In Database.php:

[mediawiki.git] / includes / db / Database.php

<?php
/**
 * @defgroup Database Database
 *
 * @file
 * @ingroup Database
 * This file deals with database interface functions
 * and query specifics/optimisations
 */

/** Number of times to re-try an operation in case of deadlock */
define( 'DEADLOCK_TRIES', 4 );
/** Minimum time to wait before retry, in microseconds */
define( 'DEADLOCK_DELAY_MIN', 500000 );
/** Maximum time to wait before retry */
define( 'DEADLOCK_DELAY_MAX', 1500000 );

/**
 * Base interface for all DBMS-specific code. At a bare minimum, all of the
 * following must be implemented to support MediaWiki
 *
 * @file
 * @ingroup Database
 */
interface DatabaseType {
        /**
         * Get the type of the DBMS, as it appears in $wgDBtype.
         *
         * @return string
         */
        function getType();

        /**
         * Open a connection to the database. Usually aborts on failure
         *
         * @param $server String: database server host
         * @param $user String: database user name
         * @param $password String: database user password
         * @param $dbName String: database name
         * @return bool
         * @throws DBConnectionError
         */
        function open( $server, $user, $password, $dbName );

        /**
         * The DBMS-dependent part of query()
         *
         * @param  $sql String: SQL query.
         * @return Result object to feed to fetchObject, fetchRow, ...; or false on failure
         */
        protected function doQuery( $sql );

        /**
         * Fetch the next row from the given result object, in object form.
         * Fields can be retrieved with $row->fieldname, with fields acting like
         * member variables.
         *
         * @param $res SQL result object as returned from DatabaseBase::query(), etc.
         * @return Row object
         * @throws DBUnexpectedError Thrown if the database returns an error
         */
        function fetchObject( $res );

        /**
         * Fetch the next row from the given result object, in associative array
         * form.  Fields are retrieved with $row['fieldname'].
         *
         * @param $res ResultWrapper result object as returned from DatabaseBase::query(), etc.
         * @return Row object
         * @throws DBUnexpectedError Thrown if the database returns an error
         */
        function fetchRow( $res );

        /**
         * Get the number of rows in a result object
         *
         * @param $res Mixed: A SQL result
         * @return int
         */
        function numRows( $res );

        /**
         * Get the number of fields in a result object
         * @see http://www.php.net/mysql_num_fields
         *
         * @param $res Mixed: A SQL result
         * @return int
         */
        function numFields( $res );

        /**
         * Get a field name in a result object
         * @see http://www.php.net/mysql_field_name
         *
         * @param $res Mixed: A SQL result
         * @param $n Integer
         * @return string
         */
        function fieldName( $res, $n );

        /**
         * Get the inserted value of an auto-increment row
         *
         * The value inserted should be fetched from nextSequenceValue()
         *
         * Example:
         * $id = $dbw->nextSequenceValue('page_page_id_seq');
         * $dbw->insert('page',array('page_id' => $id));
         * $id = $dbw->insertId();
         *
         * @return int
         */
        function insertId();

        /**
         * Change the position of the cursor in a result object
         * @see http://www.php.net/mysql_data_seek
         *
         * @param $res Mixed: A SQL result
         * @param $row Mixed: Either MySQL row or ResultWrapper
         */
        function dataSeek( $res, $row );

        /**
         * Get the last error number
         * @see http://www.php.net/mysql_errno
         *
         * @return int
         */
        function lastErrno();

        /**
         * Get a description of the last error
         * @see http://www.php.net/mysql_error
         *
         * @return string
         */
        function lastError();

        /**
         * mysql_fetch_field() wrapper
         * Returns false if the field doesn't exist
         *
         * @param $table string: table name
         * @param $field string: field name
         *
         * @return Field
         */
        function fieldInfo( $table, $field );

        /**
         * Get information about an index into an object
         * @param $table string: Table name
         * @param $index string: Index name
         * @param $fname string: Calling function name
         * @return Mixed: Database-specific index description class or false if the index does not exist
         */
        function indexInfo( $table, $index, $fname = 'Database::indexInfo' );

        /**
         * Get the number of rows affected by the last write query
         * @see http://www.php.net/mysql_affected_rows
         *
         * @return int
         */
        function affectedRows();

        /**
         * Wrapper for addslashes()
         *
         * @param $s string: to be slashed.
         * @return string: slashed string.
         */
        function strencode( $s );

        /**
         * Returns a wikitext link to the DB's website, e.g.,
         *     return "[http://www.mysql.com/ MySQL]";
         * Should at least contain plain text, if for some reason
         * your database has no website.
         *
         * @return string: wikitext of a link to the server software's web site
         */
        static function getSoftwareLink();

        /**
         * A string describing the current software version, like from
         * mysql_get_server_info().
         *
         * @return string: Version information from the database server.
         */
        function getServerVersion();

        /**
         * A string describing the current software version, and possibly
         * other details in a user-friendly way.  Will be listed on Special:Version, etc.
         * Use getServerVersion() to get machine-friendly information.
         *
         * @return string: Version information from the database server
         */
        function getServerInfo();
}

/**
 * Database abstraction object
 * @ingroup Database
 */
abstract class DatabaseBase implements DatabaseType {

# ------------------------------------------------------------------------------
# Variables
# ------------------------------------------------------------------------------

        protected $mLastQuery = '';
        protected $mDoneWrites = false;
        protected $mPHPError = false;

        protected $mServer, $mUser, $mPassword, $mDBname;

        /**
         * @var DatabaseBase
         */
        protected $mConn = null;
        protected $mOpened = false;

        protected $mTablePrefix;
        protected $mFlags;
        protected $mTrxLevel = 0;
        protected $mErrorCount = 0;
        protected $mLBInfo = array();
        protected $mFakeSlaveLag = null, $mFakeMaster = false;
        protected $mDefaultBigSelects = null;
        protected $mSchemaVars = false;

# ------------------------------------------------------------------------------
# Accessors
# ------------------------------------------------------------------------------
        # These optionally set a variable and return the previous state

        /**
         * A string describing the current software version, and possibly
         * other details in a user-friendly way.  Will be listed on Special:Version, etc.
         * Use getServerVersion() to get machine-friendly information.
         *
         * @return string: Version information from the database server
         */
        public function getServerInfo() {
                return $this->getServerVersion();
        }

        /**
         * Boolean, controls output of large amounts of debug information. 
         * @param $debug:
         *   - true to enable debugging
         *   - false to disable debugging
         *   - omitted or null to do nothing
         *
         * @return The previous value of the flag
         */
        function debug( $debug = null ) {
                return wfSetBit( $this->mFlags, DBO_DEBUG, $debug );
        }

        /**
         * Turns buffering of SQL result sets on (true) or off (false). Default is 
         * "on". 
         *
         * Unbuffered queries are very troublesome in MySQL:
         *
         *   - If another query is executed while the first query is being read 
         *     out, the first query is killed. This means you can't call normal
         *     MediaWiki functions while you are reading an unbuffered query result
         *     from a normal wfGetDB() connection.
         *
         *   - Unbuffered queries cause the MySQL server to use large amounts of 
         *     memory and to hold broad locks which block other queries.
         *
         * If you want to limit client-side memory, it's almost always better to 
         * split up queries into batches using a LIMIT clause than to switch off
         * buffering.
         *
         * @return The previous value of the flag
         */
        function bufferResults( $buffer = null ) {
                if ( is_null( $buffer ) ) {
                        return !(bool)( $this->mFlags & DBO_NOBUFFER );
                } else {
                        return !wfSetBit( $this->mFlags, DBO_NOBUFFER, !$buffer );
                }
        }

        /**
         * Turns on (false) or off (true) the automatic generation and sending
         * of a "we're sorry, but there has been a database error" page on
         * database errors. Default is on (false). When turned off, the
         * code should use lastErrno() and lastError() to handle the
         * situation as appropriate.
         *
         * @return The previous value of the flag.
         */
        function ignoreErrors( $ignoreErrors = null ) {
                return wfSetBit( $this->mFlags, DBO_IGNORE, $ignoreErrors );
        }

        /**
         * Gets or sets the current transaction level.
         *
         * Historically, transactions were allowed to be "nested". This is no 
         * longer supported, so this function really only returns a boolean.
         *
         * @param $level An integer (0 or 1), or omitted to leave it unchanged.
         * @return The previous value
         */
        function trxLevel( $level = null ) {
                return wfSetVar( $this->mTrxLevel, $level );
        }

        /**
         * Get/set the number of errors logged. Only useful when errors are ignored
         * @param $count The count to set, or omitted to leave it unchanged.
         * @return The error count
         */
        function errorCount( $count = null ) {
                return wfSetVar( $this->mErrorCount, $count );
        }

        /**
         * Get/set the table prefix.
         * @param $prefix The table prefix to set, or omitted to leave it unchanged.
         * @return The previous table prefix.
         */
        function tablePrefix( $prefix = null ) {
                return wfSetVar( $this->mTablePrefix, $prefix, true );
        }

        /**
         * Get properties passed down from the server info array of the load 
         * balancer.
         *
         * @param $name The entry of the info array to get, or null to get the 
         *   whole array
         */
        function getLBInfo( $name = null ) {
                if ( is_null( $name ) ) {
                        return $this->mLBInfo;
                } else {
                        if ( array_key_exists( $name, $this->mLBInfo ) ) {
                                return $this->mLBInfo[$name];
                        } else {
                                return null;
                        }
                }
        }

        /**
         * Set the LB info array, or a member of it. If called with one parameter, 
         * the LB info array is set to that parameter. If it is called with two 
         * parameters, the member with the given name is set to the given value.
         *
         * @param $name
         * @param $value
         * @return void
         */
        function setLBInfo( $name, $value = null ) {
                if ( is_null( $value ) ) {
                        $this->mLBInfo = $name;
                } else {
                        $this->mLBInfo[$name] = $value;
                }
        }

        /**
         * Set lag time in seconds for a fake slave
         */
        function setFakeSlaveLag( $lag ) {
                $this->mFakeSlaveLag = $lag;
        }

        /**
         * Make this connection a fake master
         */
        function setFakeMaster( $enabled = true ) {
                $this->mFakeMaster = $enabled;
        }

        /**
         * Returns true if this database supports (and uses) cascading deletes
         */
        function cascadingDeletes() {
                return false;
        }

        /**
         * Returns true if this database supports (and uses) triggers (e.g. on the page table)
         */
        function cleanupTriggers() {
                return false;
        }

        /**
         * Returns true if this database is strict about what can be put into an IP field.
         * Specifically, it uses a NULL value instead of an empty string.
         */
        function strictIPs() {
                return false;
        }

        /**
         * Returns true if this database uses timestamps rather than integers
        */
        function realTimestamps() {
                return false;
        }

        /**
         * Returns true if this database does an implicit sort when doing GROUP BY
         */
        function implicitGroupby() {
                return true;
        }

        /**
         * Returns true if this database does an implicit order by when the column has an index
         * For example: SELECT page_title FROM page LIMIT 1
         */
        function implicitOrderby() {
                return true;
        }

        /**
         * Returns true if this database requires that SELECT DISTINCT queries require that all
           ORDER BY expressions occur in the SELECT list per the SQL92 standard
         */
        function standardSelectDistinct() {
                return true;
        }

        /**
         * Returns true if this database can do a native search on IP columns
         * e.g. this works as expected: .. WHERE rc_ip = '127.42.12.102/32';
         */
        function searchableIPs() {
                return false;
        }

        /**
         * Returns true if this database can use functional indexes
         */
        function functionalIndexes() {
                return false;
        }

        /**
         * Return the last query that went through DatabaseBase::query()
         * @return String
         */
        function lastQuery() {
                return $this->mLastQuery;
        }


        /**
         * Returns true if the connection may have been used for write queries.
         * Should return true if unsure.
         *
         * @return bool
         */
        function doneWrites() {
                return $this->mDoneWrites;
        }

        /**
         * Is a connection to the database open?
         * @return Boolean
         */
        function isOpen() {
                return $this->mOpened;
        }

        /**
         * Set a flag for this connection
         *
         * @param $flag Integer: DBO_* constants from Defines.php:
         *   - DBO_DEBUG: output some debug info (same as debug())
         *   - DBO_NOBUFFER: don't buffer results (inverse of bufferResults())
         *   - DBO_IGNORE: ignore errors (same as ignoreErrors())
         *   - DBO_TRX: automatically start transactions
         *   - DBO_DEFAULT: automatically sets DBO_TRX if not in command line mode
         *       and removes it in command line mode
         *   - DBO_PERSISTENT: use persistant database connection
         */
        function setFlag( $flag ) {
                $this->mFlags |= $flag;
        }

        /**
         * Clear a flag for this connection
         *
         * @param $flag: same as setFlag()'s $flag param
         */
        function clearFlag( $flag ) {
                $this->mFlags &= ~$flag;
        }

        /**
         * Returns a boolean whether the flag $flag is set for this connection
         *
         * @param $flag: same as setFlag()'s $flag param
         * @return Boolean
         */
        function getFlag( $flag ) {
                return !!( $this->mFlags & $flag );
        }

        /**
         * General read-only accessor
         */
        function getProperty( $name ) {
                return $this->$name;
        }

        /**
         * @return string
         */
        function getWikiID() {
                if ( $this->mTablePrefix ) {
                        return "{$this->mDBname}-{$this->mTablePrefix}";
                } else {
                        return $this->mDBname;
                }
        }

        /**
         * Return a path to the DBMS-specific schema, otherwise default to tables.sql
         *
         * @return string
         */
        public function getSchema() {
                global $IP;
                if ( file_exists( "$IP/maintenance/" . $this->getType() . "/tables.sql" ) ) {
                        return "$IP/maintenance/" . $this->getType() . "/tables.sql";
                } else {
                        return "$IP/maintenance/tables.sql";
                }
        }

# ------------------------------------------------------------------------------
# Other functions
# ------------------------------------------------------------------------------

        /**
         * Constructor.
         * @param $server String: database server host
         * @param $user String: database user name
         * @param $password String: database user password
         * @param $dbName String: database name
         * @param $flags
         * @param $tablePrefix String: database table prefixes. By default use the prefix gave in LocalSettings.php
         */
        function __construct( $server = false, $user = false, $password = false, $dbName = false,
                $flags = 0, $tablePrefix = 'get from global'
        ) {
                global $wgDBprefix, $wgCommandLineMode;

                $this->mFlags = $flags;

                if ( $this->mFlags & DBO_DEFAULT ) {
                        if ( $wgCommandLineMode ) {
                                $this->mFlags &= ~DBO_TRX;
                        } else {
                                $this->mFlags |= DBO_TRX;
                        }
                }

                /** Get the default table prefix*/
                if ( $tablePrefix == 'get from global' ) {
                        $this->mTablePrefix = $wgDBprefix;
                } else {
                        $this->mTablePrefix = $tablePrefix;
                }

                if ( $user ) {
                        $this->open( $server, $user, $password, $dbName );
                }
        }

        /**
         * Same as new DatabaseMysql( ... ), kept for backward compatibility
         * @deprecated since 1.17
         *
         * @return DatabaseMysql
         */
        static function newFromParams( $server, $user, $password, $dbName, $flags = 0 ) {
                wfDeprecated( __METHOD__ );
                return new DatabaseMysql( $server, $user, $password, $dbName, $flags );
        }

        /**
         * Given a DB type, construct the name of the appropriate child class of
         * DatabaseBase. This is designed to replace all of the manual stuff like:
         *      $class = 'Database' . ucfirst( strtolower( $type ) );
         * as well as validate against the canonical list of DB types we have
         *
         * This factory function is mostly useful for when you need to connect to a
         * database other than the MediaWiki default (such as for external auth,
         * an extension, et cetera). Do not use this to connect to the MediaWiki
         * database. Example uses in core:
         * @see LoadBalancer::reallyOpenConnection()
         * @see ExternalUser_MediaWiki::initFromCond()
         * @see ForeignDBRepo::getMasterDB()
         * @see WebInstaller_DBConnect::execute()
         *
         * @param $dbType String A possible DB type
         * @param $p Array An array of options to pass to the constructor.
         *    Valid options are: host, user, password, dbname, flags, tableprefix
         * @return DatabaseBase subclass or null
         */
        public final static function newFromType( $dbType, $p = array() ) {
                $canonicalDBTypes = array(
                        'mysql', 'postgres', 'sqlite', 'oracle', 'mssql', 'ibm_db2'
                );
                $dbType = strtolower( $dbType );

                if( in_array( $dbType, $canonicalDBTypes ) ) {
                        $class = 'Database' . ucfirst( $dbType );
                        return new $class(
                                isset( $p['host'] ) ? $p['host'] : false,
                                isset( $p['user'] ) ? $p['user'] : false,
                                isset( $p['password'] ) ? $p['password'] : false,
                                isset( $p['dbname'] ) ? $p['dbname'] : false,
                                isset( $p['flags'] ) ? $p['flags'] : 0,
                                isset( $p['tableprefix'] ) ? $p['tableprefix'] : 'get from global'
                        );
                } else {
                        return null;
                }
        }

        protected function installErrorHandler() {
                $this->mPHPError = false;
                $this->htmlErrors = ini_set( 'html_errors', '0' );
                set_error_handler( array( $this, 'connectionErrorHandler' ) );
        }

        protected function restoreErrorHandler() {
                restore_error_handler();
                if ( $this->htmlErrors !== false ) {
                        ini_set( 'html_errors', $this->htmlErrors );
                }
                if ( $this->mPHPError ) {
                        $error = preg_replace( '!\[<a.*</a>\]!', '', $this->mPHPError );
                        $error = preg_replace( '!^.*?:(.*)$!', '$1', $error );
                        return $error;
                } else {
                        return false;
                }
        }

        protected function connectionErrorHandler( $errno,  $errstr ) {
                $this->mPHPError = $errstr;
        }

        /**
         * Closes a database connection.
         * if it is open : commits any open transactions
         *
         * @return Bool operation success. true if already closed.
         */
        function close() {
                # Stub, should probably be overridden
                return true;
        }

        /**
         * @param $error String: fallback error message, used if none is given by DB
         */
        function reportConnectionError( $error = 'Unknown error' ) {
                $myError = $this->lastError();
                if ( $myError ) {
                        $error = $myError;
                }

                # New method
                throw new DBConnectionError( $this, $error );
        }

        /**
         * Determine whether a query writes to the DB.
         * Should return true if unsure.
         *
         * @return bool
         */
        function isWriteQuery( $sql ) {
                return !preg_match( '/^(?:SELECT|BEGIN|COMMIT|SET|SHOW|\(SELECT)\b/i', $sql );
        }

        /**
         * Run an SQL query and return the result. Normally throws a DBQueryError 
         * on failure. If errors are ignored, returns false instead.
         *
         * In new code, the query wrappers select(), insert(), update(), delete(), 
         * etc. should be used where possible, since they give much better DBMS 
         * independence and automatically quote or validate user input in a variety
         * of contexts. This function is generally only useful for queries which are
         * explicitly DBMS-dependent and are unsupported by the query wrappers, such 
         * as CREATE TABLE.
         *
         * However, the query wrappers themselves should call this function.
         *
         * @param  $sql        String: SQL query
         * @param  $fname      String: Name of the calling function, for profiling/SHOW PROCESSLIST
         *     comment (you can use __METHOD__ or add some extra info)
         * @param  $tempIgnore Boolean:   Whether to avoid throwing an exception on errors...
         *     maybe best to catch the exception instead?
         * @return boolean|ResultWrapper. true for a successful write query, ResultWrapper object 
         *     for a successful read query, or false on failure if $tempIgnore set
         * @throws DBQueryError Thrown when the database returns an error of any kind
         */
        public function query( $sql, $fname = '', $tempIgnore = false ) {
                $isMaster = !is_null( $this->getLBInfo( 'master' ) );
                if ( !Profiler::instance()->isStub() ) {
                        # generalizeSQL will probably cut down the query to reasonable
                        # logging size most of the time. The substr is really just a sanity check.

                        if ( $isMaster ) {
                                $queryProf = 'query-m: ' . substr( DatabaseBase::generalizeSQL( $sql ), 0, 255 );
                                $totalProf = 'DatabaseBase::query-master';
                        } else {
                                $queryProf = 'query: ' . substr( DatabaseBase::generalizeSQL( $sql ), 0, 255 );
                                $totalProf = 'DatabaseBase::query';
                        }

                        wfProfileIn( $totalProf );
                        wfProfileIn( $queryProf );
                }

                $this->mLastQuery = $sql;
                if ( !$this->mDoneWrites && $this->isWriteQuery( $sql ) ) {
                        # Set a flag indicating that writes have been done
                        wfDebug( __METHOD__ . ": Writes done: $sql\n" );
                        $this->mDoneWrites = true;
                }

                # Add a comment for easy SHOW PROCESSLIST interpretation
                global $wgUser;
                if ( is_object( $wgUser ) && $wgUser->isItemLoaded( 'name' ) ) {
                        $userName = $wgUser->getName();
                        if ( mb_strlen( $userName ) > 15 ) {
                                $userName = mb_substr( $userName, 0, 15 ) . '...';
                        }
                        $userName = str_replace( '/', '', $userName );
                } else {
                        $userName = '';
                }
                $commentedSql = preg_replace( '/\s/', " /* $fname $userName */ ", $sql, 1 );

                # If DBO_TRX is set, start a transaction
                if ( ( $this->mFlags & DBO_TRX ) && !$this->trxLevel() &&
                        $sql != 'BEGIN' && $sql != 'COMMIT' && $sql != 'ROLLBACK' ) {
                        # avoid establishing transactions for SHOW and SET statements too -
                        # that would delay transaction initializations to once connection
                        # is really used by application
                        $sqlstart = substr( $sql, 0, 10 ); // very much worth it, benchmark certified(tm)
                        if ( strpos( $sqlstart, "SHOW " ) !== 0 and strpos( $sqlstart, "SET " ) !== 0 )
                                $this->begin();
                }

                if ( $this->debug() ) {
                        static $cnt = 0;

                        $cnt++;
                        $sqlx = substr( $commentedSql, 0, 500 );
                        $sqlx = strtr( $sqlx, "\t\n", '  ' );

                        if ( $isMaster ) {
                                wfDebug( "Query $cnt (master): $sqlx\n" );
                        } else {
                                wfDebug( "Query $cnt (slave): $sqlx\n" );
                        }
                }

                if ( istainted( $sql ) & TC_MYSQL ) {
                        throw new MWException( 'Tainted query found' );
                }

                # Do the query and handle errors
                $ret = $this->doQuery( $commentedSql );

                # Try reconnecting if the connection was lost
                if ( false === $ret && $this->wasErrorReissuable() ) {
                        # Transaction is gone, like it or not
                        $this->mTrxLevel = 0;
                        wfDebug( "Connection lost, reconnecting...\n" );

                        if ( $this->ping() ) {
                                wfDebug( "Reconnected\n" );
                                $sqlx = substr( $commentedSql, 0, 500 );
                                $sqlx = strtr( $sqlx, "\t\n", '  ' );
                                global $wgRequestTime;
                                $elapsed = round( microtime( true ) - $wgRequestTime, 3 );
                                if ( $elapsed < 300 ) {
                                        # Not a database error to lose a transaction after a minute or two
                                        wfLogDBError( "Connection lost and reconnected after {$elapsed}s, query: $sqlx\n" );
                                }
                                $ret = $this->doQuery( $commentedSql );
                        } else {
                                wfDebug( "Failed\n" );
                        }
                }

                if ( false === $ret ) {
                        $this->reportQueryError( $this->lastError(), $this->lastErrno(), $sql, $fname, $tempIgnore );
                }

                if ( !Profiler::instance()->isStub() ) {
                        wfProfileOut( $queryProf );
                        wfProfileOut( $totalProf );
                }

                return $this->resultObject( $ret );
        }

        /**
         * Report a query error. Log the error, and if neither the object ignore 
         * flag nor the $tempIgnore flag is set, throw a DBQueryError.
         *
         * @param $error String
         * @param $errno Integer
         * @param $sql String
         * @param $fname String
         * @param $tempIgnore Boolean
         */
        function reportQueryError( $error, $errno, $sql, $fname, $tempIgnore = false ) {
                # Ignore errors during error handling to avoid infinite recursion
                $ignore = $this->ignoreErrors( true );
                ++$this->mErrorCount;

                if ( $ignore || $tempIgnore ) {
                        wfDebug( "SQL ERROR (ignored): $error\n" );
                        $this->ignoreErrors( $ignore );
                } else {
                        $sql1line = str_replace( "\n", "\\n", $sql );
                        wfLogDBError( "$fname\t{$this->mServer}\t$errno\t$error\t$sql1line\n" );
                        wfDebug( "SQL ERROR: " . $error . "\n" );
                        throw new DBQueryError( $this, $error, $errno, $sql, $fname );
                }
        }

        /**
         * Intended to be compatible with the PEAR::DB wrapper functions.
         * http://pear.php.net/manual/en/package.database.db.intro-execute.php
         *
         * ? = scalar value, quoted as necessary
         * ! = raw SQL bit (a function for instance)
         * & = filename; reads the file and inserts as a blob
         *     (we don't use this though...)
         *
         * This function should not be used directly by new code outside of the 
         * database classes. The query wrapper functions (select() etc.) should be
         * used instead.
         *
         * @return array
         */
        function prepare( $sql, $func = 'DatabaseBase::prepare' ) {
                /* MySQL doesn't support prepared statements (yet), so just
                   pack up the query for reference. We'll manually replace
                   the bits later. */
                return array( 'query' => $sql, 'func' => $func );
        }

        /**
         * Free a prepared query, generated by prepare().
         */
        function freePrepared( $prepared ) {
                /* No-op by default */
        }

        /**
         * Execute a prepared query with the various arguments
         * @param $prepared String: the prepared sql
         * @param $args Mixed: Either an array here, or put scalars as varargs
         *
         * @return ResultWrapper
         */
        function execute( $prepared, $args = null ) {
                if ( !is_array( $args ) ) {
                        # Pull the var args
                        $args = func_get_args();
                        array_shift( $args );
                }

                $sql = $this->fillPrepared( $prepared['query'], $args );

                return $this->query( $sql, $prepared['func'] );
        }

        /**
         * Prepare & execute an SQL statement, quoting and inserting arguments
         * in the appropriate places.
         *
         * This function should not be used directly by new code outside of the 
         * database classes. The query wrapper functions (select() etc.) should be
         * used instead.
         *
         * @param $query String
         * @param $args ...
         *
         * @return ResultWrapper
         */
        function safeQuery( $query, $args = null ) {
                $prepared = $this->prepare( $query, 'DatabaseBase::safeQuery' );

                if ( !is_array( $args ) ) {
                        # Pull the var args
                        $args = func_get_args();
                        array_shift( $args );
                }

                $retval = $this->execute( $prepared, $args );
                $this->freePrepared( $prepared );

                return $retval;
        }

        /**
         * For faking prepared SQL statements on DBs that don't support
         * it directly.
         * @param $preparedQuery String: a 'preparable' SQL statement
         * @param $args Array of arguments to fill it with
         * @return string executable SQL
         */
        function fillPrepared( $preparedQuery, $args ) {
                reset( $args );
                $this->preparedArgs =& $args;

                return preg_replace_callback( '/(\\\\[?!&]|[?!&])/',
                        array( &$this, 'fillPreparedArg' ), $preparedQuery );
        }

        /**
         * preg_callback func for fillPrepared()
         * The arguments should be in $this->preparedArgs and must not be touched
         * while we're doing this.
         *
         * @param $matches Array
         * @return String
         * @private
         */
        function fillPreparedArg( $matches ) {
                switch( $matches[1] ) {
                        case '\\?': return '?';
                        case '\\!': return '!';
                        case '\\&': return '&';
                }

                list( /* $n */ , $arg ) = each( $this->preparedArgs );

                switch( $matches[1] ) {
                        case '?': return $this->addQuotes( $arg );
                        case '!': return $arg;
                        case '&':
                                # return $this->addQuotes( file_get_contents( $arg ) );
                                throw new DBUnexpectedError( $this, '& mode is not implemented. If it\'s really needed, uncomment the line above.' );
                        default:
                                throw new DBUnexpectedError( $this, 'Received invalid match. This should never happen!' );
                }
        }

        /**
         * Free a result object returned by query() or select(). It's usually not 
         * necessary to call this, just use unset() or let the variable holding 
         * the result object go out of scope.
         *
         * @param $res Mixed: A SQL result
         */
        function freeResult( $res ) {
        }

        /**
         * Simple UPDATE wrapper.
         * Usually throws a DBQueryError on failure.
         * If errors are explicitly ignored, returns success
         *
         * This function exists for historical reasons, DatabaseBase::update() has a more standard
         * calling convention and feature set
         *
         * @return bool
         */
        function set( $table, $var, $value, $cond, $fname = 'DatabaseBase::set' ) {
                $table = $this->tableName( $table );
                $sql = "UPDATE $table SET $var = '" .
                  $this->strencode( $value ) . "' WHERE ($cond)";

                return (bool)$this->query( $sql, $fname );
        }

        /**
         * A SELECT wrapper which returns a single field from a single result row.
         *
         * Usually throws a DBQueryError on failure. If errors are explicitly 
         * ignored, returns false on failure.
         *
         * If no result rows are returned from the query, false is returned.
         *
         * @param $table Table name. See DatabaseBase::select() for details.
         * @param $var The field name to select. This must be a valid SQL
         *   fragment: do not use unvalidated user input.
         * @param $cond The condition array. See DatabaseBase::select() for details.
         * @param $fname The function name of the caller.
         * @param $options The query options. See DatabaseBase::select() for details.
         *
         * @return The value from the field, or false on failure.
         */
        function selectField( $table, $var, $cond = '', $fname = 'DatabaseBase::selectField', 
                $options = array() ) 
        {
                if ( !is_array( $options ) ) {
                        $options = array( $options );
                }

                $options['LIMIT'] = 1;

                $res = $this->select( $table, $var, $cond, $fname, $options );

                if ( $res === false || !$this->numRows( $res ) ) {
                        return false;
                }

                $row = $this->fetchRow( $res );

                if ( $row !== false ) {
                        return reset( $row );
                } else {
                        return false;
                }
        }

        /**
         * Returns an optional USE INDEX clause to go after the table, and a
         * string to go at the end of the query. 
         *
         * @param $options Array: associative array of options to be turned into
         *              an SQL query, valid keys are listed in the function.
         * @return Array
         * @see DatabaseBase::select()
         */
        protected function makeSelectOptions( $options ) {
                $preLimitTail = $postLimitTail = '';
                $startOpts = '';

                $noKeyOptions = array();

                foreach ( $options as $key => $option ) {
                        if ( is_numeric( $key ) ) {
                                $noKeyOptions[$option] = true;
                        }
                }

                if ( isset( $options['GROUP BY'] ) ) {
                        $gb = is_array( $options['GROUP BY'] )
                                ? implode( ',', $options['GROUP BY'] )
                                : $options['GROUP BY'];
                        $preLimitTail .= " GROUP BY {$gb}";
                }

                if ( isset( $options['HAVING'] ) ) {
                        $preLimitTail .= " HAVING {$options['HAVING']}";
                }

                if ( isset( $options['ORDER BY'] ) ) {
                        $ob = is_array( $options['ORDER BY'] )
                                ? implode( ',', $options['ORDER BY'] )
                                : $options['ORDER BY'];
                        $preLimitTail .= " ORDER BY {$ob}";
                }

                // if (isset($options['LIMIT'])) {
                //      $tailOpts .= $this->limitResult('', $options['LIMIT'],
                //              isset($options['OFFSET']) ? $options['OFFSET']
                //              : false);
                // }

                if ( isset( $noKeyOptions['FOR UPDATE'] ) ) {
                        $postLimitTail .= ' FOR UPDATE';
                }

                if ( isset( $noKeyOptions['LOCK IN SHARE MODE'] ) ) {
                        $postLimitTail .= ' LOCK IN SHARE MODE';
                }

                if ( isset( $noKeyOptions['DISTINCT'] ) || isset( $noKeyOptions['DISTINCTROW'] ) ) {
                        $startOpts .= 'DISTINCT';
                }

                # Various MySQL extensions
                if ( isset( $noKeyOptions['STRAIGHT_JOIN'] ) ) {
                        $startOpts .= ' /*! STRAIGHT_JOIN */';
                }

                if ( isset( $noKeyOptions['HIGH_PRIORITY'] ) ) {
                        $startOpts .= ' HIGH_PRIORITY';
                }

                if ( isset( $noKeyOptions['SQL_BIG_RESULT'] ) ) {
                        $startOpts .= ' SQL_BIG_RESULT';
                }

                if ( isset( $noKeyOptions['SQL_BUFFER_RESULT'] ) ) {
                        $startOpts .= ' SQL_BUFFER_RESULT';
                }

                if ( isset( $noKeyOptions['SQL_SMALL_RESULT'] ) ) {
                        $startOpts .= ' SQL_SMALL_RESULT';
                }

                if ( isset( $noKeyOptions['SQL_CALC_FOUND_ROWS'] ) ) {
                        $startOpts .= ' SQL_CALC_FOUND_ROWS';
                }

                if ( isset( $noKeyOptions['SQL_CACHE'] ) ) {
                        $startOpts .= ' SQL_CACHE';
                }

                if ( isset( $noKeyOptions['SQL_NO_CACHE'] ) ) {
                        $startOpts .= ' SQL_NO_CACHE';
                }

                if ( isset( $options['USE INDEX'] ) && ! is_array( $options['USE INDEX'] ) ) {
                        $useIndex = $this->useIndexClause( $options['USE INDEX'] );
                } else {
                        $useIndex = '';
                }

                return array( $startOpts, $useIndex, $preLimitTail, $postLimitTail );
        }

        /**
         * Execute a SELECT query constructed using the various parameters provided.
         * See below for full details of the parameters.
         *
         * @param $table Table name
         * @param $vars Field names
         * @param $conds Conditions
         * @param $fname Caller function name
         * @param $options Query options
         * @param $join_conds Join conditions
         *
         *
         * @b    $table
         *
         * May be either an array of table names, or a single string holding a table 
         * name. If an array is given, table aliases can be specified, for example: 
         *
         *    array( 'a' => 'user' )
         *
         * This includes the user table in the query, with the alias "a" available 
         * for use in field names (e.g. a.user_name).
         *
         * All of the table names given here are automatically run through 
         * DatabaseBase::tableName(), which causes the table prefix (if any) to be 
         * added, and various other table name mappings to be performed.
         *
         *
         * @b    $vars
         *
         * May be either a field name or an array of field names. The field names 
         * here are complete fragments of SQL, for direct inclusion into the SELECT 
         * query. Expressions and aliases may be specified as in SQL, for example:
         *
         *   array( 'MAX(rev_id) AS maxrev' )
         * 
         * If an expression is given, care must be taken to ensure that it is 
         * DBMS-independent.
         *
         *
         * @b    $conds
         *
         * May be either a string containing a single condition, or an array of 
         * conditions. If an array is given, the conditions constructed from each 
         * element are combined with AND. 
         *
         * Array elements may take one of two forms:
         *
         *   - Elements with a numeric key are interpreted as raw SQL fragments.
         *   - Elements with a string key are interpreted as equality conditions, 
         *     where the key is the field name.
         *     - If the value of such an array element is a scalar (such as a 
         *       string), it will be treated as data and thus quoted appropriately. 
         *       If it is null, an IS NULL clause will be added.
         *     - If the value is an array, an IN(...) clause will be constructed, 
         *       such that the field name may match any of the elements in the 
         *       array. The elements of the array will be quoted.
         *     - If the field name ends with "!", this is taken as a flag which 
         *       inverts the comparison, allowing NOT IN clauses to be constructed, 
         *       for example: array( 'user_id!' => array( 1, 2, 3 ) )
         *
         * Note that expressions are often DBMS-dependent in their syntax. 
         * DBMS-independent wrappers are provided for constructing several types of 
         * expression commonly used in condition queries. See:
         *    - DatabaseBase::buildLike()
         *    - DatabaseBase::conditional()
         *
         *
         * @b    $options
         *
         * Optional: Array of query options. Boolean options are specified by 
         * including them in the array as a string value with a numeric key, for 
         * example:
         *
         *    array( 'FOR UPDATE' )
         *
         * The supported options are:
         *
         *   - OFFSET: Skip this many rows at the start of the result set. OFFSET 
         *     with LIMIT can theoretically be used for paging through a result set,
         *     but this is discouraged in MediaWiki for performance reasons. 
         *
         *   - LIMIT: Integer: return at most this many rows. The rows are sorted 
         *     and then the first rows are taken until the limit is reached. LIMIT
         *     is applied to a result set after OFFSET.
         *
         *   - FOR UPDATE: Boolean: lock the returned rows so that they can't be 
         *     changed until the next COMMIT.
         *
         *   - DISTINCT: Boolean: return only unique result rows.
         *
         *   - GROUP BY: May be either an SQL fragment string naming a field or 
         *     expression to group by, or an array of such SQL fragments.
         *
         *   - HAVING: A string containing a HAVING clause. 
         *
         *   - ORDER BY: May be either an SQL fragment giving a field name or 
         *     expression to order by, or an array of such SQL fragments.
         *
         *   - USE INDEX: This may be either a string giving the index name to use 
         *     for the query, or an array. If it is an associative array, each key 
         *     gives the table name (or alias), each value gives the index name to
         *     use for that table. All strings are SQL fragments and so should be 
         *     validated by the caller.
         *
         *   - EXPLAIN: In MySQL, this causes an EXPLAIN SELECT query to be run, 
         *     instead of SELECT.
         *
         * And also the following boolean MySQL extensions, see the MySQL manual 
         * for documentation:
         *
         *    - LOCK IN SHARE MODE
         *    - STRAIGHT_JOIN
         *    - HIGH_PRIORITY
         *    - SQL_BIG_RESULT
         *    - SQL_BUFFER_RESULT
         *    - SQL_SMALL_RESULT
         *    - SQL_CALC_FOUND_ROWS
         *    - SQL_CACHE
         *    - SQL_NO_CACHE
         *
         *
         * @b    $join_conds
         *
         * Optional associative array of table-specific join conditions. In the 
         * most common case, this is unnecessary, since the join condition can be 
         * in $conds. However, it is useful for doing a LEFT JOIN.
         *
         * The key of the array contains the table name or alias. The value is an 
         * array with two elements, numbered 0 and 1. The first gives the type of 
         * join, the second is an SQL fragment giving the join condition for that 
         * table. For example:
         *
         *    array( 'page' => array('LEFT JOIN','page_latest=rev_id') )
         *
         * @return ResultWrapper. If the query returned no rows, a ResultWrapper
         *   with no rows in it will be returned. If there was a query error, a 
         *   DBQueryError exception will be thrown, except if the "ignore errors"
         *   option was set, in which case false will be returned.
         */
        function select( $table, $vars, $conds = '', $fname = 'DatabaseBase::select', 
                $options = array(), $join_conds = array() ) 
        {
                $sql = $this->selectSQLText( $table, $vars, $conds, $fname, $options, $join_conds );

                return $this->query( $sql, $fname );
        }

        /**
         * The equivalent of DatabaseBase::select() except that the constructed SQL
         * is returned, instead of being immediately executed.
         *
         * @param $table Table name
         * @param $vars Field names
         * @param $conds Conditions
         * @param $fname Caller function name
         * @param $options Query options
         * @param $join_conds Join conditions
         *
         * @return SQL query string.
         * @see DatabaseBase::select()
         */
        function selectSQLText( $table, $vars, $conds = '', $fname = 'DatabaseBase::select', $options = array(), $join_conds = array() ) {
                if ( is_array( $vars ) ) {
                        $vars = implode( ',', $vars );
                }

                if ( !is_array( $options ) ) {
                        $options = array( $options );
                }

                if ( is_array( $table ) ) {
                        if ( !empty( $join_conds ) || ( isset( $options['USE INDEX'] ) && is_array( @$options['USE INDEX'] ) ) ) {
                                $from = ' FROM ' . $this->tableNamesWithUseIndexOrJOIN( $table, @$options['USE INDEX'], $join_conds );
                        } else {
                                $from = ' FROM ' . implode( ',', $this->tableNamesWithAlias( $table ) );
                        }
                } elseif ( $table != '' ) {
                        if ( $table { 0 } == ' ' ) {
                                $from = ' FROM ' . $table;
                        } else {
                                $from = ' FROM ' . $this->tableName( $table );
                        }
                } else {
                        $from = '';
                }

                list( $startOpts, $useIndex, $preLimitTail, $postLimitTail ) = $this->makeSelectOptions( $options );

                if ( !empty( $conds ) ) {
                        if ( is_array( $conds ) ) {
                                $conds = $this->makeList( $conds, LIST_AND );
                        }
                        $sql = "SELECT $startOpts $vars $from $useIndex WHERE $conds $preLimitTail";
                } else {
                        $sql = "SELECT $startOpts $vars $from $useIndex $preLimitTail";
                }

                if ( isset( $options['LIMIT'] ) )
                        $sql = $this->limitResult( $sql, $options['LIMIT'],
                                isset( $options['OFFSET'] ) ? $options['OFFSET'] : false );
                $sql = "$sql $postLimitTail";

                if ( isset( $options['EXPLAIN'] ) ) {
                        $sql = 'EXPLAIN ' . $sql;
                }

                return $sql;
        }

        /**
         * Single row SELECT wrapper. Equivalent to DatabaseBase::select(), except
         * that a single row object is returned. If the query returns no rows, 
         * false is returned.
         *
         * @param $table Table name
         * @param $vars Field names
         * @param $conds Conditions
         * @param $fname Caller function name
         * @param $options Query options
         * @param $join_conds Join conditions
         *
         * @return ResultWrapper or bool
         */
        function selectRow( $table, $vars, $conds, $fname = 'DatabaseBase::selectRow', 
                $options = array(), $join_conds = array() ) 
        {
                $options['LIMIT'] = 1;
                $res = $this->select( $table, $vars, $conds, $fname, $options, $join_conds );

                if ( $res === false ) {
                        return false;
                }

                if ( !$this->numRows( $res ) ) {
                        return false;
                }

                $obj = $this->fetchObject( $res );

                return $obj;
        }

        /**
         * Estimate rows in dataset.
         *
         * MySQL allows you to estimate the number of rows that would be returned 
         * by a SELECT query, using EXPLAIN SELECT. The estimate is provided using
         * index cardinality statistics, and is notoriously inaccurate, especially
         * when large numbers of rows have recently been added or deleted.
         *
         * For DBMSs that don't support fast result size estimation, this function
         * will actually perform the SELECT COUNT(*). 
         *
         * Takes the same arguments as DatabaseBase::select().
         *
         * @param $table String: table name
         * @param $vars Array: unused
         * @param $conds Array: filters on the table
         * @param $fname String: function name for profiling
         * @param $options Array: options for select
         * @return Integer: row count
         */
        public function estimateRowCount( $table, $vars = '*', $conds = '', 
                $fname = 'DatabaseBase::estimateRowCount', $options = array() ) 
        {
                $rows = 0;
                $res = $this->select ( $table, 'COUNT(*) AS rowcount', $conds, $fname, $options );

                if ( $res ) {
                        $row = $this->fetchRow( $res );
                        $rows = ( isset( $row['rowcount'] ) ) ? $row['rowcount'] : 0;
                }

                return $rows;
        }

        /**
         * Removes most variables from an SQL query and replaces them with X or N for numbers.
         * It's only slightly flawed. Don't use for anything important.
         *
         * @param $sql String: A SQL Query
         */
        static function generalizeSQL( $sql ) {
                # This does the same as the regexp below would do, but in such a way
                # as to avoid crashing php on some large strings.
                # $sql = preg_replace ( "/'([^\\\\']|\\\\.)*'|\"([^\\\\\"]|\\\\.)*\"/", "'X'", $sql);

                $sql = str_replace ( "\\\\", '', $sql );
                $sql = str_replace ( "\\'", '', $sql );
                $sql = str_replace ( "\\\"", '', $sql );
                $sql = preg_replace ( "/'.*'/s", "'X'", $sql );
                $sql = preg_replace ( '/".*"/s', "'X'", $sql );

                # All newlines, tabs, etc replaced by single space
                $sql = preg_replace ( '/\s+/', ' ', $sql );

                # All numbers => N
                $sql = preg_replace ( '/-?[0-9]+/s', 'N', $sql );

                return $sql;
        }

        /**
         * Determines whether a field exists in a table
         *
         * @param $table String: table name
         * @param $field String: filed to check on that table
         * @param $fname String: calling function name (optional)
         * @return Boolean: whether $table has filed $field
         */
        function fieldExists( $table, $field, $fname = 'DatabaseBase::fieldExists' ) {
                $info = $this->fieldInfo( $table, $field );

                return (bool)$info;
        }

        /**
         * Determines whether an index exists
         * Usually throws a DBQueryError on failure
         * If errors are explicitly ignored, returns NULL on failure
         *
         * @return bool|null
         */
        function indexExists( $table, $index, $fname = 'DatabaseBase::indexExists' ) {
                $info = $this->indexInfo( $table, $index, $fname );
                if ( is_null( $info ) ) {
                        return null;
                } else {
                        return $info !== false;
                }
        }

        /**
         * Query whether a given table exists
         *
         * @string table
         *
         * @return bool
         */
        function tableExists( $table ) {
                $table = $this->tableName( $table );
                $old = $this->ignoreErrors( true );
                $res = $this->query( "SELECT 1 FROM $table LIMIT 1", __METHOD__ );
                $this->ignoreErrors( $old );

                return (bool)$res;
        }

        /**
         * mysql_field_type() wrapper
         */
        function fieldType( $res, $index ) {
                if ( $res instanceof ResultWrapper ) {
                        $res = $res->result;
                }

                return mysql_field_type( $res, $index );
        }

        /**
         * Determines if a given index is unique
         *
         * @param $table string
         * @param $index string
         *
         * @return bool
         */
        function indexUnique( $table, $index ) {
                $indexInfo = $this->indexInfo( $table, $index );

                if ( !$indexInfo ) {
                        return null;
                }

                return !$indexInfo[0]->Non_unique;
        }

        /**
         * Helper for DatabaseBase::insert().
         *
         * @param $options array
         * @return string
         */
        protected function makeInsertOptions( $options ) {
                return implode( ' ', $options );
        }

        /**
         * INSERT wrapper, inserts an array into a table.
         *
         * $a may be either:
         *
         *   - A single associative array. The array keys are the field names, and
         *     the values are the values to insert. The values are treated as data 
         *     and will be quoted appropriately. If NULL is inserted, this will be 
         *     converted to a database NULL.
         *   - An array with numeric keys, holding a list of associative arrays. 
         *     This causes a multi-row INSERT on DBMSs that support it. The keys in
         *     each subarray must be identical to each other, and in the same order.
         *
         * Usually throws a DBQueryError on failure. If errors are explicitly ignored, 
         * returns success.
         *
         * $options is an array of options, with boolean options encoded as values 
         * with numeric keys, in the same style as $options in 
         * DatabaseBase::select(). Supported options are:
         *
         *   - IGNORE: Boolean: if present, duplicate key errors are ignored, and 
         *     any rows which cause duplicate key errors are not inserted. It's 
         *     possible to determine how many rows were successfully inserted using 
         *     DatabaseBase::affectedRows().
         *
         * @param $table   Table name. This will be passed through 
         *                 DatabaseBase::tableName().
         * @param $a       Array of rows to insert
         * @param $fname   Calling function name (use __METHOD__) for logs/profiling
         * @param $options Array of options
         *
         * @return bool
         */
        function insert( $table, $a, $fname = 'DatabaseBase::insert', $options = array() ) {
                # No rows to insert, easy just return now
                if ( !count( $a ) ) {
                        return true;
                }

                $table = $this->tableName( $table );

                if ( !is_array( $options ) ) {
                        $options = array( $options );
                }

                $options = $this->makeInsertOptions( $options );

                if ( isset( $a[0] ) && is_array( $a[0] ) ) {
                        $multi = true;
                        $keys = array_keys( $a[0] );
                } else {
                        $multi = false;
                        $keys = array_keys( $a );
                }

                $sql = 'INSERT ' . $options .
                        " INTO $table (" . implode( ',', $keys ) . ') VALUES ';

                if ( $multi ) {
                        $first = true;
                        foreach ( $a as $row ) {
                                if ( $first ) {
                                        $first = false;
                                } else {
                                        $sql .= ',';
                                }
                                $sql .= '(' . $this->makeList( $row ) . ')';
                        }
                } else {
                        $sql .= '(' . $this->makeList( $a ) . ')';
                }

                return (bool)$this->query( $sql, $fname );
        }

        /**
         * Make UPDATE options for the DatabaseBase::update function
         *
         * @param $options Array: The options passed to DatabaseBase::update
         * @return string
         */
        protected function makeUpdateOptions( $options ) {
                if ( !is_array( $options ) ) {
                        $options = array( $options );
                }

                $opts = array();

                if ( in_array( 'LOW_PRIORITY', $options ) ) {
                        $opts[] = $this->lowPriorityOption();
                }

                if ( in_array( 'IGNORE', $options ) ) {
                        $opts[] = 'IGNORE';
                }

                return implode( ' ', $opts );
        }

        /**
         * UPDATE wrapper. Takes a condition array and a SET array.
         *
         * @param $table  The name of the table to UPDATE. This will be passed through 
         *                DatabaseBase::tableName().
         *
         * @param $values Array:  An array of values to SET. For each array element, 
         *                the key gives the field name, and the value gives the data
         *                to set that field to. The data will be quoted by 
         *                DatabaseBase::addQuotes().
         *
         * @param $conds  Array:  An array of conditions (WHERE). See 
         *                DatabaseBase::select() for the details of the format of 
         *                condition arrays. Use '*' to update all rows.
         *
         * @param $fname  String: The function name of the caller (from __METHOD__),
         *                for logging and profiling.
         *
         * @param $options Array: An array of UPDATE options, can be:
         *                   - IGNORE: Ignore unique key conflicts
         *                   - LOW_PRIORITY: MySQL-specific, see MySQL manual.
         * @return Boolean
         */
        function update( $table, $values, $conds, $fname = 'DatabaseBase::update', $options = array() ) {
                $table = $this->tableName( $table );
                $opts = $this->makeUpdateOptions( $options );
                $sql = "UPDATE $opts $table SET " . $this->makeList( $values, LIST_SET );

                if ( $conds != '*' ) {
                        $sql .= " WHERE " . $this->makeList( $conds, LIST_AND );
                }

                return $this->query( $sql, $fname );
        }

        /**
         * Makes an encoded list of strings from an array
         * @param $a Array containing the data
         * @param $mode:
         *      - LIST_COMMA:          comma separated, no field names
         *      - LIST_AND:            ANDed WHERE clause (without the WHERE). See 
         *        the documentation for $conds in DatabaseBase::select().
         *      - LIST_OR:             ORed WHERE clause (without the WHERE)
         *      - LIST_SET:            comma separated with field names, like a SET clause
         *      - LIST_NAMES:          comma separated field names
         *
         * In LIST_AND or LIST_OR modes, you can suffix a field with an exclamation
         * mark to generate a 'NOT IN' structure.
         *
         * Example:
         *  $db->makeList( array( 'field!' => array( 1,2,3 ) );
         *
         *  outputs:
         *    'field' NOT IN ('1', '2', '3' );

         * @return string
         */
        function makeList( $a, $mode = LIST_COMMA ) {
                if ( !is_array( $a ) ) {
                        throw new DBUnexpectedError( $this, 'DatabaseBase::makeList called with incorrect parameters' );
                }

                $first = true;
                $list = '';

                foreach ( $a as $field => $value ) {
                        if ( !$first ) {
                                if ( $mode == LIST_AND ) {
                                        $list .= ' AND ';
                                } elseif ( $mode == LIST_OR ) {
                                        $list .= ' OR ';
                                } else {
                                        $list .= ',';
                                }
                        } else {
                                $first = false;
                        }

                        // Support 'NOT IN' by suffixing fieldname with an exclamation mark
                        $not = false;
                        if( substr($field,-1) == '!' ) {
                                $not = true;
                                $field = substr($field, 0, -1 );
                        }

                        if ( ( $mode == LIST_AND || $mode == LIST_OR ) && is_numeric( $field ) ) {
                                $list .= "($value)";
                        } elseif ( ( $mode == LIST_SET ) && is_numeric( $field ) ) {
                                $list .= "$value";
                        } elseif ( ( $mode == LIST_AND || $mode == LIST_OR ) && is_array( $value ) ) {
                                if ( count( $value ) == 0 ) {
                                        throw new MWException( __METHOD__ . ': empty input' );
                                } elseif ( count( $value ) == 1 ) {
                                        // Special-case single values, as IN isn't terribly efficient
                                        // Don't necessarily assume the single key is 0; we don't
                                        // enforce linear numeric ordering on other arrays here.
                                        $value = array_values( $value );

                                        $operator = $not ? ' != ' : ' = ';
                                        $list .= $field . $operator . $this->addQuotes( $value[0] );
                                } else {
                                        $operator = $not ? ' NOT IN ' : ' IN ';
                                        $list .= $field . $operator . "(" . $this->makeList( $value ) . ")";
                                }
                        } elseif ( $value === null ) {
                                if ( $mode == LIST_AND || $mode == LIST_OR ) {
                                        $list .= "$field IS ";
                                        if ( $not ) {
                                                $list .= "NOT ";
                                        }
                                } elseif ( $mode == LIST_SET ) {
                                        $list .= "$field = ";
                                }
                                $list .= 'NULL';
                        } else {
                                if ( $mode == LIST_AND || $mode == LIST_OR || $mode == LIST_SET ) {
                                        $list .= "$field = ";
                                }
                                $list .= $mode == LIST_NAMES ? $value : $this->addQuotes( $value );
                        }
                }

                return $list;
        }

        /**
         * Build a partial where clause from a 2-d array such as used for LinkBatch.
         * The keys on each level may be either integers or strings.
         *
         * @param $data Array: organized as 2-d
         *              array(baseKeyVal => array(subKeyVal => <ignored>, ...), ...)
         * @param $baseKey String: field name to match the base-level keys to (eg 'pl_namespace')
         * @param $subKey String: field name to match the sub-level keys to (eg 'pl_title')
         * @return Mixed: string SQL fragment, or false if no items in array.
         */
        function makeWhereFrom2d( $data, $baseKey, $subKey ) {
                $conds = array();

                foreach ( $data as $base => $sub ) {
                        if ( count( $sub ) ) {
                                $conds[] = $this->makeList(
                                        array( $baseKey => $base, $subKey => array_keys( $sub ) ),
                                        LIST_AND );
                        }
                }

                if ( $conds ) {
                        return $this->makeList( $conds, LIST_OR );
                } else {
                        // Nothing to search for...
                        return false;
                }
        }

        /**
         * Bitwise operations
         */

        /**
         * @param $field
         * @return string
         */
        function bitNot( $field ) {
                return "(~$field)";
        }

        /**
         * @param $fieldLeft
         * @param $fieldRight
         * @return string
         */
        function bitAnd( $fieldLeft, $fieldRight ) {
                return "($fieldLeft & $fieldRight)";
        }

        /**
         * @param  $fieldLeft
         * @param  $fieldRight
         * @return string
         */
        function bitOr( $fieldLeft, $fieldRight ) {
                return "($fieldLeft | $fieldRight)";
        }

        /**
         * Change the current database
         *
         * @todo Explain what exactly will fail if this is not overridden.
         * @return bool Success or failure
         */
        function selectDB( $db ) {
                # Stub.  Shouldn't cause serious problems if it's not overridden, but
                # if your database engine supports a concept similar to MySQL's
                # databases you may as well.
                $this->mDBname = $db;
                return true;
        }

        /**
         * Get the current DB name
         */
        function getDBname() {
                return $this->mDBname;
        }

        /**
         * Get the server hostname or IP address
         */
        function getServer() {
                return $this->mServer;
        }

        /**
         * Format a table name ready for use in constructing an SQL query
         *
         * This does two important things: it quotes the table names to clean them up,
         * and it adds a table prefix if only given a table name with no quotes.
         *
         * All functions of this object which require a table name call this function
         * themselves. Pass the canonical name to such functions. This is only needed
         * when calling query() directly.
         *
         * @param $name String: database table name
         * @param $quoted Boolean: Automatically pass the table name through
         *          addIdentifierQuotes() so that it can be used in a query.
         * @return String: full database name
         */
        function tableName( $name, $quoted = true ) {
                global $wgSharedDB, $wgSharedPrefix, $wgSharedTables;
                # Skip the entire process when we have a string quoted on both ends.
                # Note that we check the end so that we will still quote any use of
                # use of `database`.table. But won't break things if someone wants
                # to query a database table with a dot in the name.
                if ( $this->isQuotedIdentifier( $name ) ) {
                        return $name;
                }

                # Lets test for any bits of text that should never show up in a table
                # name. Basically anything like JOIN or ON which are actually part of
                # SQL queries, but may end up inside of the table value to combine
                # sql. Such as how the API is doing.
                # Note that we use a whitespace test rather than a \b test to avoid
                # any remote case where a word like on may be inside of a table name
                # surrounded by symbols which may be considered word breaks.
                if ( preg_match( '/(^|\s)(DISTINCT|JOIN|ON|AS)(\s|$)/i', $name ) !== 0 ) {
                        return $name;
                }

                # Split database and table into proper variables.
                # We reverse the explode so that database.table and table both output
                # the correct table.
                $dbDetails = array_reverse( explode( '.', $name, 2 ) );
                if ( isset( $dbDetails[1] ) ) {
                        @list( $table, $database ) = $dbDetails;
                } else {
                        @list( $table ) = $dbDetails;
                }
                $prefix = $this->mTablePrefix; # Default prefix

                # Note that we use the long format because php will complain in in_array if
                # the input is not an array, and will complain in is_array if it is not set.
                if ( !isset( $database ) # Don't use shared database if pre selected.
                 && isset( $wgSharedDB ) # We have a shared database
                 && !$this->isQuotedIdentifier( $table ) # Paranoia check to prevent shared tables listing '`table`'
                 && isset( $wgSharedTables )
                 && is_array( $wgSharedTables )
                 && in_array( $table, $wgSharedTables ) ) { # A shared table is selected
                        $database = $wgSharedDB;
                        $prefix   = isset( $wgSharedPrefix ) ? $wgSharedPrefix : $prefix;
                }

                # Quote the $database and $table and apply the prefix if not quoted.
                if ( isset( $database ) ) {
                        $database = ( !$quoted || $this->isQuotedIdentifier( $database ) ? $database : $this->addIdentifierQuotes( $database ) );
                        $prefix = '';
                }

                $table = "{$prefix}{$table}";
                if ( $quoted && !$this->isQuotedIdentifier( $table ) ) {
                        $table = $this->addIdentifierQuotes( "{$table}" );
                }

                # Merge our database and table into our final table name.
                $tableName = ( isset( $database ) ? "{$database}.{$table}" : "{$table}" );

                return $tableName;
        }

        /**
         * Fetch a number of table names into an array
         * This is handy when you need to construct SQL for joins
         *
         * Example:
         * extract($dbr->tableNames('user','watchlist'));
         * $sql = "SELECT wl_namespace,wl_title FROM $watchlist,$user
         *         WHERE wl_user=user_id AND wl_user=$nameWithQuotes";
         */
        public function tableNames() {
                $inArray = func_get_args();
                $retVal = array();

                foreach ( $inArray as $name ) {
                        $retVal[$name] = $this->tableName( $name );
                }

                return $retVal;
        }

        /**
         * Fetch a number of table names into an zero-indexed numerical array
         * This is handy when you need to construct SQL for joins
         *
         * Example:
         * list( $user, $watchlist ) = $dbr->tableNamesN('user','watchlist');
         * $sql = "SELECT wl_namespace,wl_title FROM $watchlist,$user
         *         WHERE wl_user=user_id AND wl_user=$nameWithQuotes";
         */
        public function tableNamesN() {
                $inArray = func_get_args();
                $retVal = array();

                foreach ( $inArray as $name ) {
                        $retVal[] = $this->tableName( $name );
                }

                return $retVal;
        }

        /**
         * Get an aliased table name
         * e.g. tableName AS newTableName
         *
         * @param $name string Table name, see tableName()
         * @param $alias string Alias (optional)
         * @return string SQL name for aliased table. Will not alias a table to its own name
         */
        public function tableNameWithAlias( $name, $alias = false ) {
                if ( !$alias || $alias == $name ) {
                        return $this->tableName( $name );
                } else {
                        return $this->tableName( $name ) . ' ' . $this->addIdentifierQuotes( $alias );
                }
        }

        /**
         * Gets an array of aliased table names
         *
         * @param $tables array( [alias] => table )
         * @return array of strings, see tableNameWithAlias()
         */
        public function tableNamesWithAlias( $tables ) {
                $retval = array();
                foreach ( $tables as $alias => $table ) {
                        if ( is_numeric( $alias ) ) {
                                $alias = $table;
                        }
                        $retval[] = $this->tableNameWithAlias( $table, $alias );
                }
                return $retval;
        }

        /**
         * @return string
         */
        function tableNamesWithUseIndexOrJOIN( $tables, $use_index = array(), $join_conds = array() ) {
                $ret = array();
                $retJOIN = array();
                $use_index_safe = is_array( $use_index ) ? $use_index : array();
                $join_conds_safe = is_array( $join_conds ) ? $join_conds : array();

                foreach ( $tables as $alias => $table ) {
                        if ( !is_string( $alias ) ) {
                                // No alias? Set it equal to the table name
                                $alias = $table;
                        }
                        // Is there a JOIN and INDEX clause for this table?
                        if ( isset( $join_conds_safe[$alias] ) && isset( $use_index_safe[$alias] ) ) {
                                $tableClause = $join_conds_safe[$alias][0] . ' ' . $this->tableNameWithAlias( $table, $alias );
                                $tableClause .= ' ' . $this->useIndexClause( implode( ',', (array)$use_index_safe[$alias] ) );
                                $on = $this->makeList( (array)$join_conds_safe[$alias][1], LIST_AND );
                                if ( $on != '' ) {
                                        $tableClause .= ' ON (' . $on . ')';
                                }

                                $retJOIN[] = $tableClause;
                        // Is there an INDEX clause?
                        } elseif ( isset( $use_index_safe[$alias] ) ) {
                                $tableClause = $this->tableNameWithAlias( $table, $alias );
                                $tableClause .= ' ' . $this->useIndexClause( implode( ',', (array)$use_index_safe[$alias] ) );
                                $ret[] = $tableClause;
                        // Is there a JOIN clause?
                        } elseif ( isset( $join_conds_safe[$alias] ) ) {
                                $tableClause = $join_conds_safe[$alias][0] . ' ' . $this->tableNameWithAlias( $table, $alias );
                                $on = $this->makeList( (array)$join_conds_safe[$alias][1], LIST_AND );
                                if ( $on != '' ) {
                                        $tableClause .= ' ON (' . $on . ')';
                                }

                                $retJOIN[] = $tableClause;
                        } else {
                                $tableClause = $this->tableNameWithAlias( $table, $alias );
                                $ret[] = $tableClause;
                        }
                }

                // We can't separate explicit JOIN clauses with ',', use ' ' for those
                $straightJoins = !empty( $ret ) ? implode( ',', $ret ) : "";
                $otherJoins = !empty( $retJOIN ) ? implode( ' ', $retJOIN ) : "";

                // Compile our final table clause
                return implode( ' ', array( $straightJoins, $otherJoins ) );
        }

        /**
         * Get the name of an index in a given table
         *
         * @param $index
         *
         * @return string
         */
        function indexName( $index ) {
                // Backwards-compatibility hack
                $renamed = array(
                        'ar_usertext_timestamp' => 'usertext_timestamp',
                        'un_user_id'            => 'user_id',
                        'un_user_ip'            => 'user_ip',
                );

                if ( isset( $renamed[$index] ) ) {
                        return $renamed[$index];
                } else {
                        return $index;
                }
        }

        /**
         * If it's a string, adds quotes and backslashes
         * Otherwise returns as-is
         *
         * @param $s string
         *
         * @return string
         */
        function addQuotes( $s ) {
                if ( $s === null ) {
                        return 'NULL';
                } else {
                        # This will also quote numeric values. This should be harmless,
                        # and protects against weird problems that occur when they really
                        # _are_ strings such as article titles and string->number->string
                        # conversion is not 1:1.
                        return "'" . $this->strencode( $s ) . "'";
                }
        }

        /**
         * Quotes an identifier using `backticks` or "double quotes" depending on the database type.
         * MySQL uses `backticks` while basically everything else uses double quotes.
         * Since MySQL is the odd one out here the double quotes are our generic
         * and we implement backticks in DatabaseMysql.
         *
         * @return string
         */
        public function addIdentifierQuotes( $s ) {
                return '"' . str_replace( '"', '""', $s ) . '"';
        }

        /**
         * Returns if the given identifier looks quoted or not according to
         * the database convention for quoting identifiers .
         *
         * @param $name string
         *
         * @return boolean
         */
        public function isQuotedIdentifier( $name ) {
                return $name[0] == '"' && substr( $name, -1, 1 ) == '"';
        }

        /**
         * Backwards compatibility, identifier quoting originated in DatabasePostgres
         * which used quote_ident which does not follow our naming conventions
         * was renamed to addIdentifierQuotes.
         * @deprecated since 1.18 use addIdentifierQuotes
         *
         * @return string
         */
        function quote_ident( $s ) {
                wfDeprecated( __METHOD__ );
                return $this->addIdentifierQuotes( $s );
        }

        /**
         * Escape string for safe LIKE usage.
         * WARNING: you should almost never use this function directly,
         * instead use buildLike() that escapes everything automatically
         * @deprecated since 1.17, warnings in 1.17, removed in ???
         */
        public function escapeLike( $s ) {
                wfDeprecated( __METHOD__ );
                return $this->escapeLikeInternal( $s );
        }

        protected function escapeLikeInternal( $s ) {
                $s = str_replace( '\\', '\\\\', $s );
                $s = $this->strencode( $s );
                $s = str_replace( array( '%', '_' ), array( '\%', '\_' ), $s );

                return $s;
        }

        /**
         * LIKE statement wrapper, receives a variable-length argument list with parts of pattern to match
         * containing either string literals that will be escaped or tokens returned by anyChar() or anyString().
         * Alternatively, the function could be provided with an array of aforementioned parameters.
         *
         * Example: $dbr->buildLike( 'My_page_title/', $dbr->anyString() ) returns a LIKE clause that searches
         * for subpages of 'My page title'.
         * Alternatively: $pattern = array( 'My_page_title/', $dbr->anyString() ); $query .= $dbr->buildLike( $pattern );
         *
         * @since 1.16
         * @return String: fully built LIKE statement
         */
        function buildLike() {
                $params = func_get_args();

                if ( count( $params ) > 0 && is_array( $params[0] ) ) {
                        $params = $params[0];
                }

                $s = '';

                foreach ( $params as $value ) {
                        if ( $value instanceof LikeMatch ) {
                                $s .= $value->toString();
                        } else {
                                $s .= $this->escapeLikeInternal( $value );
                        }
                }

                return " LIKE '" . $s . "' ";
        }

        /**
         * Returns a token for buildLike() that denotes a '_' to be used in a LIKE query
         *
         * @return LikeMatch
         */
        function anyChar() {
                return new LikeMatch( '_' );
        }

        /**
         * Returns a token for buildLike() that denotes a '%' to be used in a LIKE query
         *
         * @rerturn LikeMatch
         */
        function anyString() {
                return new LikeMatch( '%' );
        }

        /**
         * Returns an appropriately quoted sequence value for inserting a new row.
         * MySQL has autoincrement fields, so this is just NULL. But the PostgreSQL
         * subclass will return an integer, and save the value for insertId()
         */
        function nextSequenceValue( $seqName ) {
                return null;
        }

        /**
         * USE INDEX clause.  Unlikely to be useful for anything but MySQL.  This
         * is only needed because a) MySQL must be as efficient as possible due to
         * its use on Wikipedia, and b) MySQL 4.0 is kind of dumb sometimes about
         * which index to pick.  Anyway, other databases might have different
         * indexes on a given table.  So don't bother overriding this unless you're
         * MySQL.
         */
        function useIndexClause( $index ) {
                return '';
        }

        /**
         * REPLACE query wrapper
         * PostgreSQL simulates this with a DELETE followed by INSERT
         * $row is the row to insert, an associative array
         * $uniqueIndexes is an array of indexes. Each element may be either a
         * field name or an array of field names
         *
         * It may be more efficient to leave off unique indexes which are unlikely to collide.
         * However if you do this, you run the risk of encountering errors which wouldn't have
         * occurred in MySQL
         *
         * @param $table String: The table to replace the row(s) in.
         * @param $uniqueIndexes Array: An associative array of indexes
         * @param $rows Array: Array of rows to replace
         * @param $fname String: Calling function name (use __METHOD__) for logs/profiling
         */
        function replace( $table, $uniqueIndexes, $rows, $fname = 'DatabaseBase::replace' ) {
                $table = $this->tableName( $table );

                # Single row case
                if ( !is_array( reset( $rows ) ) ) {
                        $rows = array( $rows );
                }

                $sql = "REPLACE INTO $table (" . implode( ',', array_keys( $rows[0] ) ) . ') VALUES ';
                $first = true;

                foreach ( $rows as $row ) {
                        if ( $first ) {
                                $first = false;
                        } else {
                                $sql .= ',';
                        }

                        $sql .= '(' . $this->makeList( $row ) . ')';
                }

                return $this->query( $sql, $fname );
        }

        /**
         * DELETE where the condition is a join.
         *
         * MySQL overrides this to use a multi-table DELETE syntax, in other databases 
         * we use sub-selects
         *
         * For safety, an empty $conds will not delete everything. If you want to 
         * delete all rows where the join condition matches, set $conds='*'.
         *
         * DO NOT put the join condition in $conds.
         *
         * @param $delTable   String: The table to delete from.
         * @param $joinTable  String: The other table.
         * @param $delVar     String: The variable to join on, in the first table.
         * @param $joinVar    String: The variable to join on, in the second table.
         * @param $conds      Array: Condition array of field names mapped to variables, 
         *                    ANDed together in the WHERE clause
         * @param $fname      String: Calling function name (use __METHOD__) for 
         *                    logs/profiling
         */
        function deleteJoin( $delTable, $joinTable, $delVar, $joinVar, $conds, 
                $fname = 'DatabaseBase::deleteJoin' ) 
        {
                if ( !$conds ) {
                        throw new DBUnexpectedError( $this, 
                                'DatabaseBase::deleteJoin() called with empty $conds' );
                }

                $delTable = $this->tableName( $delTable );
                $joinTable = $this->tableName( $joinTable );
                $sql = "DELETE FROM $delTable WHERE $delVar IN (SELECT $joinVar FROM $joinTable ";
                if ( $conds != '*' ) {
                        $sql .= 'WHERE ' . $this->makeList( $conds, LIST_AND );
                }
                $sql .= ')';

                $this->query( $sql, $fname );
        }

        /**
         * Returns the size of a text field, or -1 for "unlimited"
         */
        function textFieldSize( $table, $field ) {
                $table = $this->tableName( $table );
                $sql = "SHOW COLUMNS FROM $table LIKE \"$field\";";
                $res = $this->query( $sql, 'DatabaseBase::textFieldSize' );
                $row = $this->fetchObject( $res );

                $m = array();

                if ( preg_match( '/\((.*)\)/', $row->Type, $m ) ) {
                        $size = $m[1];
                } else {
                        $size = -1;
                }

                return $size;
        }

        /**
         * A string to insert into queries to show that they're low-priority, like
         * MySQL's LOW_PRIORITY.  If no such feature exists, return an empty
         * string and nothing bad should happen.
         *
         * @return string Returns the text of the low priority option if it is 
         *   supported, or a blank string otherwise
         */
        function lowPriorityOption() {
                return '';
        }

        /**
         * DELETE query wrapper.
         *
         * @param $table Table name
         * @param $conds Condition array. See $conds in DatabaseBase::select() for 
         *               the format. Use $conds == "*" to delete all rows
         *
         * @return bool
         */
        function delete( $table, $conds, $fname = 'DatabaseBase::delete' ) {
                if ( !$conds ) {
                        throw new DBUnexpectedError( $this, 'DatabaseBase::delete() called with no conditions' );
                }

                $table = $this->tableName( $table );
                $sql = "DELETE FROM $table";

                if ( $conds != '*' ) {
                        $sql .= ' WHERE ' . $this->makeList( $conds, LIST_AND );
                }

                return $this->query( $sql, $fname );
        }

        /**
         * INSERT SELECT wrapper. Takes data from a SELECT query and inserts it
         * into another table.
         *
         * @param $destTable The table name to insert into
         * @param $srcTable May be either a table name, or an array of table names
         *    to include in a join.
         *
         * @param $varMap must be an associative array of the form 
         *    array( 'dest1' => 'source1', ...). Source items may be literals 
         *    rather than field names, but strings should be quoted with 
         *    DatabaseBase::addQuotes()
         *
         * @param $conds Condition array. See $conds in DatabaseBase::select() for 
         *    the details of the format of condition arrays. May be "*" to copy the 
         *    whole table.
         *
         * @param $fname The function name of the caller, from __METHOD__
         *
         * @param $insertOptions Options for the INSERT part of the query, see 
         *    DatabaseBase::insert() for details.
         * @param $selectOptions Options for the SELECT part of the query, see
         *    DatabaseBase::select() for details.
         *
         * @return ResultWrapper
         */
        function insertSelect( $destTable, $srcTable, $varMap, $conds, 
                $fname = 'DatabaseBase::insertSelect',
                $insertOptions = array(), $selectOptions = array() )
        {
                $destTable = $this->tableName( $destTable );

                if ( is_array( $insertOptions ) ) {
                        $insertOptions = implode( ' ', $insertOptions );
                }

                if ( !is_array( $selectOptions ) ) {
                        $selectOptions = array( $selectOptions );
                }

                list( $startOpts, $useIndex, $tailOpts ) = $this->makeSelectOptions( $selectOptions );

                if ( is_array( $srcTable ) ) {
                        $srcTable =  implode( ',', array_map( array( &$this, 'tableName' ), $srcTable ) );
                } else {
                        $srcTable = $this->tableName( $srcTable );
                }

                $sql = "INSERT $insertOptions INTO $destTable (" . implode( ',', array_keys( $varMap ) ) . ')' .
                        " SELECT $startOpts " . implode( ',', $varMap ) .
                        " FROM $srcTable $useIndex ";

                if ( $conds != '*' ) {
                        $sql .= ' WHERE ' . $this->makeList( $conds, LIST_AND );
                }

                $sql .= " $tailOpts";

                return $this->query( $sql, $fname );
        }

        /**
         * Construct a LIMIT query with optional offset.  This is used for query
         * pages.  The SQL should be adjusted so that only the first $limit rows
         * are returned.  If $offset is provided as well, then the first $offset
         * rows should be discarded, and the next $limit rows should be returned.
         * If the result of the query is not ordered, then the rows to be returned
         * are theoretically arbitrary.
         *
         * $sql is expected to be a SELECT, if that makes a difference.  For
         * UPDATE, limitResultForUpdate should be used.
         *
         * The version provided by default works in MySQL and SQLite.  It will very
         * likely need to be overridden for most other DBMSes.
         *
         * @param $sql String: SQL query we will append the limit too
         * @param $limit Integer: the SQL limit
         * @param $offset Integer the SQL offset (default false)
         *
         * @return string
         */
        function limitResult( $sql, $limit, $offset = false ) {
                if ( !is_numeric( $limit ) ) {
                        throw new DBUnexpectedError( $this, "Invalid non-numeric limit passed to limitResult()\n" );
                }

                return "$sql LIMIT "
                                . ( ( is_numeric( $offset ) && $offset != 0 ) ? "{$offset}," : "" )
                                . "{$limit} ";
        }

        function limitResultForUpdate( $sql, $num ) {
                return $this->limitResult( $sql, $num, 0 );
        }

        /**
         * Returns true if current database backend supports ORDER BY or LIMIT for separate subqueries
         * within the UNION construct.
         * @return Boolean
         */
        function unionSupportsOrderAndLimit() {
                return true; // True for almost every DB supported
        }

        /**
         * Construct a UNION query
         * This is used for providing overload point for other DB abstractions
         * not compatible with the MySQL syntax.
         * @param $sqls Array: SQL statements to combine
         * @param $all Boolean: use UNION ALL
         * @return String: SQL fragment
         */
        function unionQueries( $sqls, $all ) {
                $glue = $all ? ') UNION ALL (' : ') UNION (';
                return '(' . implode( $glue, $sqls ) . ')';
        }

        /**
         * Returns an SQL expression for a simple conditional.  This doesn't need
         * to be overridden unless CASE isn't supported in your DBMS.
         *
         * @param $cond String: SQL expression which will result in a boolean value
         * @param $trueVal String: SQL expression to return if true
         * @param $falseVal String: SQL expression to return if false
         * @return String: SQL fragment
         */
        function conditional( $cond, $trueVal, $falseVal ) {
                return " (CASE WHEN $cond THEN $trueVal ELSE $falseVal END) ";
        }

        /**
         * Returns a comand for str_replace function in SQL query.
         * Uses REPLACE() in MySQL
         *
         * @param $orig String: column to modify
         * @param $old String: column to seek
         * @param $new String: column to replace with
         *
         * @return string
         */
        function strreplace( $orig, $old, $new ) {
                return "REPLACE({$orig}, {$old}, {$new})";
        }

        /**
         * Determines if the last failure was due to a deadlock
         * STUB
         *
         * @return bool
         */
        function wasDeadlock() {
                return false;
        }

        /**
         * Determines if the last query error was something that should be dealt
         * with by pinging the connection and reissuing the query.
         * STUB
         *
         * @return bool
         */
        function wasErrorReissuable() {
                return false;
        }

        /**
         * Determines if the last failure was due to the database being read-only.
         * STUB
         *
         * @return bool
         */
        function wasReadOnlyError() {
                return false;
        }

        /**
         * Perform a deadlock-prone transaction.
         *
         * This function invokes a callback function to perform a set of write
         * queries. If a deadlock occurs during the processing, the transaction
         * will be rolled back and the callback function will be called again.
         *
         * Usage:
         *   $dbw->deadlockLoop( callback, ... );
         *
         * Extra arguments are passed through to the specified callback function.
         *
         * Returns whatever the callback function returned on its successful,
         * iteration, or false on error, for example if the retry limit was
         * reached.
         */
        function deadlockLoop() {
                $myFname = 'DatabaseBase::deadlockLoop';

                $this->begin();
                $args = func_get_args();
                $function = array_shift( $args );
                $oldIgnore = $this->ignoreErrors( true );
                $tries = DEADLOCK_TRIES;

                if ( is_array( $function ) ) {
                        $fname = $function[0];
                } else {
                        $fname = $function;
                }

                do {
                        $retVal = call_user_func_array( $function, $args );
                        $error = $this->lastError();
                        $errno = $this->lastErrno();
                        $sql = $this->lastQuery();

                        if ( $errno ) {
                                if ( $this->wasDeadlock() ) {
                                        # Retry
                                        usleep( mt_rand( DEADLOCK_DELAY_MIN, DEADLOCK_DELAY_MAX ) );
                                } else {
                                        $this->reportQueryError( $error, $errno, $sql, $fname );
                                }
                        }
                } while ( $this->wasDeadlock() && --$tries > 0 );

                $this->ignoreErrors( $oldIgnore );

                if ( $tries <= 0 ) {
                        $this->rollback( $myFname );
                        $this->reportQueryError( $error, $errno, $sql, $fname );
                        return false;
                } else {
                        $this->commit( $myFname );
                        return $retVal;
                }
        }

        /**
         * Do a SELECT MASTER_POS_WAIT()
         *
         * @param $pos MySQLMasterPos object
         * @param $timeout Integer: the maximum number of seconds to wait for synchronisation
         */
        function masterPosWait( MySQLMasterPos $pos, $timeout ) {
                $fname = 'DatabaseBase::masterPosWait';
                wfProfileIn( $fname );

                # Commit any open transactions
                if ( $this->mTrxLevel ) {
                        $this->commit();
                }

                if ( !is_null( $this->mFakeSlaveLag ) ) {
                        $wait = intval( ( $pos->pos - microtime( true ) + $this->mFakeSlaveLag ) * 1e6 );

                        if ( $wait > $timeout * 1e6 ) {
                                wfDebug( "Fake slave timed out waiting for $pos ($wait us)\n" );
                                wfProfileOut( $fname );
                                return -1;
                        } elseif ( $wait > 0 ) {
                                wfDebug( "Fake slave waiting $wait us\n" );
                                usleep( $wait );
                                wfProfileOut( $fname );
                                return 1;
                        } else {
                                wfDebug( "Fake slave up to date ($wait us)\n" );
                                wfProfileOut( $fname );
                                return 0;
                        }
                }

                # Call doQuery() directly, to avoid opening a transaction if DBO_TRX is set
                $encFile = $this->addQuotes( $pos->file );
                $encPos = intval( $pos->pos );
                $sql = "SELECT MASTER_POS_WAIT($encFile, $encPos, $timeout)";
                $res = $this->doQuery( $sql );

                if ( $res && $row = $this->fetchRow( $res ) ) {
                        wfProfileOut( $fname );
                        return $row[0];
                } else {
                        wfProfileOut( $fname );
                        return false;
                }
        }

        /**
         * Get the position of the master from SHOW SLAVE STATUS
         *
         * @return MySQLMasterPos|false
         */
        function getSlavePos() {
                if ( !is_null( $this->mFakeSlaveLag ) ) {
                        $pos = new MySQLMasterPos( 'fake', microtime( true ) - $this->mFakeSlaveLag );
                        wfDebug( __METHOD__ . ": fake slave pos = $pos\n" );
                        return $pos;
                }

                $res = $this->query( 'SHOW SLAVE STATUS', 'DatabaseBase::getSlavePos' );
                $row = $this->fetchObject( $res );

                if ( $row ) {
                        $pos = isset( $row->Exec_master_log_pos ) ? $row->Exec_master_log_pos : $row->Exec_Master_Log_Pos;
                        return new MySQLMasterPos( $row->Relay_Master_Log_File, $pos );
                } else {
                        return false;
                }
        }

        /**
         * Get the position of the master from SHOW MASTER STATUS
         *
         * @return MySQLMasterPos|false
         */
        function getMasterPos() {
                if ( $this->mFakeMaster ) {
                        return new MySQLMasterPos( 'fake', microtime( true ) );
                }

                $res = $this->query( 'SHOW MASTER STATUS', 'DatabaseBase::getMasterPos' );
                $row = $this->fetchObject( $res );

                if ( $row ) {
                        return new MySQLMasterPos( $row->File, $row->Position );
                } else {
                        return false;
                }
        }

        /**
         * Begin a transaction, committing any previously open transaction
         */
        function begin( $fname = 'DatabaseBase::begin' ) {
                $this->query( 'BEGIN', $fname );
                $this->mTrxLevel = 1;
        }

        /**
         * End a transaction
         */
        function commit( $fname = 'DatabaseBase::commit' ) {
                if ( $this->mTrxLevel ) {
                        $this->query( 'COMMIT', $fname );
                        $this->mTrxLevel = 0;
                }
        }

        /**
         * Rollback a transaction.
         * No-op on non-transactional databases.
         */
        function rollback( $fname = 'DatabaseBase::rollback' ) {
                if ( $this->mTrxLevel ) {
                        $this->query( 'ROLLBACK', $fname, true );
                        $this->mTrxLevel = 0;
                }
        }

        /**
         * Creates a new table with structure copied from existing table
         * Note that unlike most database abstraction functions, this function does not
         * automatically append database prefix, because it works at a lower
         * abstraction level.
         * The table names passed to this function shall not be quoted (this
         * function calls addIdentifierQuotes when needed).
         *
         * @param $oldName String: name of table whose structure should be copied
         * @param $newName String: name of table to be created
         * @param $temporary Boolean: whether the new table should be temporary
         * @param $fname String: calling function name
         * @return Boolean: true if operation was successful
         */
        function duplicateTableStructure( $oldName, $newName, $temporary = false, 
                $fname = 'DatabaseBase::duplicateTableStructure' ) 
        {
                throw new MWException( 
                        'DatabaseBase::duplicateTableStructure is not implemented in descendant class' );
        }

        /**
         * List all tables on the database
         *
         * @param $prefix Only show tables with this prefix, e.g. mw_
         * @param $fname String: calling function name
         */
        function listTables( $prefix = null, $fname = 'DatabaseBase::listTables' ) {
                throw new MWException( 'DatabaseBase::listTables is not implemented in descendant class' );
        }

        /**
         * Convert a timestamp in one of the formats accepted by wfTimestamp() 
         * to the format used for inserting into timestamp fields in this DBMS.
         *
         * The result is unquoted, and needs to be passed through addQuotes() 
         * before it can be included in raw SQL.
         *
         * @return string
         */
        function timestamp( $ts = 0 ) {
                return wfTimestamp( TS_MW, $ts );
        }

        /**
         * Convert a timestamp in one of the formats accepted by wfTimestamp() 
         * to the format used for inserting into timestamp fields in this DBMS. If 
         * NULL is input, it is passed through, allowing NULL values to be inserted 
         * into timestamp fields.
         *
         * The result is unquoted, and needs to be passed through addQuotes() 
         * before it can be included in raw SQL.
         *
         * @return string
         */
        function timestampOrNull( $ts = null ) {
                if ( is_null( $ts ) ) {
                        return null;
                } else {
                        return $this->timestamp( $ts );
                }
        }

        /**
         * Take the result from a query, and wrap it in a ResultWrapper if 
         * necessary. Boolean values are passed through as is, to indicate success 
         * of write queries or failure. ResultWrapper objects are also passed 
         * through.
         */
        protected function resultObject( $result ) {
                if ( empty( $result ) ) {
                        return false;
                } elseif ( $result instanceof ResultWrapper ) {
                        return $result;
                } elseif ( $result === true ) {
                        // Successful write query
                        return $result;
                } else {
                        return new ResultWrapper( $this, $result );
                }
        }

        /**
         * Return aggregated value alias
         */
        function aggregateValue ( $valuedata, $valuename = 'value' ) {
                return $valuename;
        }

        /**
         * Ping the server and try to reconnect if it there is no connection
         *
         * @return bool Success or failure
         */
        function ping() {
                # Stub.  Not essential to override.
                return true;
        }

        /**
         * Get slave lag.
         * Currently supported only by MySQL
         * @return Database replication lag in seconds
         */
        function getLag() {
                return intval( $this->mFakeSlaveLag );
        }

        /**
         * Get status information from SHOW STATUS in an associative array
         *
         * @return array
         */
        function getStatus( $which = "%" ) {
                $res = $this->query( "SHOW STATUS LIKE '{$which}'" );
                $status = array();

                foreach ( $res as $row ) {
                        $status[$row->Variable_name] = $row->Value;
                }

                return $status;
        }

        /**
         * Return the maximum number of items allowed in a list, or 0 for unlimited.
         *
         * return int
         */
        function maxListLen() {
                return 0;
        }

        /**
         * Some DBMSs have a special format for inserting into blob fields, they 
         * don't allow simple quoted strings to be inserted. To insert into such 
         * a field, pass the data through this function before passing it to 
         * DatabaseBase::insert(). 
         */
        function encodeBlob( $b ) {
                return $b;
        }

        /**
         * Some DBMSs return a special placeholder object representing blob fields
         * in result objects. Pass the object through this function to return the 
         * original string.
         */
        function decodeBlob( $b ) {
                return $b;
        }

        /**
         * Override database's default connection timeout.  May be useful for very
         * long batch queries such as full-wiki dumps, where a single query reads
         * out over hours or days.  May or may not be necessary for non-MySQL
         * databases.  For most purposes, leaving it as a no-op should be fine.
         *
         * @param $timeout Integer in seconds
         */
        public function setTimeout( $timeout ) {}

        /**
         * Read and execute SQL commands from a file.
         *
         * Returns true on success, error string or exception on failure (depending 
         * on object's error ignore settings).
         *
         * @param $filename String: File name to open
         * @param $lineCallback Callback: Optional function called before reading each line
         * @param $resultCallback Callback: Optional function called for each MySQL result
         * @param $fname String: Calling function name or false if name should be 
         *      generated dynamically using $filename
         */
        function sourceFile( $filename, $lineCallback = false, $resultCallback = false, $fname = false ) {
                wfSuppressWarnings();
                $fp = fopen( $filename, 'r' );
                wfRestoreWarnings();

                if ( false === $fp ) {
                        throw new MWException( "Could not open \"{$filename}\".\n" );
                }

                if ( !$fname ) {
                        $fname = __METHOD__ . "( $filename )";
                }

                try {
                        $error = $this->sourceStream( $fp, $lineCallback, $resultCallback, $fname );
                }
                catch ( MWException $e ) {
                        fclose( $fp );
                        throw $e;
                }

                fclose( $fp );

                return $error;
        }

        /**
         * Get the full path of a patch file. Originally based on archive()
         * from updaters.inc. Keep in mind this always returns a patch, as
         * it fails back to MySQL if no DB-specific patch can be found
         *
         * @param $patch String The name of the patch, like patch-something.sql
         * @return String Full path to patch file
         */
        public function patchPath( $patch ) {
                global $IP;

                $dbType = $this->getType();
                if ( file_exists( "$IP/maintenance/$dbType/archives/$patch" ) ) {
                        return "$IP/maintenance/$dbType/archives/$patch";
                } else {
                        return "$IP/maintenance/archives/$patch";
                }
        }

        /**
         * Set variables to be used in sourceFile/sourceStream, in preference to the
         * ones in $GLOBALS. If an array is set here, $GLOBALS will not be used at
         * all. If it's set to false, $GLOBALS will be used.
         *
         * @param $vars False, or array mapping variable name to value.
         */
        function setSchemaVars( $vars ) {
                $this->mSchemaVars = $vars;
        }

        /**
         * Read and execute commands from an open file handle.
         *
         * Returns true on success, error string or exception on failure (depending 
         * on object's error ignore settings).
         *
         * @param $fp Resource: File handle
         * @param $lineCallback Callback: Optional function called before reading each line
         * @param $resultCallback Callback: Optional function called for each MySQL result
         * @param $fname String: Calling function name
         */
        function sourceStream( $fp, $lineCallback = false, $resultCallback = false, 
                $fname = 'DatabaseBase::sourceStream' ) 
        {
                $cmd = "";
                $done = false;
                $dollarquote = false;

                while ( ! feof( $fp ) ) {
                        if ( $lineCallback ) {
                                call_user_func( $lineCallback );
                        }

                        $line = trim( fgets( $fp ) );
                        $sl = strlen( $line ) - 1;

                        if ( $sl < 0 ) {
                                continue;
                        }

                        if ( '-' == $line { 0 } && '-' == $line { 1 } ) {
                                continue;
                        }

                        # # Allow dollar quoting for function declarations
                        if ( substr( $line, 0, 4 ) == '$mw$' ) {
                                if ( $dollarquote ) {
                                        $dollarquote = false;
                                        $done = true;
                                }
                                else {
                                        $dollarquote = true;
                                }
                        }
                        elseif ( !$dollarquote ) {
                                if ( ';' == $line { $sl } && ( $sl < 2 || ';' != $line { $sl - 1 } ) ) {
                                        $done = true;
                                        $line = substr( $line, 0, $sl );
                                }
                        }

                        if ( $cmd != '' ) {
                                $cmd .= ' ';
                        }

                        $cmd .= "$line\n";

                        if ( $done ) {
                                $cmd = str_replace( ';;', ";", $cmd );
                                $cmd = $this->replaceVars( $cmd );
                                $res = $this->query( $cmd, $fname );

                                if ( $resultCallback ) {
                                        call_user_func( $resultCallback, $res, $this );
                                }

                                if ( false === $res ) {
                                        $err = $this->lastError();
                                        return "Query \"{$cmd}\" failed with error code \"$err\".\n";
                                }

                                $cmd = '';
                                $done = false;
                        }
                }

                return true;
        }

        /**
         * Database independent variable replacement. Replaces a set of variables
         * in an SQL statement with their contents as given by $this->getSchemaVars().
         *
         * Supports '{$var}' `{$var}` and / *$var* / (without the spaces) style variables.
         *
         * - '{$var}' should be used for text and is passed through the database's 
         *   addQuotes method.
         * - `{$var}` should be used for identifiers (eg: table and database names), 
         *   it is passed through the database's addIdentifierQuotes method which 
         *   can be overridden if the database uses something other than backticks.
         * - / *$var* / is just encoded, besides traditional table prefix and 
         *   table options its use should be avoided.
         *
         * @param $ins String: SQL statement to replace variables in
         * @return String The new SQL statement with variables replaced
         */
        protected function replaceSchemaVars( $ins ) {
                $vars = $this->getSchemaVars();
                foreach ( $vars as $var => $value ) {
                        // replace '{$var}'
                        $ins = str_replace( '\'{$' . $var . '}\'', $this->addQuotes( $value ), $ins );
                        // replace `{$var}`
                        $ins = str_replace( '`{$' . $var . '}`', $this->addIdentifierQuotes( $value ), $ins );
                        // replace /*$var*/
                        $ins = str_replace( '/*$' . $var . '*/', $this->strencode( $value ) , $ins );
                }
                return $ins;
        }

        /**
         * Replace variables in sourced SQL
         */
        protected function replaceVars( $ins ) {
                $ins = $this->replaceSchemaVars( $ins );

                // Table prefixes
                $ins = preg_replace_callback( '!/\*(?:\$wgDBprefix|_)\*/([a-zA-Z_0-9]*)!',
                        array( $this, 'tableNameCallback' ), $ins );

                // Index names
                $ins = preg_replace_callback( '!/\*i\*/([a-zA-Z_0-9]*)!',
                        array( $this, 'indexNameCallback' ), $ins );

                return $ins;
        }

        /**
         * Get schema variables. If none have been set via setSchemaVars(), then
         * use some defaults from the current object.
         */
        protected function getSchemaVars() {
                if ( $this->mSchemaVars ) {
                        return $this->mSchemaVars;
                } else {
                        return $this->getDefaultSchemaVars();
                }
        }

        /**
         * Get schema variables to use if none have been set via setSchemaVars().
         *
         * Override this in derived classes to provide variables for tables.sql
         * and SQL patch files.
         */
        protected function getDefaultSchemaVars() {
                return array();
        }

        /**
         * Table name callback
         * @private
         */
        protected function tableNameCallback( $matches ) {
                return $this->tableName( $matches[1] );
        }

        /**
         * Index name callback
         */
        protected function indexNameCallback( $matches ) {
                return $this->indexName( $matches[1] );
        }

        /**
         * Build a concatenation list to feed into a SQL query
         * @param $stringList Array: list of raw SQL expressions; caller is responsible for any quoting
         * @return String
         */
        function buildConcat( $stringList ) {
                return 'CONCAT(' . implode( ',', $stringList ) . ')';
        }

        /**
         * Acquire a named lock
         *
         * Abstracted from Filestore::lock() so child classes can implement for
         * their own needs.
         *
         * @param $lockName String: name of lock to aquire
         * @param $method String: name of method calling us
         * @param $timeout Integer: timeout
         * @return Boolean
         */
        public function lock( $lockName, $method, $timeout = 5 ) {
                return true;
        }

        /**
         * Release a lock.
         *
         * @param $lockName String: Name of lock to release
         * @param $method String: Name of method calling us
         *
         * @return Returns 1 if the lock was released, 0 if the lock was not established
         * by this thread (in which case the lock is not released), and NULL if the named
         * lock did not exist
         */
        public function unlock( $lockName, $method ) {
                return true;
        }

        /**
         * Lock specific tables
         *
         * @param $read Array of tables to lock for read access
         * @param $write Array of tables to lock for write access
         * @param $method String name of caller
         * @param $lowPriority bool Whether to indicate writes to be LOW PRIORITY
         */
        public function lockTables( $read, $write, $method, $lowPriority = true ) {
                return true;
        }

        /**
         * Unlock specific tables
         *
         * @param $method String the caller
         */
        public function unlockTables( $method ) {
                return true;
        }

        /**
         * Delete a table
         */
        public function dropTable( $tableName, $fName = 'DatabaseBase::dropTable' ) {
                if( !$this->tableExists( $tableName ) ) {
                        return false;
                }
                $sql = "DROP TABLE " . $this->tableName( $tableName );
                if( $this->cascadingDeletes() ) {
                        $sql .= " CASCADE";
                }
                return $this->query( $sql, $fName );
        }

        /**
         * Get search engine class. All subclasses of this need to implement this
         * if they wish to use searching.
         *
         * @return String
         */
        public function getSearchEngine() {
                return 'SearchEngineDummy';
        }

        /**
         * Find out when 'infinity' is. Most DBMSes support this. This is a special
         * keyword for timestamps in PostgreSQL, and works with CHAR(14) as well
         * because "i" sorts after all numbers.
         *
         * @return String
         */
        public function getInfinity() {
                return 'infinity';
        }

        /**
         * Encode an expiry time
         *
         * @param $expiry String: timestamp for expiry, or the 'infinity' string
         * @return String
         */
        public function encodeExpiry( $expiry ) {
                if ( $expiry == '' || $expiry == $this->getInfinity() ) {
                        return $this->getInfinity();
                } else {
                        return $this->timestamp( $expiry );
                }
        }

        /**
         * Allow or deny "big selects" for this session only. This is done by setting
         * the sql_big_selects session variable.
         *
         * This is a MySQL-specific feature.
         *
         * @param $value Mixed: true for allow, false for deny, or "default" to 
         *   restore the initial value
         */
        public function setBigSelects( $value = true ) {
                // no-op
        }
}