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 = array( 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 slave DB for the current wiki
 149                 $this->mDb = $this->mDb ?: wfGetDB( DB_SLAVE );
 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                                 : array();
 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                                 : array();
 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 DatabaseBase
 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                 wfProfileIn( $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                 wfProfileOut( $fname );
 231         }
 232
 233         /**
 234          * @return ResultWrapper The result wrapper.
 235          */
 236         function getResult() {
 237                 return $this->mResult;
 238         }
 239
 240         /**
 241          * Set the offset from an other source than the request
 242          *
 243          * @param int|string $offset
 244          */
 245         function setOffset( $offset ) {
 246                 $this->mOffset = $offset;
 247         }
 248
 249         /**
 250          * Set the limit from an other source than the request
 251          *
 252          * Verifies limit is between 1 and 5000
 253          *
 254          * @param int|string $limit
 255          */
 256         function setLimit( $limit ) {
 257                 $limit = (int)$limit;
 258                 // WebRequest::getLimitOffset() puts a cap of 5000, so do same here.
 259                 if ( $limit > 5000 ) {
 260                         $limit = 5000;
 261                 }
 262                 if ( $limit > 0 ) {
 263                         $this->mLimit = $limit;
 264                 }
 265         }
 266
 267         /**
 268          * Get the current limit
 269          *
 270          * @return int
 271          */
 272         function getLimit() {
 273                 return $this->mLimit;
 274         }
 275
 276         /**
 277          * Set whether a row matching exactly the offset should be also included
 278          * in the result or not. By default this is not the case, but when the
 279          * offset is user-supplied this might be wanted.
 280          *
 281          * @param bool $include
 282          */
 283         public function setIncludeOffset( $include ) {
 284                 $this->mIncludeOffset = $include;
 285         }
 286
 287         /**
 288          * Extract some useful data from the result object for use by
 289          * the navigation bar, put it into $this
 290          *
 291          * @param bool $isFirst False if there are rows before those fetched (i.e.
 292          *     if a "previous" link would make sense)
 293          * @param int $limit Exact query limit
 294          * @param ResultWrapper $res
 295          */
 296         function extractResultInfo( $isFirst, $limit, ResultWrapper $res ) {
 297                 $numRows = $res->numRows();
 298                 if ( $numRows ) {
 299                         # Remove any table prefix from index field
 300                         $parts = explode( '.', $this->mIndexField );
 301                         $indexColumn = end( $parts );
 302
 303                         $row = $res->fetchRow();
 304                         $firstIndex = $row[$indexColumn];
 305
 306                         # Discard the extra result row if there is one
 307                         if ( $numRows > $this->mLimit && $numRows > 1 ) {
 308                                 $res->seek( $numRows - 1 );
 309                                 $this->mPastTheEndRow = $res->fetchObject();
 310                                 $this->mPastTheEndIndex = $this->mPastTheEndRow->$indexColumn;
 311                                 $res->seek( $numRows - 2 );
 312                                 $row = $res->fetchRow();
 313                                 $lastIndex = $row[$indexColumn];
 314                         } else {
 315                                 $this->mPastTheEndRow = null;
 316                                 # Setting indexes to an empty string means that they will be
 317                                 # omitted if they would otherwise appear in URLs. It just so
 318                                 # happens that this  is the right thing to do in the standard
 319                                 # UI, in all the relevant cases.
 320                                 $this->mPastTheEndIndex = '';
 321                                 $res->seek( $numRows - 1 );
 322                                 $row = $res->fetchRow();
 323                                 $lastIndex = $row[$indexColumn];
 324                         }
 325                 } else {
 326                         $firstIndex = '';
 327                         $lastIndex = '';
 328                         $this->mPastTheEndRow = null;
 329                         $this->mPastTheEndIndex = '';
 330                 }
 331
 332                 if ( $this->mIsBackwards ) {
 333                         $this->mIsFirst = ( $numRows < $limit );
 334                         $this->mIsLast = $isFirst;
 335                         $this->mLastShown = $firstIndex;
 336                         $this->mFirstShown = $lastIndex;
 337                 } else {
 338                         $this->mIsFirst = $isFirst;
 339                         $this->mIsLast = ( $numRows < $limit );
 340                         $this->mLastShown = $lastIndex;
 341                         $this->mFirstShown = $firstIndex;
 342                 }
 343         }
 344
 345         /**
 346          * Get some text to go in brackets in the "function name" part of the SQL comment
 347          *
 348          * @return string
 349          */
 350         function getSqlComment() {
 351                 return get_class( $this );
 352         }
 353
 354         /**
 355          * Do a query with specified parameters, rather than using the object
 356          * context
 357          *
 358          * @param string $offset Index offset, inclusive
 359          * @param int $limit Exact query limit
 360          * @param bool $descending Query direction, false for ascending, true for descending
 361          * @return ResultWrapper
 362          */
 363         public function reallyDoQuery( $offset, $limit, $descending ) {
 364                 list( $tables, $fields, $conds, $fname, $options, $join_conds ) =
 365                         $this->buildQueryInfo( $offset, $limit, $descending );
 366
 367                 return $this->mDb->select( $tables, $fields, $conds, $fname, $options, $join_conds );
 368         }
 369
 370         /**
 371          * Build variables to use by the database wrapper.
 372          *
 373          * @param string $offset Index offset, inclusive
 374          * @param int $limit Exact query limit
 375          * @param bool $descending Query direction, false for ascending, true for descending
 376          * @return array
 377          */
 378         protected function buildQueryInfo( $offset, $limit, $descending ) {
 379                 $fname = __METHOD__ . ' (' . $this->getSqlComment() . ')';
 380                 $info = $this->getQueryInfo();
 381                 $tables = $info['tables'];
 382                 $fields = $info['fields'];
 383                 $conds = isset( $info['conds'] ) ? $info['conds'] : array();
 384                 $options = isset( $info['options'] ) ? $info['options'] : array();
 385                 $join_conds = isset( $info['join_conds'] ) ? $info['join_conds'] : array();
 386                 $sortColumns = array_merge( array( $this->mIndexField ), $this->mExtraSortFields );
 387                 if ( $descending ) {
 388                         $options['ORDER BY'] = $sortColumns;
 389                         $operator = $this->mIncludeOffset ? '>=' : '>';
 390                 } else {
 391                         $orderBy = array();
 392                         foreach ( $sortColumns as $col ) {
 393                                 $orderBy[] = $col . ' DESC';
 394                         }
 395                         $options['ORDER BY'] = $orderBy;
 396                         $operator = $this->mIncludeOffset ? '<=' : '<';
 397                 }
 398                 if ( $offset != '' ) {
 399                         $conds[] = $this->mIndexField . $operator . $this->mDb->addQuotes( $offset );
 400                 }
 401                 $options['LIMIT'] = intval( $limit );
 402                 return array( $tables, $fields, $conds, $fname, $options, $join_conds );
 403         }
 404
 405         /**
 406          * Pre-process results; useful for performing batch existence checks, etc.
 407          *
 408          * @param ResultWrapper $result
 409          */
 410         protected function preprocessResults( $result ) {
 411         }
 412
 413         /**
 414          * Get the formatted result list. Calls getStartBody(), formatRow() and
 415          * getEndBody(), concatenates the results and returns them.
 416          *
 417          * @return string
 418          */
 419         public function getBody() {
 420                 if ( !$this->mQueryDone ) {
 421                         $this->doQuery();
 422                 }
 423
 424                 if ( $this->mResult->numRows() ) {
 425                         # Do any special query batches before display
 426                         $this->doBatchLookups();
 427                 }
 428
 429                 # Don't use any extra rows returned by the query
 430                 $numRows = min( $this->mResult->numRows(), $this->mLimit );
 431
 432                 $s = $this->getStartBody();
 433                 if ( $numRows ) {
 434                         if ( $this->mIsBackwards ) {
 435                                 for ( $i = $numRows - 1; $i >= 0; $i-- ) {
 436                                         $this->mResult->seek( $i );
 437                                         $row = $this->mResult->fetchObject();
 438                                         $s .= $this->formatRow( $row );
 439                                 }
 440                         } else {
 441                                 $this->mResult->seek( 0 );
 442                                 for ( $i = 0; $i < $numRows; $i++ ) {
 443                                         $row = $this->mResult->fetchObject();
 444                                         $s .= $this->formatRow( $row );
 445                                 }
 446                         }
 447                 } else {
 448                         $s .= $this->getEmptyBody();
 449                 }
 450                 $s .= $this->getEndBody();
 451                 return $s;
 452         }
 453
 454         /**
 455          * Make a self-link
 456          *
 457          * @param string $text Text displayed on the link
 458          * @param array $query Associative array of parameter to be in the query string
 459          * @param string $type Link type used to create additional attributes, like "rel", "class" or
 460          *  "title". Valid values (non-exhaustive list): 'first', 'last', 'prev', 'next', 'asc', 'desc'.
 461          * @return string HTML fragment
 462          */
 463         function makeLink( $text, array $query = null, $type = null ) {
 464                 if ( $query === null ) {
 465                         return $text;
 466                 }
 467
 468                 $attrs = array();
 469                 if ( in_array( $type, array( 'prev', 'next' ) ) ) {
 470                         $attrs['rel'] = $type;
 471                 }
 472
 473                 if ( in_array( $type, array( 'asc', 'desc' ) ) ) {
 474                         $attrs['title'] = wfMessage( $type == 'asc' ? 'sort-ascending' : 'sort-descending' )->text();
 475                 }
 476
 477                 if ( $type ) {
 478                         $attrs['class'] = "mw-{$type}link";
 479                 }
 480
 481
 482                 return Linker::linkKnown(
 483                         $this->getTitle(),
 484                         $text,
 485                         $attrs,
 486                         $query + $this->getDefaultQuery()
 487                 );
 488         }
 489
 490         /**
 491          * Called from getBody(), before getStartBody() is called and
 492          * after doQuery() was called. This will be called only if there
 493          * are rows in the result set.
 494          *
 495          * @return void
 496          */
 497         protected function doBatchLookups() {
 498         }
 499
 500         /**
 501          * Hook into getBody(), allows text to be inserted at the start. This
 502          * will be called even if there are no rows in the result set.
 503          *
 504          * @return string
 505          */
 506         protected function getStartBody() {
 507                 return '';
 508         }
 509
 510         /**
 511          * Hook into getBody() for the end of the list
 512          *
 513          * @return string
 514          */
 515         protected function getEndBody() {
 516                 return '';
 517         }
 518
 519         /**
 520          * Hook into getBody(), for the bit between the start and the
 521          * end when there are no rows
 522          *
 523          * @return string
 524          */
 525         protected function getEmptyBody() {
 526                 return '';
 527         }
 528
 529         /**
 530          * Get an array of query parameters that should be put into self-links.
 531          * By default, all parameters passed in the URL are used, except for a
 532          * short blacklist.
 533          *
 534          * @return array Associative array
 535          */
 536         function getDefaultQuery() {
 537                 if ( !isset( $this->mDefaultQuery ) ) {
 538                         $this->mDefaultQuery = $this->getRequest()->getQueryValues();
 539                         unset( $this->mDefaultQuery['title'] );
 540                         unset( $this->mDefaultQuery['dir'] );
 541                         unset( $this->mDefaultQuery['offset'] );
 542                         unset( $this->mDefaultQuery['limit'] );
 543                         unset( $this->mDefaultQuery['order'] );
 544                         unset( $this->mDefaultQuery['month'] );
 545                         unset( $this->mDefaultQuery['year'] );
 546                 }
 547                 return $this->mDefaultQuery;
 548         }
 549
 550         /**
 551          * Get the number of rows in the result set
 552          *
 553          * @return int
 554          */
 555         function getNumRows() {
 556                 if ( !$this->mQueryDone ) {
 557                         $this->doQuery();
 558                 }
 559                 return $this->mResult->numRows();
 560         }
 561
 562         /**
 563          * Get a URL query array for the prev, next, first and last links.
 564          *
 565          * @return array
 566          */
 567         function getPagingQueries() {
 568                 if ( !$this->mQueryDone ) {
 569                         $this->doQuery();
 570                 }
 571
 572                 # Don't announce the limit everywhere if it's the default
 573                 $urlLimit = $this->mLimit == $this->mDefaultLimit ? null : $this->mLimit;
 574
 575                 if ( $this->mIsFirst ) {
 576                         $prev = false;
 577                         $first = false;
 578                 } else {
 579                         $prev = array(
 580                                 'dir' => 'prev',
 581                                 'offset' => $this->mFirstShown,
 582                                 'limit' => $urlLimit
 583                         );
 584                         $first = array( 'limit' => $urlLimit );
 585                 }
 586                 if ( $this->mIsLast ) {
 587                         $next = false;
 588                         $last = false;
 589                 } else {
 590                         $next = array( 'offset' => $this->mLastShown, 'limit' => $urlLimit );
 591                         $last = array( 'dir' => 'prev', 'limit' => $urlLimit );
 592                 }
 593                 return array(
 594                         'prev' => $prev,
 595                         'next' => $next,
 596                         'first' => $first,
 597                         'last' => $last
 598                 );
 599         }
 600
 601         /**
 602          * Returns whether to show the "navigation bar"
 603          *
 604          * @return bool
 605          */
 606         function isNavigationBarShown() {
 607                 if ( !$this->mQueryDone ) {
 608                         $this->doQuery();
 609                 }
 610                 // Hide navigation by default if there is nothing to page
 611                 return !( $this->mIsFirst && $this->mIsLast );
 612         }
 613
 614         /**
 615          * Get paging links. If a link is disabled, the item from $disabledTexts
 616          * will be used. If there is no such item, the unlinked text from
 617          * $linkTexts will be used. Both $linkTexts and $disabledTexts are arrays
 618          * of HTML.
 619          *
 620          * @param array $linkTexts
 621          * @param array $disabledTexts
 622          * @return array
 623          */
 624         function getPagingLinks( $linkTexts, $disabledTexts = array() ) {
 625                 $queries = $this->getPagingQueries();
 626                 $links = array();
 627
 628                 foreach ( $queries as $type => $query ) {
 629                         if ( $query !== false ) {
 630                                 $links[$type] = $this->makeLink(
 631                                         $linkTexts[$type],
 632                                         $queries[$type],
 633                                         $type
 634                                 );
 635                         } elseif ( isset( $disabledTexts[$type] ) ) {
 636                                 $links[$type] = $disabledTexts[$type];
 637                         } else {
 638                                 $links[$type] = $linkTexts[$type];
 639                         }
 640                 }
 641
 642                 return $links;
 643         }
 644
 645         function getLimitLinks() {
 646                 $links = array();
 647                 if ( $this->mIsBackwards ) {
 648                         $offset = $this->mPastTheEndIndex;
 649                 } else {
 650                         $offset = $this->mOffset;
 651                 }
 652                 foreach ( $this->mLimitsShown as $limit ) {
 653                         $links[] = $this->makeLink(
 654                                 $this->getLanguage()->formatNum( $limit ),
 655                                 array( 'offset' => $offset, 'limit' => $limit ),
 656                                 'num'
 657                         );
 658                 }
 659                 return $links;
 660         }
 661
 662         /**
 663          * Abstract formatting function. This should return an HTML string
 664          * representing the result row $row. Rows will be concatenated and
 665          * returned by getBody()
 666          *
 667          * @param array|stdClass $row Database row
 668          * @return string
 669          */
 670         abstract function formatRow( $row );
 671
 672         /**
 673          * This function should be overridden to provide all parameters
 674          * needed for the main paged query. It returns an associative
 675          * array with the following elements:
 676          *    tables => Table(s) for passing to Database::select()
 677          *    fields => Field(s) for passing to Database::select(), may be *
 678          *    conds => WHERE conditions
 679          *    options => option array
 680          *    join_conds => JOIN conditions
 681          *
 682          * @return array
 683          */
 684         abstract function getQueryInfo();
 685
 686         /**
 687          * This function should be overridden to return the name of the index fi-
 688          * eld.  If the pager supports multiple orders, it may return an array of
 689          * 'querykey' => 'indexfield' pairs, so that a request with &count=querykey
 690          * will use indexfield to sort.  In this case, the first returned key is
 691          * the default.
 692          *
 693          * Needless to say, it's really not a good idea to use a non-unique index
 694          * for this!  That won't page right.
 695          *
 696          * @return string|array
 697          */
 698         abstract function getIndexField();
 699
 700         /**
 701          * This function should be overridden to return the names of secondary columns
 702          * to order by in addition to the column in getIndexField(). These fields will
 703          * not be used in the pager offset or in any links for users.
 704          *
 705          * If getIndexField() returns an array of 'querykey' => 'indexfield' pairs then
 706          * this must return a corresponding array of 'querykey' => array( fields...) pairs
 707          * in order for a request with &count=querykey to use array( fields...) to sort.
 708          *
 709          * This is useful for pagers that GROUP BY a unique column (say page_id)
 710          * and ORDER BY another (say page_len). Using GROUP BY and ORDER BY both on
 711          * page_len,page_id avoids temp tables (given a page_len index). This would
 712          * also work if page_id was non-unique but we had a page_len,page_id index.
 713          *
 714          * @return array
 715          */
 716         protected function getExtraSortFields() {
 717                 return array();
 718         }
 719
 720         /**
 721          * Return the default sorting direction: DIR_ASCENDING or DIR_DESCENDING.
 722          * You can also have an associative array of ordertype => dir,
 723          * if multiple order types are supported.  In this case getIndexField()
 724          * must return an array, and the keys of that must exactly match the keys
 725          * of this.
 726          *
 727          * For backward compatibility, this method's return value will be ignored
 728          * if $this->mDefaultDirection is already set when the constructor is
 729          * called, for instance if it's statically initialized.  In that case the
 730          * value of that variable (which must be a boolean) will be used.
 731          *
 732          * Note that despite its name, this does not return the value of the
 733          * $this->mDefaultDirection member variable.  That's the default for this
 734          * particular instantiation, which is a single value.  This is the set of
 735          * all defaults for the class.
 736          *
 737          * @return bool
 738          */
 739         protected function getDefaultDirections() {
 740                 return IndexPager::DIR_ASCENDING;
 741         }
 742 }