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         public $mRequest;
  68         public $mLimitsShown = array( 20, 50, 100, 250, 500 );
  69         public $mDefaultLimit = 50;
  70         public $mOffset, $mLimit;
  71         public $mQueryDone = false;
  72         public $mDb;
  73         public $mPastTheEndRow;
  74
  75         /**
  76          * The index to actually be used for ordering. This is a single column,
  77          * for one ordering, even if multiple orderings are supported.
  78          */
  79         protected $mIndexField;
  80         /**
  81          * An array of secondary columns to order by. These fields are not part of the offset.
  82          * This is a column list for one ordering, even if multiple orderings are supported.
  83          */
  84         protected $mExtraSortFields;
  85         /** For pages that support multiple types of ordering, which one to use.
  86          */
  87         protected $mOrderType;
  88         /**
  89          * $mDefaultDirection gives the direction to use when sorting results:
  90          * false for ascending, true for descending.  If $mIsBackwards is set, we
  91          * start from the opposite end, but we still sort the page itself according
  92          * to $mDefaultDirection.  E.g., if $mDefaultDirection is false but we're
  93          * going backwards, we'll display the last page of results, but the last
  94          * result will be at the bottom, not the top.
  95          *
  96          * Like $mIndexField, $mDefaultDirection will be a single value even if the
  97          * class supports multiple default directions for different order types.
  98          */
  99         public $mDefaultDirection;
 100         public $mIsBackwards;
 101
 102         /** True if the current result set is the first one */
 103         public $mIsFirst;
 104         public $mIsLast;
 105
 106         protected $mLastShown, $mFirstShown, $mPastTheEndIndex, $mDefaultQuery, $mNavigationBar;
 107
 108         /**
 109          * Whether to include the offset in the query
 110          */
 111         protected $mIncludeOffset = false;
 112
 113         /**
 114          * Result object for the query. Warning: seek before use.
 115          *
 116          * @var ResultWrapper
 117          */
 118         public $mResult;
 119
 120         public function __construct( IContextSource $context = null ) {
 121                 if ( $context ) {
 122                         $this->setContext( $context );
 123                 }
 124
 125                 $this->mRequest = $this->getRequest();
 126
 127                 # NB: the offset is quoted, not validated. It is treated as an
 128                 # arbitrary string to support the widest variety of index types. Be
 129                 # careful outputting it into HTML!
 130                 $this->mOffset = $this->mRequest->getText( 'offset' );
 131
 132                 # Use consistent behavior for the limit options
 133                 $this->mDefaultLimit = $this->getUser()->getIntOption( 'rclimit' );
 134                 if ( !$this->mLimit ) {
 135                         // Don't override if a subclass calls $this->setLimit() in its constructor.
 136                         list( $this->mLimit, /* $offset */ ) = $this->mRequest->getLimitOffset();
 137                 }
 138
 139                 $this->mIsBackwards = ( $this->mRequest->getVal( 'dir' ) == 'prev' );
 140                 # Let the subclass set the DB here; otherwise use a slave DB for the current wiki
 141                 $this->mDb = $this->mDb ?: wfGetDB( DB_SLAVE );
 142
 143                 $index = $this->getIndexField(); // column to sort on
 144                 $extraSort = $this->getExtraSortFields(); // extra columns to sort on for query planning
 145                 $order = $this->mRequest->getVal( 'order' );
 146                 if ( is_array( $index ) && isset( $index[$order] ) ) {
 147                         $this->mOrderType = $order;
 148                         $this->mIndexField = $index[$order];
 149                         $this->mExtraSortFields = isset( $extraSort[$order] )
 150                                 ? (array)$extraSort[$order]
 151                                 : array();
 152                 } elseif ( is_array( $index ) ) {
 153                         # First element is the default
 154                         reset( $index );
 155                         list( $this->mOrderType, $this->mIndexField ) = each( $index );
 156                         $this->mExtraSortFields = isset( $extraSort[$this->mOrderType] )
 157                                 ? (array)$extraSort[$this->mOrderType]
 158                                 : array();
 159                 } else {
 160                         # $index is not an array
 161                         $this->mOrderType = null;
 162                         $this->mIndexField = $index;
 163                         $this->mExtraSortFields = (array)$extraSort;
 164                 }
 165
 166                 if ( !isset( $this->mDefaultDirection ) ) {
 167                         $dir = $this->getDefaultDirections();
 168                         $this->mDefaultDirection = is_array( $dir )
 169                                 ? $dir[$this->mOrderType]
 170                                 : $dir;
 171                 }
 172         }
 173
 174         /**
 175          * Get the Database object in use
 176          *
 177          * @return DatabaseBase
 178          */
 179         public function getDatabase() {
 180                 return $this->mDb;
 181         }
 182
 183         /**
 184          * Do the query, using information from the object context. This function
 185          * has been kept minimal to make it overridable if necessary, to allow for
 186          * result sets formed from multiple DB queries.
 187          */
 188         public function doQuery() {
 189                 # Use the child class name for profiling
 190                 $fname = __METHOD__ . ' (' . get_class( $this ) . ')';
 191                 wfProfileIn( $fname );
 192
 193                 $descending = ( $this->mIsBackwards == $this->mDefaultDirection );
 194                 # Plus an extra row so that we can tell the "next" link should be shown
 195                 $queryLimit = $this->mLimit + 1;
 196
 197                 if ( $this->mOffset == '' ) {
 198                         $isFirst = true;
 199                 } else {
 200                         // If there's an offset, we may or may not be at the first entry.
 201                         // The only way to tell is to run the query in the opposite
 202                         // direction see if we get a row.
 203                         $oldIncludeOffset = $this->mIncludeOffset;
 204                         $this->mIncludeOffset = !$this->mIncludeOffset;
 205                         $isFirst = !$this->reallyDoQuery( $this->mOffset, 1, !$descending )->numRows();
 206                         $this->mIncludeOffset = $oldIncludeOffset;
 207                 }
 208
 209                 $this->mResult = $this->reallyDoQuery(
 210                         $this->mOffset,
 211                         $queryLimit,
 212                         $descending
 213                 );
 214
 215                 $this->extractResultInfo( $isFirst, $queryLimit, $this->mResult );
 216                 $this->mQueryDone = true;
 217
 218                 $this->preprocessResults( $this->mResult );
 219                 $this->mResult->rewind(); // Paranoia
 220
 221                 wfProfileOut( $fname );
 222         }
 223
 224         /**
 225          * @return ResultWrapper The result wrapper.
 226          */
 227         function getResult() {
 228                 return $this->mResult;
 229         }
 230
 231         /**
 232          * Set the offset from an other source than the request
 233          *
 234          * @param int|string $offset
 235          */
 236         function setOffset( $offset ) {
 237                 $this->mOffset = $offset;
 238         }
 239
 240         /**
 241          * Set the limit from an other source than the request
 242          *
 243          * Verifies limit is between 1 and 5000
 244          *
 245          * @param int|string $limit
 246          */
 247         function setLimit( $limit ) {
 248                 $limit = (int)$limit;
 249                 // WebRequest::getLimitOffset() puts a cap of 5000, so do same here.
 250                 if ( $limit > 5000 ) {
 251                         $limit = 5000;
 252                 }
 253                 if ( $limit > 0 ) {
 254                         $this->mLimit = $limit;
 255                 }
 256         }
 257
 258         /**
 259          * Get the current limit
 260          *
 261          * @return int
 262          */
 263         function getLimit() {
 264                 return $this->mLimit;
 265         }
 266
 267         /**
 268          * Set whether a row matching exactly the offset should be also included
 269          * in the result or not. By default this is not the case, but when the
 270          * offset is user-supplied this might be wanted.
 271          *
 272          * @param bool $include
 273          */
 274         public function setIncludeOffset( $include ) {
 275                 $this->mIncludeOffset = $include;
 276         }
 277
 278         /**
 279          * Extract some useful data from the result object for use by
 280          * the navigation bar, put it into $this
 281          *
 282          * @param bool $isFirst False if there are rows before those fetched (i.e.
 283          *     if a "previous" link would make sense)
 284          * @param int $limit Exact query limit
 285          * @param ResultWrapper $res
 286          */
 287         function extractResultInfo( $isFirst, $limit, ResultWrapper $res ) {
 288                 $numRows = $res->numRows();
 289                 if ( $numRows ) {
 290                         # Remove any table prefix from index field
 291                         $parts = explode( '.', $this->mIndexField );
 292                         $indexColumn = end( $parts );
 293
 294                         $row = $res->fetchRow();
 295                         $firstIndex = $row[$indexColumn];
 296
 297                         # Discard the extra result row if there is one
 298                         if ( $numRows > $this->mLimit && $numRows > 1 ) {
 299                                 $res->seek( $numRows - 1 );
 300                                 $this->mPastTheEndRow = $res->fetchObject();
 301                                 $this->mPastTheEndIndex = $this->mPastTheEndRow->$indexColumn;
 302                                 $res->seek( $numRows - 2 );
 303                                 $row = $res->fetchRow();
 304                                 $lastIndex = $row[$indexColumn];
 305                         } else {
 306                                 $this->mPastTheEndRow = null;
 307                                 # Setting indexes to an empty string means that they will be
 308                                 # omitted if they would otherwise appear in URLs. It just so
 309                                 # happens that this  is the right thing to do in the standard
 310                                 # UI, in all the relevant cases.
 311                                 $this->mPastTheEndIndex = '';
 312                                 $res->seek( $numRows - 1 );
 313                                 $row = $res->fetchRow();
 314                                 $lastIndex = $row[$indexColumn];
 315                         }
 316                 } else {
 317                         $firstIndex = '';
 318                         $lastIndex = '';
 319                         $this->mPastTheEndRow = null;
 320                         $this->mPastTheEndIndex = '';
 321                 }
 322
 323                 if ( $this->mIsBackwards ) {
 324                         $this->mIsFirst = ( $numRows < $limit );
 325                         $this->mIsLast = $isFirst;
 326                         $this->mLastShown = $firstIndex;
 327                         $this->mFirstShown = $lastIndex;
 328                 } else {
 329                         $this->mIsFirst = $isFirst;
 330                         $this->mIsLast = ( $numRows < $limit );
 331                         $this->mLastShown = $lastIndex;
 332                         $this->mFirstShown = $firstIndex;
 333                 }
 334         }
 335
 336         /**
 337          * Get some text to go in brackets in the "function name" part of the SQL comment
 338          *
 339          * @return string
 340          */
 341         function getSqlComment() {
 342                 return get_class( $this );
 343         }
 344
 345         /**
 346          * Do a query with specified parameters, rather than using the object
 347          * context
 348          *
 349          * @param string $offset Index offset, inclusive
 350          * @param int $limit Exact query limit
 351          * @param bool $descending Query direction, false for ascending, true for descending
 352          * @return ResultWrapper
 353          */
 354         public function reallyDoQuery( $offset, $limit, $descending ) {
 355                 list( $tables, $fields, $conds, $fname, $options, $join_conds ) =
 356                         $this->buildQueryInfo( $offset, $limit, $descending );
 357
 358                 return $this->mDb->select( $tables, $fields, $conds, $fname, $options, $join_conds );
 359         }
 360
 361         /**
 362          * Build variables to use by the database wrapper.
 363          *
 364          * @param string $offset Index offset, inclusive
 365          * @param int $limit Exact query limit
 366          * @param bool $descending Query direction, false for ascending, true for descending
 367          * @return array
 368          */
 369         protected function buildQueryInfo( $offset, $limit, $descending ) {
 370                 $fname = __METHOD__ . ' (' . $this->getSqlComment() . ')';
 371                 $info = $this->getQueryInfo();
 372                 $tables = $info['tables'];
 373                 $fields = $info['fields'];
 374                 $conds = isset( $info['conds'] ) ? $info['conds'] : array();
 375                 $options = isset( $info['options'] ) ? $info['options'] : array();
 376                 $join_conds = isset( $info['join_conds'] ) ? $info['join_conds'] : array();
 377                 $sortColumns = array_merge( array( $this->mIndexField ), $this->mExtraSortFields );
 378                 if ( $descending ) {
 379                         $options['ORDER BY'] = $sortColumns;
 380                         $operator = $this->mIncludeOffset ? '>=' : '>';
 381                 } else {
 382                         $orderBy = array();
 383                         foreach ( $sortColumns as $col ) {
 384                                 $orderBy[] = $col . ' DESC';
 385                         }
 386                         $options['ORDER BY'] = $orderBy;
 387                         $operator = $this->mIncludeOffset ? '<=' : '<';
 388                 }
 389                 if ( $offset != '' ) {
 390                         $conds[] = $this->mIndexField . $operator . $this->mDb->addQuotes( $offset );
 391                 }
 392                 $options['LIMIT'] = intval( $limit );
 393                 return array( $tables, $fields, $conds, $fname, $options, $join_conds );
 394         }
 395
 396         /**
 397          * Pre-process results; useful for performing batch existence checks, etc.
 398          *
 399          * @param ResultWrapper $result
 400          */
 401         protected function preprocessResults( $result ) {
 402         }
 403
 404         /**
 405          * Get the formatted result list. Calls getStartBody(), formatRow() and
 406          * getEndBody(), concatenates the results and returns them.
 407          *
 408          * @return string
 409          */
 410         public function getBody() {
 411                 if ( !$this->mQueryDone ) {
 412                         $this->doQuery();
 413                 }
 414
 415                 if ( $this->mResult->numRows() ) {
 416                         # Do any special query batches before display
 417                         $this->doBatchLookups();
 418                 }
 419
 420                 # Don't use any extra rows returned by the query
 421                 $numRows = min( $this->mResult->numRows(), $this->mLimit );
 422
 423                 $s = $this->getStartBody();
 424                 if ( $numRows ) {
 425                         if ( $this->mIsBackwards ) {
 426                                 for ( $i = $numRows - 1; $i >= 0; $i-- ) {
 427                                         $this->mResult->seek( $i );
 428                                         $row = $this->mResult->fetchObject();
 429                                         $s .= $this->formatRow( $row );
 430                                 }
 431                         } else {
 432                                 $this->mResult->seek( 0 );
 433                                 for ( $i = 0; $i < $numRows; $i++ ) {
 434                                         $row = $this->mResult->fetchObject();
 435                                         $s .= $this->formatRow( $row );
 436                                 }
 437                         }
 438                 } else {
 439                         $s .= $this->getEmptyBody();
 440                 }
 441                 $s .= $this->getEndBody();
 442                 return $s;
 443         }
 444
 445         /**
 446          * Make a self-link
 447          *
 448          * @param string $text Text displayed on the link
 449          * @param array $query Associative array of parameter to be in the query string
 450          * @param string $type Value of the "rel" attribute
 451          *
 452          * @return string HTML fragment
 453          */
 454         function makeLink( $text, array $query = null, $type = null ) {
 455                 if ( $query === null ) {
 456                         return $text;
 457                 }
 458
 459                 $attrs = array();
 460                 if ( in_array( $type, array( 'first', 'prev', 'next', 'last' ) ) ) {
 461                         # HTML5 rel attributes
 462                         $attrs['rel'] = $type;
 463                 }
 464
 465                 if ( $type ) {
 466                         $attrs['class'] = "mw-{$type}link";
 467                 }
 468
 469                 return Linker::linkKnown(
 470                         $this->getTitle(),
 471                         $text,
 472                         $attrs,
 473                         $query + $this->getDefaultQuery()
 474                 );
 475         }
 476
 477         /**
 478          * Called from getBody(), before getStartBody() is called and
 479          * after doQuery() was called. This will be called only if there
 480          * are rows in the result set.
 481          *
 482          * @return void
 483          */
 484         protected function doBatchLookups() {
 485         }
 486
 487         /**
 488          * Hook into getBody(), allows text to be inserted at the start. This
 489          * will be called even if there are no rows in the result set.
 490          *
 491          * @return string
 492          */
 493         protected function getStartBody() {
 494                 return '';
 495         }
 496
 497         /**
 498          * Hook into getBody() for the end of the list
 499          *
 500          * @return string
 501          */
 502         protected function getEndBody() {
 503                 return '';
 504         }
 505
 506         /**
 507          * Hook into getBody(), for the bit between the start and the
 508          * end when there are no rows
 509          *
 510          * @return string
 511          */
 512         protected function getEmptyBody() {
 513                 return '';
 514         }
 515
 516         /**
 517          * Get an array of query parameters that should be put into self-links.
 518          * By default, all parameters passed in the URL are used, except for a
 519          * short blacklist.
 520          *
 521          * @return array Associative array
 522          */
 523         function getDefaultQuery() {
 524                 if ( !isset( $this->mDefaultQuery ) ) {
 525                         $this->mDefaultQuery = $this->getRequest()->getQueryValues();
 526                         unset( $this->mDefaultQuery['title'] );
 527                         unset( $this->mDefaultQuery['dir'] );
 528                         unset( $this->mDefaultQuery['offset'] );
 529                         unset( $this->mDefaultQuery['limit'] );
 530                         unset( $this->mDefaultQuery['order'] );
 531                         unset( $this->mDefaultQuery['month'] );
 532                         unset( $this->mDefaultQuery['year'] );
 533                 }
 534                 return $this->mDefaultQuery;
 535         }
 536
 537         /**
 538          * Get the number of rows in the result set
 539          *
 540          * @return int
 541          */
 542         function getNumRows() {
 543                 if ( !$this->mQueryDone ) {
 544                         $this->doQuery();
 545                 }
 546                 return $this->mResult->numRows();
 547         }
 548
 549         /**
 550          * Get a URL query array for the prev, next, first and last links.
 551          *
 552          * @return array
 553          */
 554         function getPagingQueries() {
 555                 if ( !$this->mQueryDone ) {
 556                         $this->doQuery();
 557                 }
 558
 559                 # Don't announce the limit everywhere if it's the default
 560                 $urlLimit = $this->mLimit == $this->mDefaultLimit ? null : $this->mLimit;
 561
 562                 if ( $this->mIsFirst ) {
 563                         $prev = false;
 564                         $first = false;
 565                 } else {
 566                         $prev = array(
 567                                 'dir' => 'prev',
 568                                 'offset' => $this->mFirstShown,
 569                                 'limit' => $urlLimit
 570                         );
 571                         $first = array( 'limit' => $urlLimit );
 572                 }
 573                 if ( $this->mIsLast ) {
 574                         $next = false;
 575                         $last = false;
 576                 } else {
 577                         $next = array( 'offset' => $this->mLastShown, 'limit' => $urlLimit );
 578                         $last = array( 'dir' => 'prev', 'limit' => $urlLimit );
 579                 }
 580                 return array(
 581                         'prev' => $prev,
 582                         'next' => $next,
 583                         'first' => $first,
 584                         'last' => $last
 585                 );
 586         }
 587
 588         /**
 589          * Returns whether to show the "navigation bar"
 590          *
 591          * @return bool
 592          */
 593         function isNavigationBarShown() {
 594                 if ( !$this->mQueryDone ) {
 595                         $this->doQuery();
 596                 }
 597                 // Hide navigation by default if there is nothing to page
 598                 return !( $this->mIsFirst && $this->mIsLast );
 599         }
 600
 601         /**
 602          * Get paging links. If a link is disabled, the item from $disabledTexts
 603          * will be used. If there is no such item, the unlinked text from
 604          * $linkTexts will be used. Both $linkTexts and $disabledTexts are arrays
 605          * of HTML.
 606          *
 607          * @param array $linkTexts
 608          * @param array $disabledTexts
 609          * @return array
 610          */
 611         function getPagingLinks( $linkTexts, $disabledTexts = array() ) {
 612                 $queries = $this->getPagingQueries();
 613                 $links = array();
 614
 615                 foreach ( $queries as $type => $query ) {
 616                         if ( $query !== false ) {
 617                                 $links[$type] = $this->makeLink(
 618                                         $linkTexts[$type],
 619                                         $queries[$type],
 620                                         $type
 621                                 );
 622                         } elseif ( isset( $disabledTexts[$type] ) ) {
 623                                 $links[$type] = $disabledTexts[$type];
 624                         } else {
 625                                 $links[$type] = $linkTexts[$type];
 626                         }
 627                 }
 628
 629                 return $links;
 630         }
 631
 632         function getLimitLinks() {
 633                 $links = array();
 634                 if ( $this->mIsBackwards ) {
 635                         $offset = $this->mPastTheEndIndex;
 636                 } else {
 637                         $offset = $this->mOffset;
 638                 }
 639                 foreach ( $this->mLimitsShown as $limit ) {
 640                         $links[] = $this->makeLink(
 641                                 $this->getLanguage()->formatNum( $limit ),
 642                                 array( 'offset' => $offset, 'limit' => $limit ),
 643                                 'num'
 644                         );
 645                 }
 646                 return $links;
 647         }
 648
 649         /**
 650          * Abstract formatting function. This should return an HTML string
 651          * representing the result row $row. Rows will be concatenated and
 652          * returned by getBody()
 653          *
 654          * @param array|stdClass $row Database row
 655          * @return string
 656          */
 657         abstract function formatRow( $row );
 658
 659         /**
 660          * This function should be overridden to provide all parameters
 661          * needed for the main paged query. It returns an associative
 662          * array with the following elements:
 663          *    tables => Table(s) for passing to Database::select()
 664          *    fields => Field(s) for passing to Database::select(), may be *
 665          *    conds => WHERE conditions
 666          *    options => option array
 667          *    join_conds => JOIN conditions
 668          *
 669          * @return array
 670          */
 671         abstract function getQueryInfo();
 672
 673         /**
 674          * This function should be overridden to return the name of the index fi-
 675          * eld.  If the pager supports multiple orders, it may return an array of
 676          * 'querykey' => 'indexfield' pairs, so that a request with &count=querykey
 677          * will use indexfield to sort.  In this case, the first returned key is
 678          * the default.
 679          *
 680          * Needless to say, it's really not a good idea to use a non-unique index
 681          * for this!  That won't page right.
 682          *
 683          * @return string|array
 684          */
 685         abstract function getIndexField();
 686
 687         /**
 688          * This function should be overridden to return the names of secondary columns
 689          * to order by in addition to the column in getIndexField(). These fields will
 690          * not be used in the pager offset or in any links for users.
 691          *
 692          * If getIndexField() returns an array of 'querykey' => 'indexfield' pairs then
 693          * this must return a corresponding array of 'querykey' => array( fields...) pairs
 694          * in order for a request with &count=querykey to use array( fields...) to sort.
 695          *
 696          * This is useful for pagers that GROUP BY a unique column (say page_id)
 697          * and ORDER BY another (say page_len). Using GROUP BY and ORDER BY both on
 698          * page_len,page_id avoids temp tables (given a page_len index). This would
 699          * also work if page_id was non-unique but we had a page_len,page_id index.
 700          *
 701          * @return array
 702          */
 703         protected function getExtraSortFields() {
 704                 return array();
 705         }
 706
 707         /**
 708          * Return the default sorting direction: false for ascending, true for
 709          * descending.  You can also have an associative array of ordertype => dir,
 710          * if multiple order types are supported.  In this case getIndexField()
 711          * must return an array, and the keys of that must exactly match the keys
 712          * of this.
 713          *
 714          * For backward compatibility, this method's return value will be ignored
 715          * if $this->mDefaultDirection is already set when the constructor is
 716          * called, for instance if it's statically initialized.  In that case the
 717          * value of that variable (which must be a boolean) will be used.
 718          *
 719          * Note that despite its name, this does not return the value of the
 720          * $this->mDefaultDirection member variable.  That's the default for this
 721          * particular instantiation, which is a single value.  This is the set of
 722          * all defaults for the class.
 723          *
 724          * @return bool
 725          */
 726         protected function getDefaultDirections() {
 727                 return false;
 728         }
 729 }