
 170                                 : [];
 171                 } else {
 172                         # $index is not an array
 173                         $this->mOrderType = null;
 174                         $this->mIndexField = $index;
 175                         $this->mExtraSortFields = (array)$extraSort;
 176                 }
 177
 178                 if ( !isset( $this->mDefaultDirection ) ) {
 179                         $dir = $this->getDefaultDirections();
 180                         $this->mDefaultDirection = is_array( $dir )
 181                                 ? $dir[$this->mOrderType]
 182                                 : $dir;
 183                 }
 184         }
 185
 186         /**
 187          * Get the Database object in use
 188          *
 189          * @return IDatabase
 190          */
 191         public function getDatabase() {
 192                 return $this->mDb;
 193         }
 194
 195         /**
 196          * Do the query, using information from the object context. This function
 197          * has been kept minimal to make it overridable if necessary, to allow for
 198          * result sets formed from multiple DB queries.
 199          */
 200         public function doQuery() {
 201                 # Use the child class name for profiling
 202                 $fname = __METHOD__ . ' (' . static::class . ')';
 203                 $section = Profiler::instance()->scopedProfileIn( $fname );
 204
 205                 // @todo This should probably compare to DIR_DESCENDING and DIR_ASCENDING constants
 206                 $descending = ( $this->mIsBackwards == $this->mDefaultDirection );
 207                 # Plus an extra row so that we can tell the "next" link should be shown
 208                 $queryLimit = $this->mLimit + 1;
 209
 210                 if ( $this->mOffset == '' ) {
 211                         $isFirst = true;
 212                 } else {
 213                         // If there's an offset, we may or may not be at the first entry.
 214                         // The only way to tell is to run the query in the opposite
 215                         // direction see if we get a row.
 216                         $oldIncludeOffset = $this->mIncludeOffset;
 217                         $this->mIncludeOffset = !$this->mIncludeOffset;
 218                         $isFirst = !$this->reallyDoQuery( $this->mOffset, 1, !$descending )->numRows();
 219                         $this->mIncludeOffset = $oldIncludeOffset;
 220                 }
 221
 222                 $this->mResult = $this->reallyDoQuery(
 223                         $this->mOffset,
 224                         $queryLimit,
 225                         $descending
 226                 );
 227
 228                 $this->extractResultInfo( $isFirst, $queryLimit, $this->mResult );
 229                 $this->mQueryDone = true;
 230
 231                 $this->preprocessResults( $this->mResult );
 232                 $this->mResult->rewind(); // Paranoia
 233         }
 234
 235         /**
 236          * @return IResultWrapper The result wrapper.
 237          */
 238         function getResult() {
 239                 return $this->mResult;
 240         }
 241
 242         /**
 243          * Set the offset from an other source than the request
 244          *
 245          * @param int|string $offset
 246          */
 247         function setOffset( $offset ) {
 248                 $this->mOffset = $offset;
 249         }
 250
 251         /**
 252          * Set the limit from an other source than the request
 253          *
 254          * Verifies limit is between 1 and 5000
 255          *
 256          * @param int|string $limit
 257          */
 258         function setLimit( $limit ) {
 259                 $limit = (int)$limit;
 260                 // WebRequest::getLimitOffset() puts a cap of 5000, so do same here.
 261                 if ( $limit > 5000 ) {
 262                         $limit = 5000;
 263                 }
 264                 if ( $limit > 0 ) {
 265                         $this->mLimit = $limit;
 266                 }
 267         }
 268
 269         /**
 270          * Get the current limit
 271          *
 272          * @return int
 273          */
 274         function getLimit() {
 275                 return $this->mLimit;
 276         }
 277
 278         /**
 279          * Set whether a row matching exactly the offset should be also included
 280          * in the result or not. By default this is not the case, but when the
 281          * offset is user-supplied this might be wanted.
 282          *
 283          * @param bool $include
 284          */
 285         public function setIncludeOffset( $include ) {
 286                 $this->mIncludeOffset = $include;
 287         }
 288
 289         /**
 290          * Extract some useful data from the result object for use by
 291          * the navigation bar, put it into $this
 292          *
 293          * @param bool $isFirst False if there are rows before those fetched (i.e.
 294          *     if a "previous" link would make sense)
 295          * @param int $limit Exact query limit
 296          * @param IResultWrapper $res
 297          */
 298         function extractResultInfo( $isFirst, $limit, IResultWrapper $res ) {
 299                 $numRows = $res->numRows();
 300                 if ( $numRows ) {
 301                         # Remove any table prefix from index field
 302                         $parts = explode( '.', $this->mIndexField );
 303                         $indexColumn = end( $parts );
 304
 305                         $row = $res->fetchRow();
 306                         $firstIndex = $row[$indexColumn];
 307
 308                         # Discard the extra result row if there is one
 309                         if ( $numRows > $this->mLimit && $numRows > 1 ) {
 310                                 $res->seek( $numRows - 1 );
 311                                 $this->mPastTheEndRow = $res->fetchObject();
 312                                 $this->mPastTheEndIndex = $this->mPastTheEndRow->$indexColumn;
 313                                 $res->seek( $numRows - 2 );
 314                                 $row = $res->fetchRow();
 315                                 $lastIndex = $row[$indexColumn];
 316                         } else {
 317                                 $this->mPastTheEndRow = null;
 318                                 # Setting indexes to an empty string means that they will be
 319                                 # omitted if they would otherwise appear in URLs. It just so
 320                                 # happens that this  is the right thing to do in the standard
 321                                 # UI, in all the relevant cases.
 322                                 $this->mPastTheEndIndex = '';
 323                                 $res->seek( $numRows - 1 );
 324                                 $row = $res->fetchRow();
 325                                 $lastIndex = $row[$indexColumn];
 326                         }
 327                 } else {
 328                         $firstIndex = '';
 329                         $lastIndex = '';
 330                         $this->mPastTheEndRow = null;
 331                         $this->mPastTheEndIndex = '';
 332                 }
 333
 334                 if ( $this->mIsBackwards ) {
 335                         $this->mIsFirst = ( $numRows < $limit );
 336                         $this->mIsLast = $isFirst;
 337                         $this->mLastShown = $firstIndex;
 338                         $this->mFirstShown = $lastIndex;
 339                 } else {
 340                         $this->mIsFirst = $isFirst;
 341                         $this->mIsLast = ( $numRows < $limit );
 342                         $this->mLastShown = $lastIndex;
 343                         $this->mFirstShown = $firstIndex;
 344                 }
 345         }
 346
 347         /**
 348          * Get some text to go in brackets in the "function name" part of the SQL comment
 349          *
 350          * @return string
 351          */
 352         function getSqlComment() {
 353                 return static::class;
 354         }
 355
 356         /**
 357          * Do a query with specified parameters, rather than using the object
 358          * context
 359          *
 360          * @param string $offset Index offset, inclusive
 361          * @param int $limit Exact query limit
 362          * @param bool $descending Query direction, false for ascending, true for descending
 363          * @return IResultWrapper
 364          */
 365         public function reallyDoQuery( $offset, $limit, $descending ) {
 366                 list( $tables, $fields, $conds, $fname, $options, $join_conds ) =
 367                         $this->buildQueryInfo( $offset, $limit, $descending );
 368
 369                 return $this->mDb->select( $tables, $fields, $conds, $fname, $options, $join_conds );
 370         }
 371
 372         /**
 373          * Build variables to use by the database wrapper.
 374          *
 375          * @param string $offset Index offset, inclusive
 376          * @param int $limit Exact query limit
 377          * @param bool $descending Query direction, false for ascending, true for descending
 378          * @return array
 379          */
 380         protected function buildQueryInfo( $offset, $limit, $descending ) {
 381                 $fname = __METHOD__ . ' (' . $this->getSqlComment() . ')';
 382                 $info = $this->getQueryInfo();
 383                 $tables = $info['tables'];
 384                 $fields = $info['fields'];
 385                 $conds = $info['conds'] ?? [];
 386                 $options = $info['options'] ?? [];
 387                 $join_conds = $info['join_conds'] ?? [];
 388                 $sortColumns = array_merge( [ $this->mIndexField ], $this->mExtraSortFields );
 389                 if ( $descending ) {
 390                         $options['ORDER BY'] = $sortColumns;
 391                         $operator = $this->mIncludeOffset ? '>=' : '>';
 392                 } else {
 393                         $orderBy = [];
 394                         foreach ( $sortColumns as $col ) {
 395                                 $orderBy[] = $col . ' DESC';
 396                         }
 397                         $options['ORDER BY'] = $orderBy;
 398                         $operator = $this->mIncludeOffset ? '<=' : '<';
 399                 }
 400                 if ( $offset != '' ) {
 401                         $conds[] = $this->mIndexField . $operator . $this->mDb->addQuotes( $offset );
 402                 }
 403                 $options['LIMIT'] = intval( $limit );
 404                 return [ $tables, $fields, $conds, $fname, $options, $join_conds ];
 405         }
 406
 407         /**
 408          * Pre-process results; useful for performing batch existence checks, etc.
 409          *
 410          * @param IResultWrapper $result
 411          */
 412         protected function preprocessResults( $result ) {
 413         }
 414
 415         /**
 416          * Get the formatted result list. Calls getStartBody(), formatRow() and
 417          * getEndBody(), concatenates the results and returns them.
 418          *
 419          * @return string
 420          */
 421         public function getBody() {
 422                 if ( !$this->mQueryDone ) {
 423                         $this->doQuery();
 424                 }
 425
 426                 if ( $this->mResult->numRows() ) {
 427                         # Do any special query batches before display
 428                         $this->doBatchLookups();
 429                 }
 430
 431                 # Don't use any extra rows returned by the query
 432                 $numRows = min( $this->mResult->numRows(), $this->mLimit );
 433
 434                 $s = $this->getStartBody();
 435                 if ( $numRows ) {
 436                         if ( $this->mIsBackwards ) {
 437                                 for ( $i = $numRows - 1; $i >= 0; $i-- ) {
 438                                         $this->mResult->seek( $i );
 439                                         $row = $this->mResult->fetchObject();
 440                                         $s .= $this->formatRow( $row );
 441                                 }
 442                         } else {
 443                                 $this->mResult->seek( 0 );
 444                                 for ( $i = 0; $i < $numRows; $i++ ) {
 445                                         $row = $this->mResult->fetchObject();
 446                                         $s .= $this->formatRow( $row );
 447                                 }
 448                         }
 449                 } else {
 450                         $s .= $this->getEmptyBody();
 451                 }
 452                 $s .= $this->getEndBody();
 453                 return $s;
 454         }
 455
 456         /**
 457          * Make a self-link
 458          *
 459          * @param string $text Text displayed on the link
 460          * @param array|null $query Associative array of parameter to be in the query string
 461          * @param string|null $type Link type used to create additional attributes, like "rel", "class" or
 462          *  "title". Valid values (non-exhaustive list): 'first', 'last', 'prev', 'next', 'asc', 'desc'.
 463          * @return string HTML fragment
 464          */
 465         function makeLink( $text, array $query = null, $type = null ) {
 466                 if ( $query === null ) {
 467                         return $text;
 468                 }
 469
 470                 $attrs = [];
 471                 if ( in_array( $type, [ 'prev', 'next' ] ) ) {
 472                         $attrs['rel'] = $type;
 473                 }
 474
 475                 if ( in_array( $type, [ 'asc', 'desc' ] ) ) {
 476                         $attrs['title'] = $this->msg( $type == 'asc' ? 'sort-ascending' : 'sort-descending' )->text();
 477                 }
 478
 479                 if ( $type ) {
 480                         $attrs['class'] = "mw-{$type}link";
 481                 }
 482
 483                 return Linker::linkKnown(
 484                         $this->getTitle(),
 485                         $text,
 486                         $attrs,
 487                         $query + $this->getDefaultQuery()
 488                 );
 489         }
 490
 491         /**
 492          * Called from getBody(), before getStartBody() is called and
 493          * after doQuery() was called. This will be called only if there
 494          * are rows in the result set.
 495          *
 496          * @return void
 497          */
 498         protected function doBatchLookups() {
 499         }
 500
 501         /**
 502          * Hook into getBody(), allows text to be inserted at the start. This
 503          * will be called even if there are no rows in the result set.
 504          *
 505          * @return string
 506          */
 507         protected function getStartBody() {
 508                 return '';
 509         }
 510
 511         /**
 512          * Hook into getBody() for the end of the list
 513          *
 514          * @return string
 515          */
 516         protected function getEndBody() {
 517                 return '';
 518         }
 519
 520         /**
 521          * Hook into getBody(), for the bit between the start and the
 522          * end when there are no rows
 523          *
 524          * @return string
 525          */
 526         protected function getEmptyBody() {
 527                 return '';
 528         }
 529
 530         /**
 531          * Get an array of query parameters that should be put into self-links.
 532          * By default, all parameters passed in the URL are used, except for a
 533          * short blacklist.
 534          *
 535          * @return array Associative array
 536          */
 537         function getDefaultQuery() {
 538                 if ( !isset( $this->mDefaultQuery ) ) {
 539                         $this->mDefaultQuery = $this->getRequest()->getQueryValues();
 540                         unset( $this->mDefaultQuery['title'] );
 541                         unset( $this->mDefaultQuery['dir'] );
 542                         unset( $this->mDefaultQuery['offset'] );
 543                         unset( $this->mDefaultQuery['limit'] );
 544                         unset( $this->mDefaultQuery['order'] );
 545                         unset( $this->mDefaultQuery['month'] );
 546                         unset( $this->mDefaultQuery['year'] );
 547                 }
 548                 return $this->mDefaultQuery;
 549         }
 550
 551         /**
 552          * Get the number of rows in the result set
 553          *
 554          * @return int
 555          */
 556         function getNumRows() {
 557                 if ( !$this->mQueryDone ) {
 558                         $this->doQuery();
 559                 }
 560                 return $this->mResult->numRows();
 561         }
 562
 563         /**
 564          * Get a URL query array for the prev, next, first and last links.
 565          *
 566          * @return array
 567          */
 568         function getPagingQueries() {
 569                 if ( !$this->mQueryDone ) {
 570                         $this->doQuery();
 571                 }
 572
 573                 # Don't announce the limit everywhere if it's the default
 574                 $urlLimit = $this->mLimit == $this->mDefaultLimit ? null : $this->mLimit;
 575
 576                 if ( $this->mIsFirst ) {
 577                         $prev = false;
 578                         $first = false;
 579                 } else {
 580                         $prev = [
 581                                 'dir' => 'prev',
 582                                 'offset' => $this->mFirstShown,
 583                                 'limit' => $urlLimit
 584                         ];
 585                         $first = [ 'limit' => $urlLimit ];
 586                 }
 587                 if ( $this->mIsLast ) {
 588                         $next = false;
 589                         $last = false;
 590                 } else {
 591                         $next = [ 'offset' => $this->mLastShown, 'limit' => $urlLimit ];
 592                         $last = [ 'dir' => 'prev', 'limit' => $urlLimit ];
 593                 }
 594                 return [
 595                         'prev' => $prev,
 596                         'next' => $next,
 597                         'first' => $first,
 598                         'last' => $last
 599                 ];
 600         }
 601
 602         /**
 603          * Returns whether to show the "navigation bar"
 604          *
 605          * @return bool
 606          */
 607         function isNavigationBarShown() {
 608                 if ( !$this->mQueryDone ) {
 609                         $this->doQuery();
 610                 }
 611                 // Hide navigation by default if there is nothing to page
 612                 return !( $this->mIsFirst && $this->mIsLast );
 613         }
 614
 615         /**
 616          * Get paging links. If a link is disabled, the item from $disabledTexts
 617          * will be used. If there is no such item, the unlinked text from
 618          * $linkTexts will be used. Both $linkTexts and $disabledTexts are arrays
 619          * of HTML.
 620          *
 621          * @param array $linkTexts
 622          * @param array $disabledTexts
 623          * @return array
 624          */
 625         function getPagingLinks( $linkTexts, $disabledTexts = [] ) {
 626                 $queries = $this->getPagingQueries();
 627                 $links = [];
 628
 629                 foreach ( $queries as $type => $query ) {
 630                         if ( $query !== false ) {
 631                                 $links[$type] = $this->makeLink(
 632                                         $linkTexts[$type],
 633                                         $queries[$type],
 634                                         $type
 635                                 );
 636                         } elseif ( isset( $disabledTexts[$type] ) ) {
 637                                 $links[$type] = $disabledTexts[$type];
 638                         } else {
 639                                 $links[$type] = $linkTexts[$type];
 640                         }
 641                 }
 642
 643                 return $links;
 644         }
 645
 646         function getLimitLinks() {
 647                 $links = [];
 648                 if ( $this->mIsBackwards ) {
 649                         $offset = $this->mPastTheEndIndex;
 650                 } else {
 651                         $offset = $this->mOffset;
 652                 }
 653                 foreach ( $this->mLimitsShown as $limit ) {
 654                         $links[] = $this->makeLink(
 655                                 $this->getLanguage()->formatNum( $limit ),
 656                                 [ 'offset' => $offset, 'limit' => $limit ],
 657                                 'num'
 658                         );
 659                 }
 660                 return $links;
 661         }
 662
 663         /**
 664          * Abstract formatting function. This should return an HTML string
 665          * representing the result row $row. Rows will be concatenated and
 666          * returned by getBody()
 667          *
 668          * @param array|stdClass $row Database row
 669          * @return string
 670          */
 671         abstract function formatRow( $row );
 672
 673         /**
 674          * This function should be overridden to provide all parameters
 675          * needed for the main paged query. It returns an associative
 676          * array with the following elements:
 677          *    tables => Table(s) for passing to Database::select()
 678          *    fields => Field(s) for passing to Database::select(), may be *
 679          *    conds => WHERE conditions
 680          *    options => option array
 681          *    join_conds => JOIN conditions
 682          *
 683          * @return array
 684          */
 685         abstract function getQueryInfo();
 686
 687         /**
 688          * This function should be overridden to return the name of the index fi-
 689          * eld.  If the pager supports multiple orders, it may return an array of
 690          * 'querykey' => 'indexfield' pairs, so that a request with &count=querykey
 691          * will use indexfield to sort.  In this case, the first returned key is
 692          * the default.
 693          *
 694          * Needless to say, it's really not a good idea to use a non-unique index
 695          * for this!  That won't page right.
 696          *
 697          * @return string|array
 698          */
 699         abstract function getIndexField();
 700
 701         /**
 702          * This function should be overridden to return the names of secondary columns
 703          * to order by in addition to the column in getIndexField(). These fields will
 704          * not be used in the pager offset or in any links for users.
 705          *
 706          * If getIndexField() returns an array of 'querykey' => 'indexfield' pairs then
 707          * this must return a corresponding array of 'querykey' => [ fields... ] pairs
 708          * in order for a request with &count=querykey to use [ fields... ] to sort.
 709          *
 710          * This is useful for pagers that GROUP BY a unique column (say page_id)
 711          * and ORDER BY another (say page_len). Using GROUP BY and ORDER BY both on
 712          * page_len,page_id avoids temp tables (given a page_len index). This would
 713          * also work if page_id was non-unique but we had a page_len,page_id index.
 714          *
 715          * @return array
 716          */
 717         protected function getExtraSortFields() {
 718                 return [];
 719         }
 720
 721         /**
 722          * Return the default sorting direction: DIR_ASCENDING or DIR_DESCENDING.
 723          * You can also have an associative array of ordertype => dir,
 724          * if multiple order types are supported.  In this case getIndexField()
 725          * must return an array, and the keys of that must exactly match the keys
 726          * of this.
 727          *
 728          * For backward compatibility, this method's return value will be ignored
 729          * if $this->mDefaultDirection is already set when the constructor is
 730          * called, for instance if it's statically initialized.  In that case the
 731          * value of that variable (which must be a boolean) will be used.
 732          *
 733          * Note that despite its name, this does not return the value of the
 734          * $this->mDefaultDirection member variable.  That's the default for this
 735          * particular instantiation, which is a single value.  This is the set of
 736          * all defaults for the class.
 737          *
 738          * @return bool
 739          */
 740         protected function getDefaultDirections() {
 741                 return self::DIR_ASCENDING;
 742         }
 743 }