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class DatabaseSchema_mysql in Drupal 7

Hierarchy

Expanded class hierarchy of DatabaseSchema_mysql

Related topics

Schema API
API to handle database schemas.

File

includes/database/mysql/schema.inc, line 14
Database schema code for MySQL database servers.

View source 
class DatabaseSchema_mysql extends DatabaseSchema {

  /**
   * Maximum length of a table comment in MySQL.
   */
  const COMMENT_MAX_TABLE = 60;

  /**
   * Maximum length of a column comment in MySQL.
   */
  const COMMENT_MAX_COLUMN = 255;

  /**
   * Get information about the table and database name from the prefix.
   *
   * @return
   *   A keyed array with information about the database, table name and prefix.
   */
  protected function getPrefixInfo($table = 'default', $add_prefix = TRUE) {
    $info = array(
      'prefix' => $this->connection
        ->tablePrefix($table),
    );
    if ($add_prefix) {
      $table = $info['prefix'] . $table;
    }
    if (($pos = strpos($table, '.')) !== FALSE) {
      $info['database'] = substr($table, 0, $pos);
      $info['table'] = substr($table, ++$pos);
    }
    else {
      $db_info = $this->connection
        ->getConnectionOptions();
      $info['database'] = $db_info['database'];
      $info['table'] = $table;
    }
    return $info;
  }

  /**
   * Build a condition to match a table name against a standard information_schema.
   *
   * MySQL uses databases like schemas rather than catalogs so when we build
   * a condition to query the information_schema.tables, we set the default
   * database as the schema unless specified otherwise, and exclude table_catalog
   * from the condition criteria.
   */
  protected function buildTableNameCondition($table_name, $operator = '=', $add_prefix = TRUE) {
    $info = $this->connection
      ->getConnectionOptions();

    // Ensure the table name is not surrounded with quotes as that is not
    // appropriate for schema queries.
    $quote_char = variable_get('mysql_identifier_quote_character', MYSQL_IDENTIFIER_QUOTE_CHARACTER_DEFAULT);
    $table_name = str_replace($quote_char, '', $table_name);
    $table_info = $this
      ->getPrefixInfo($table_name, $add_prefix);
    $condition = new DatabaseCondition('AND');
    $condition
      ->condition('table_schema', $table_info['database']);
    $condition
      ->condition('table_name', $table_info['table'], $operator);
    return $condition;
  }

  /**
   * Generate SQL to create a new table from a Drupal schema definition.
   *
   * @param $name
   *   The name of the table to create.
   * @param $table
   *   A Schema API table definition array.
   * @return
   *   An array of SQL statements to create the table.
   */
  protected function createTableSql($name, $table) {
    $info = $this->connection
      ->getConnectionOptions();

    // Provide defaults if needed.
    $table += array(
      'mysql_engine' => 'InnoDB',
      // Allow the default charset to be overridden in settings.php.
      'mysql_character_set' => $this->connection
        ->utf8mb4IsActive() ? 'utf8mb4' : 'utf8',
    );
    $sql = "CREATE TABLE {" . $name . "} (\n";

    // Add the SQL statement for each field.
    foreach ($table['fields'] as $field_name => $field) {
      $sql .= $this
        ->createFieldSql($field_name, $this
        ->processField($field)) . ", \n";
    }

    // Process keys & indexes.
    $keys = $this
      ->createKeysSql($table);
    if (count($keys)) {
      $sql .= implode(", \n", $keys) . ", \n";
    }

    // Remove the last comma and space.
    $sql = substr($sql, 0, -3) . "\n) ";
    $sql .= 'ENGINE = ' . $table['mysql_engine'] . ' DEFAULT CHARACTER SET ' . $table['mysql_character_set'];

    // By default, MySQL uses the default collation for new tables, which is
    // 'utf8_general_ci' for utf8. If an alternate collation has been set, it
    // needs to be explicitly specified.
    // @see DatabaseConnection_mysql
    if (!empty($info['collation'])) {
      $sql .= ' COLLATE ' . $info['collation'];
    }

    // The row format needs to be either DYNAMIC or COMPRESSED in order to allow
    // for the innodb_large_prefix setting to take effect, see
    // https://dev.mysql.com/doc/refman/5.6/en/create-table.html
    if ($this->connection
      ->utf8mb4IsActive()) {
      $sql .= ' ROW_FORMAT=DYNAMIC';
    }

    // Add table comment.
    if (!empty($table['description'])) {
      $sql .= ' COMMENT ' . $this
        ->prepareComment($table['description'], self::COMMENT_MAX_TABLE);
    }
    return array(
      $sql,
    );
  }

  /**
   * Create an SQL string for a field to be used in table creation or alteration.
   *
   * Before passing a field out of a schema definition into this function it has
   * to be processed by _db_process_field().
   *
   * @param $name
   *   Name of the field.
   * @param $spec
   *   The field specification, as per the schema data structure format.
   */
  protected function createFieldSql($name, $spec) {
    $sql = "`" . $name . "` " . $spec['mysql_type'];
    if (in_array($spec['mysql_type'], array(
      'VARCHAR',
      'CHAR',
      'TINYTEXT',
      'MEDIUMTEXT',
      'LONGTEXT',
      'TEXT',
    ))) {
      if (isset($spec['length'])) {
        $sql .= '(' . $spec['length'] . ')';
      }
      if (!empty($spec['binary'])) {
        $sql .= ' BINARY';
      }
    }
    elseif (isset($spec['precision']) && isset($spec['scale'])) {
      $sql .= '(' . $spec['precision'] . ', ' . $spec['scale'] . ')';
    }
    if (!empty($spec['unsigned'])) {
      $sql .= ' unsigned';
    }
    if (isset($spec['not null'])) {
      if ($spec['not null']) {
        $sql .= ' NOT NULL';
      }
      else {
        $sql .= ' NULL';
      }
    }
    if (!empty($spec['auto_increment'])) {
      $sql .= ' auto_increment';
    }

    // $spec['default'] can be NULL, so we explicitly check for the key here.
    if (array_key_exists('default', $spec)) {
      if (is_string($spec['default'])) {
        $spec['default'] = "'" . $spec['default'] . "'";
      }
      elseif (!isset($spec['default'])) {
        $spec['default'] = 'NULL';
      }
      $sql .= ' DEFAULT ' . $spec['default'];
    }
    if (empty($spec['not null']) && !isset($spec['default'])) {
      $sql .= ' DEFAULT NULL';
    }

    // Add column comment.
    if (!empty($spec['description'])) {
      $sql .= ' COMMENT ' . $this
        ->prepareComment($spec['description'], self::COMMENT_MAX_COLUMN);
    }
    return $sql;
  }

  /**
   * Set database-engine specific properties for a field.
   *
   * @param $field
   *   A field description array, as specified in the schema documentation.
   */
  protected function processField($field) {
    if (!isset($field['size'])) {
      $field['size'] = 'normal';
    }

    // Set the correct database-engine specific datatype.
    // In case one is already provided, force it to uppercase.
    if (isset($field['mysql_type'])) {
      $field['mysql_type'] = drupal_strtoupper($field['mysql_type']);
    }
    else {
      $map = $this
        ->getFieldTypeMap();
      $field['mysql_type'] = $map[$field['type'] . ':' . $field['size']];
    }
    if (isset($field['type']) && $field['type'] == 'serial') {
      $field['auto_increment'] = TRUE;
    }
    return $field;
  }
  public function getFieldTypeMap() {

    // Put :normal last so it gets preserved by array_flip. This makes
    // it much easier for modules (such as schema.module) to map
    // database types back into schema types.
    // $map does not use drupal_static as its value never changes.
    static $map = array(
      'varchar:normal' => 'VARCHAR',
      'char:normal' => 'CHAR',
      'text:tiny' => 'TINYTEXT',
      'text:small' => 'TINYTEXT',
      'text:medium' => 'MEDIUMTEXT',
      'text:big' => 'LONGTEXT',
      'text:normal' => 'TEXT',
      'serial:tiny' => 'TINYINT',
      'serial:small' => 'SMALLINT',
      'serial:medium' => 'MEDIUMINT',
      'serial:big' => 'BIGINT',
      'serial:normal' => 'INT',
      'int:tiny' => 'TINYINT',
      'int:small' => 'SMALLINT',
      'int:medium' => 'MEDIUMINT',
      'int:big' => 'BIGINT',
      'int:normal' => 'INT',
      'float:tiny' => 'FLOAT',
      'float:small' => 'FLOAT',
      'float:medium' => 'FLOAT',
      'float:big' => 'DOUBLE',
      'float:normal' => 'FLOAT',
      'numeric:normal' => 'DECIMAL',
      'blob:big' => 'LONGBLOB',
      'blob:normal' => 'BLOB',
    );
    return $map;
  }
  protected function createKeysSql($spec) {
    $keys = array();
    if (!empty($spec['primary key'])) {
      $keys[] = 'PRIMARY KEY (' . $this
        ->createKeysSqlHelper($spec['primary key']) . ')';
    }
    if (!empty($spec['unique keys'])) {
      foreach ($spec['unique keys'] as $key => $fields) {
        $keys[] = 'UNIQUE KEY `' . $key . '` (' . $this
          ->createKeysSqlHelper($fields) . ')';
      }
    }
    if (!empty($spec['indexes'])) {
      foreach ($spec['indexes'] as $index => $fields) {
        $keys[] = 'INDEX `' . $index . '` (' . $this
          ->createKeysSqlHelper($fields) . ')';
      }
    }
    return $keys;
  }
  protected function createKeySql($fields) {
    $return = array();
    foreach ($fields as $field) {
      if (is_array($field)) {
        $return[] = '`' . $field[0] . '`(' . $field[1] . ')';
      }
      else {
        $return[] = '`' . $field . '`';
      }
    }
    return implode(', ', $return);
  }
  protected function createKeysSqlHelper($fields) {
    $return = array();
    foreach ($fields as $field) {
      if (is_array($field)) {
        $return[] = '`' . $field[0] . '`(' . $field[1] . ')';
      }
      else {
        $return[] = '`' . $field . '`';
      }
    }
    return implode(', ', $return);
  }
  public function renameTable($table, $new_name) {
    if (!$this
      ->tableExists($table)) {
      throw new DatabaseSchemaObjectDoesNotExistException(t("Cannot rename @table to @table_new: table @table doesn't exist.", array(
        '@table' => $table,
        '@table_new' => $new_name,
      )));
    }
    if ($this
      ->tableExists($new_name)) {
      throw new DatabaseSchemaObjectExistsException(t("Cannot rename @table to @table_new: table @table_new already exists.", array(
        '@table' => $table,
        '@table_new' => $new_name,
      )));
    }
    $info = $this
      ->getPrefixInfo($new_name);
    return $this->connection
      ->query('ALTER TABLE {' . $table . '} RENAME TO `' . $info['table'] . '`');
  }
  public function dropTable($table) {
    if (!$this
      ->tableExists($table)) {
      return FALSE;
    }
    $this->connection
      ->query('DROP TABLE {' . $table . '}');
    return TRUE;
  }
  public function addField($table, $field, $spec, $keys_new = array()) {
    if (!$this
      ->tableExists($table)) {
      throw new DatabaseSchemaObjectDoesNotExistException(t("Cannot add field @table.@field: table doesn't exist.", array(
        '@field' => $field,
        '@table' => $table,
      )));
    }
    if ($this
      ->fieldExists($table, $field)) {
      throw new DatabaseSchemaObjectExistsException(t("Cannot add field @table.@field: field already exists.", array(
        '@field' => $field,
        '@table' => $table,
      )));
    }
    $fixnull = FALSE;
    if (!empty($spec['not null']) && !isset($spec['default'])) {
      $fixnull = TRUE;
      $spec['not null'] = FALSE;
    }
    $query = 'ALTER TABLE {' . $table . '} ADD ';
    $query .= $this
      ->createFieldSql($field, $this
      ->processField($spec));
    if ($keys_sql = $this
      ->createKeysSql($keys_new)) {
      $query .= ', ADD ' . implode(', ADD ', $keys_sql);
    }
    $this->connection
      ->query($query);
    if (isset($spec['initial'])) {
      $this->connection
        ->update($table)
        ->fields(array(
        $field => $spec['initial'],
      ))
        ->execute();
    }
    if ($fixnull) {
      $spec['not null'] = TRUE;
      $this
        ->changeField($table, $field, $field, $spec);
    }
  }
  public function dropField($table, $field) {
    if (!$this
      ->fieldExists($table, $field)) {
      return FALSE;
    }
    $this->connection
      ->query('ALTER TABLE {' . $table . '} DROP `' . $field . '`');
    return TRUE;
  }
  public function fieldSetDefault($table, $field, $default) {
    if (!$this
      ->fieldExists($table, $field)) {
      throw new DatabaseSchemaObjectDoesNotExistException(t("Cannot set default value of field @table.@field: field doesn't exist.", array(
        '@table' => $table,
        '@field' => $field,
      )));
    }
    if (!isset($default)) {
      $default = 'NULL';
    }
    else {
      $default = is_string($default) ? "'{$default}'" : $default;
    }
    $this->connection
      ->query('ALTER TABLE {' . $table . '} ALTER COLUMN `' . $field . '` SET DEFAULT ' . $default);
  }
  public function fieldSetNoDefault($table, $field) {
    if (!$this
      ->fieldExists($table, $field)) {
      throw new DatabaseSchemaObjectDoesNotExistException(t("Cannot remove default value of field @table.@field: field doesn't exist.", array(
        '@table' => $table,
        '@field' => $field,
      )));
    }
    $this->connection
      ->query('ALTER TABLE {' . $table . '} ALTER COLUMN `' . $field . '` DROP DEFAULT');
  }
  public function indexExists($table, $name) {

    // Returns one row for each column in the index. Result is string or FALSE.
    // Details at http://dev.mysql.com/doc/refman/5.0/en/show-index.html
    $row = $this->connection
      ->query('SHOW INDEX FROM {' . $table . "} WHERE key_name = '{$name}'")
      ->fetchAssoc();
    return isset($row['Key_name']);
  }
  public function addPrimaryKey($table, $fields) {
    if (!$this
      ->tableExists($table)) {
      throw new DatabaseSchemaObjectDoesNotExistException(t("Cannot add primary key to table @table: table doesn't exist.", array(
        '@table' => $table,
      )));
    }
    if ($this
      ->indexExists($table, 'PRIMARY')) {
      throw new DatabaseSchemaObjectExistsException(t("Cannot add primary key to table @table: primary key already exists.", array(
        '@table' => $table,
      )));
    }
    $this->connection
      ->query('ALTER TABLE {' . $table . '} ADD PRIMARY KEY (' . $this
      ->createKeySql($fields) . ')');
  }
  public function dropPrimaryKey($table) {
    if (!$this
      ->indexExists($table, 'PRIMARY')) {
      return FALSE;
    }
    $this->connection
      ->query('ALTER TABLE {' . $table . '} DROP PRIMARY KEY');
    return TRUE;
  }
  public function addUniqueKey($table, $name, $fields) {
    if (!$this
      ->tableExists($table)) {
      throw new DatabaseSchemaObjectDoesNotExistException(t("Cannot add unique key @name to table @table: table doesn't exist.", array(
        '@table' => $table,
        '@name' => $name,
      )));
    }
    if ($this
      ->indexExists($table, $name)) {
      throw new DatabaseSchemaObjectExistsException(t("Cannot add unique key @name to table @table: unique key already exists.", array(
        '@table' => $table,
        '@name' => $name,
      )));
    }
    $this->connection
      ->query('ALTER TABLE {' . $table . '} ADD UNIQUE KEY `' . $name . '` (' . $this
      ->createKeySql($fields) . ')');
  }
  public function dropUniqueKey($table, $name) {
    if (!$this
      ->indexExists($table, $name)) {
      return FALSE;
    }
    $this->connection
      ->query('ALTER TABLE {' . $table . '} DROP KEY `' . $name . '`');
    return TRUE;
  }
  public function addIndex($table, $name, $fields) {
    if (!$this
      ->tableExists($table)) {
      throw new DatabaseSchemaObjectDoesNotExistException(t("Cannot add index @name to table @table: table doesn't exist.", array(
        '@table' => $table,
        '@name' => $name,
      )));
    }
    if ($this
      ->indexExists($table, $name)) {
      throw new DatabaseSchemaObjectExistsException(t("Cannot add index @name to table @table: index already exists.", array(
        '@table' => $table,
        '@name' => $name,
      )));
    }
    $this->connection
      ->query('ALTER TABLE {' . $table . '} ADD INDEX `' . $name . '` (' . $this
      ->createKeySql($fields) . ')');
  }
  public function dropIndex($table, $name) {
    if (!$this
      ->indexExists($table, $name)) {
      return FALSE;
    }
    $this->connection
      ->query('ALTER TABLE {' . $table . '} DROP INDEX `' . $name . '`');
    return TRUE;
  }
  public function changeField($table, $field, $field_new, $spec, $keys_new = array()) {
    if (!$this
      ->fieldExists($table, $field)) {
      throw new DatabaseSchemaObjectDoesNotExistException(t("Cannot change the definition of field @table.@name: field doesn't exist.", array(
        '@table' => $table,
        '@name' => $field,
      )));
    }
    if ($field != $field_new && $this
      ->fieldExists($table, $field_new)) {
      throw new DatabaseSchemaObjectExistsException(t("Cannot rename field @table.@name to @name_new: target field already exists.", array(
        '@table' => $table,
        '@name' => $field,
        '@name_new' => $field_new,
      )));
    }
    $sql = 'ALTER TABLE {' . $table . '} CHANGE `' . $field . '` ' . $this
      ->createFieldSql($field_new, $this
      ->processField($spec));
    if ($keys_sql = $this
      ->createKeysSql($keys_new)) {
      $sql .= ', ADD ' . implode(', ADD ', $keys_sql);
    }
    $this->connection
      ->query($sql);
  }
  public function prepareComment($comment, $length = NULL) {

    // Work around a bug in some versions of PDO, see http://bugs.php.net/bug.php?id=41125
    $comment = str_replace("'", '’', $comment);

    // Truncate comment to maximum comment length.
    if (isset($length)) {

      // Add table prefixes before truncating.
      $comment = truncate_utf8($this->connection
        ->prefixTables($comment), $length, TRUE, TRUE);
    }
    return $this->connection
      ->quote($comment);
  }

  /**
   * Retrieve a table or column comment.
   */
  public function getComment($table, $column = NULL) {
    $condition = $this
      ->buildTableNameCondition($table);
    if (isset($column)) {
      $condition
        ->condition('column_name', $column);
      $condition
        ->compile($this->connection, $this);

      // Don't use {} around information_schema.columns table.
      return $this->connection
        ->query("SELECT column_comment AS column_comment FROM information_schema.columns WHERE " . (string) $condition, $condition
        ->arguments())
        ->fetchField();
    }
    $condition
      ->compile($this->connection, $this);

    // Don't use {} around information_schema.tables table.
    $comment = $this->connection
      ->query("SELECT table_comment AS table_comment FROM information_schema.tables WHERE " . (string) $condition, $condition
      ->arguments())
      ->fetchField();

    // Work-around for MySQL 5.0 bug http://bugs.mysql.com/bug.php?id=11379
    return preg_replace('/; InnoDB free:.*$/', '', $comment);
  }
  public function tableExists($table) {

    // The information_schema table is very slow to query under MySQL 5.0.
    // Instead, we try to select from the table in question.  If it fails,
    // the most likely reason is that it does not exist. That is dramatically
    // faster than using information_schema.
    // @link http://bugs.mysql.com/bug.php?id=19588
    // @todo: This override should be removed once we require a version of MySQL
    // that has that bug fixed.
    try {
      $this->connection
        ->queryRange("SELECT 1 FROM {" . $table . "}", 0, 1);
      return TRUE;
    } catch (Exception $e) {
      return FALSE;
    }
  }
  public function fieldExists($table, $column) {

    // The information_schema table is very slow to query under MySQL 5.0.
    // Instead, we try to select from the table and field in question. If it
    // fails, the most likely reason is that it does not exist. That is
    // dramatically faster than using information_schema.
    // @link http://bugs.mysql.com/bug.php?id=19588
    // @todo: This override should be removed once we require a version of MySQL
    // that has that bug fixed.
    try {
      $this->connection
        ->queryRange("SELECT {$column} FROM {" . $table . "}", 0, 1);
      return TRUE;
    } catch (Exception $e) {
      return FALSE;
    }
  }

}

Members

Namesort descending Modifiers Type Description Overrides
DatabaseSchema::$connection protected property The database connection.
DatabaseSchema::$defaultSchema protected property Definition of prefixInfo array structure. 1
DatabaseSchema::$placeholder protected property The placeholder counter.
DatabaseSchema::$uniqueIdentifier protected property A unique identifier for this query object.
DatabaseSchema::createTable public function Create a new table from a Drupal table definition.
DatabaseSchema::fieldNames public function Return an array of field names from an array of key/index column specifiers.
DatabaseSchema::findTables public function Find all tables that are like the specified base table name. 1
DatabaseSchema::findTablesD8 public function Finds all tables that are like the specified base table name. This is a backport of the change made to findTables in Drupal 8 to work with virtual, un-prefixed table names. The original function is retained for Backwards Compatibility. 1
DatabaseSchema::nextPlaceholder public function Implements QueryPlaceHolderInterface::nextPlaceholder(). Overrides QueryPlaceholderInterface::nextPlaceholder
DatabaseSchema::prefixNonTable function Create names for indexes, primary keys and constraints.
DatabaseSchema::uniqueIdentifier public function Implements QueryPlaceHolderInterface::uniqueIdentifier(). Overrides QueryPlaceholderInterface::uniqueIdentifier
DatabaseSchema::__clone public function Implements the magic __clone function.
DatabaseSchema::__construct public function
DatabaseSchema_mysql::addField public function Add a new field to a table. Overrides DatabaseSchema::addField
DatabaseSchema_mysql::addIndex public function Add an index. Overrides DatabaseSchema::addIndex
DatabaseSchema_mysql::addPrimaryKey public function Add a primary key. Overrides DatabaseSchema::addPrimaryKey
DatabaseSchema_mysql::addUniqueKey public function Add a unique key. Overrides DatabaseSchema::addUniqueKey
DatabaseSchema_mysql::buildTableNameCondition protected function Build a condition to match a table name against a standard information_schema. Overrides DatabaseSchema::buildTableNameCondition
DatabaseSchema_mysql::changeField public function Change a field definition. Overrides DatabaseSchema::changeField
DatabaseSchema_mysql::COMMENT_MAX_COLUMN constant Maximum length of a column comment in MySQL.
DatabaseSchema_mysql::COMMENT_MAX_TABLE constant Maximum length of a table comment in MySQL.
DatabaseSchema_mysql::createFieldSql protected function Create an SQL string for a field to be used in table creation or alteration.
DatabaseSchema_mysql::createKeySql protected function
DatabaseSchema_mysql::createKeysSql protected function
DatabaseSchema_mysql::createKeysSqlHelper protected function
DatabaseSchema_mysql::createTableSql protected function Generate SQL to create a new table from a Drupal schema definition.
DatabaseSchema_mysql::dropField public function Drop a field. Overrides DatabaseSchema::dropField
DatabaseSchema_mysql::dropIndex public function Drop an index. Overrides DatabaseSchema::dropIndex
DatabaseSchema_mysql::dropPrimaryKey public function Drop the primary key. Overrides DatabaseSchema::dropPrimaryKey
DatabaseSchema_mysql::dropTable public function Drop a table. Overrides DatabaseSchema::dropTable
DatabaseSchema_mysql::dropUniqueKey public function Drop a unique key. Overrides DatabaseSchema::dropUniqueKey
DatabaseSchema_mysql::fieldExists public function Check if a column exists in the given table. Overrides DatabaseSchema::fieldExists
DatabaseSchema_mysql::fieldSetDefault public function Set the default value for a field. Overrides DatabaseSchema::fieldSetDefault
DatabaseSchema_mysql::fieldSetNoDefault public function Set a field to have no default value. Overrides DatabaseSchema::fieldSetNoDefault
DatabaseSchema_mysql::getComment public function Retrieve a table or column comment.
DatabaseSchema_mysql::getFieldTypeMap public function Returns a mapping of Drupal schema field names to DB-native field types. Overrides DatabaseSchema::getFieldTypeMap
DatabaseSchema_mysql::getPrefixInfo protected function Get information about the table and database name from the prefix. Overrides DatabaseSchema::getPrefixInfo
DatabaseSchema_mysql::indexExists public function Checks if an index exists in the given table. Overrides DatabaseSchema::indexExists
DatabaseSchema_mysql::prepareComment public function Prepare a table or column comment for database query. Overrides DatabaseSchema::prepareComment
DatabaseSchema_mysql::processField protected function Set database-engine specific properties for a field.
DatabaseSchema_mysql::renameTable public function Rename a table. Overrides DatabaseSchema::renameTable
DatabaseSchema_mysql::tableExists public function Check if a table exists. Overrides DatabaseSchema::tableExists