class Schema

1. 8.x drupal/core/lib/Drupal/Core/Database/Schema.php \Drupal\Core\Database\Schema
2. 8.x drupal/core/lib/Drupal/Core/Database/Driver/mysql/Schema.php \Drupal\Core\Database\Driver\mysql\Schema
3. 8.x drupal/core/lib/Drupal/Core/Database/Driver/sqlite/Schema.php \Drupal\Core\Database\Driver\sqlite\Schema
4. 8.x drupal/core/lib/Drupal/Core/Database/Driver/pgsql/Schema.php \Drupal\Core\Database\Driver\pgsql\Schema

Connection::schema in drupal/core/lib/Drupal/Core/Database/Connection.php

Returns a DatabaseSchema object for manipulating the schema.

class Schema extends DatabaseSchema {

  /**
   * A cache of information about blob columns and sequences of tables.
   *
   * This is collected by DatabaseConnection_pgsql->queryTableInformation(),
   * by introspecting the database.
   *
   * @see DatabaseConnection_pgsql->queryTableInformation()
   * @var array
   */
  protected $tableInformation = array();

  /**
   * Fetch the list of blobs and sequences used on a table.
   *
   * We introspect the database to collect the information required by insert
   * and update queries.
   *
   * @param $table_name
   *   The non-prefixed name of the table.
   * @return
   *   An object with two member variables:
   *     - 'blob_fields' that lists all the blob fields in the table.
   *     - 'sequences' that lists the sequences used in that table.
   */
  public function queryTableInformation($table) {

    // Generate a key to reference this table's information on.
    $key = $this->connection
      ->prefixTables('{' . $table . '}');
    if (!strpos($key, '.')) {
      $key = 'public.' . $key;
    }
    if (!isset($this->tableInformation[$key])) {

      // Split the key into schema and table for querying.
      list($schema, $table_name) = explode('.', $key);
      $table_information = (object) array(
        'blob_fields' => array(),
        'sequences' => array(),
      );

      // Don't use {} around information_schema.columns table.
      $result = $this->connection
        ->query("SELECT column_name, data_type, column_default FROM information_schema.columns WHERE table_schema = :schema AND table_name = :table AND (data_type = 'bytea' OR (numeric_precision IS NOT NULL AND column_default LIKE :default))", array(
        ':schema' => $schema,
        ':table' => $table_name,
        ':default' => '%nextval%',
      ));
      foreach ($result as $column) {
        if ($column->data_type == 'bytea') {
          $table_information->blob_fields[$column->column_name] = TRUE;
        }
        elseif (preg_match("/nextval\\('([^']+)'/", $column->column_default, $matches)) {

          // We must know of any sequences in the table structure to help us
          // return the last insert id. If there is more than 1 sequences the
          // first one (index 0 of the sequences array) will be used.
          $table_information->sequences[] = $matches[1];
          $table_information->serial_fields[] = $column->column_name;
        }
      }
      $this->tableInformation[$key] = $table_information;
    }
    return $this->tableInformation[$key];
  }

  /**
   * Fetch the list of CHECK constraints used on a field.
   *
   * We introspect the database to collect the information required by field
   * alteration.
   *
   * @param $table
   *   The non-prefixed name of the table.
   * @param $field
   *   The name of the field.
   * @return
   *   An array of all the checks for the field.
   */
  public function queryFieldInformation($table, $field) {
    $prefixInfo = $this
      ->getPrefixInfo($table, TRUE);

    // Split the key into schema and table for querying.
    $schema = $prefixInfo['schema'];
    $table_name = $prefixInfo['table'];
    $field_information = (object) array(
      'checks' => array(),
    );
    $checks = $this->connection
      ->query("SELECT conname FROM pg_class cl INNER JOIN pg_constraint co ON co.conrelid = cl.oid INNER JOIN pg_attribute attr ON attr.attrelid = cl.oid AND attr.attnum = ANY (co.conkey) INNER JOIN pg_namespace ns ON cl.relnamespace = ns.oid WHERE co.contype = 'c' AND ns.nspname = :schema AND cl.relname = :table AND attr.attname = :column", array(
      ':schema' => $schema,
      ':table' => $table_name,
      ':column' => $field,
    ));
    $field_information = $checks
      ->fetchCol();
    return $field_information;
  }

  /**
   * 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) {
    $sql_fields = array();
    foreach ($table['fields'] as $field_name => $field) {
      $sql_fields[] = $this
        ->createFieldSql($field_name, $this
        ->processField($field));
    }
    $sql_keys = array();
    if (isset($table['primary key']) && is_array($table['primary key'])) {
      $sql_keys[] = 'PRIMARY KEY (' . implode(', ', $table['primary key']) . ')';
    }
    if (isset($table['unique keys']) && is_array($table['unique keys'])) {
      foreach ($table['unique keys'] as $key_name => $key) {
        $sql_keys[] = 'CONSTRAINT ' . $this
          ->prefixNonTable($name, $key_name, 'key') . ' UNIQUE (' . implode(', ', $key) . ')';
      }
    }
    $sql = "CREATE TABLE {" . $name . "} (\n\t";
    $sql .= implode(",\n\t", $sql_fields);
    if (count($sql_keys) > 0) {
      $sql .= ",\n\t";
    }
    $sql .= implode(",\n\t", $sql_keys);
    $sql .= "\n)";
    $statements[] = $sql;
    if (isset($table['indexes']) && is_array($table['indexes'])) {
      foreach ($table['indexes'] as $key_name => $key) {
        $statements[] = $this
          ->_createIndexSql($name, $key_name, $key);
      }
    }

    // Add table comment.
    if (!empty($table['description'])) {
      $statements[] = 'COMMENT ON TABLE {' . $name . '} IS ' . $this
        ->prepareComment($table['description']);
    }

    // Add column comments.
    foreach ($table['fields'] as $field_name => $field) {
      if (!empty($field['description'])) {
        $statements[] = 'COMMENT ON COLUMN {' . $name . '}.' . $field_name . ' IS ' . $this
          ->prepareComment($field['description']);
      }
    }
    return $statements;
  }

  /**
   * 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 . ' ' . $spec['pgsql_type'];
    if (isset($spec['type']) && $spec['type'] == 'serial') {
      unset($spec['not null']);
    }
    if (in_array($spec['pgsql_type'], array(
      'varchar',
      'character',
      'text',
    )) && isset($spec['length'])) {
      $sql .= '(' . $spec['length'] . ')';
    }
    elseif (isset($spec['precision']) && isset($spec['scale'])) {
      $sql .= '(' . $spec['precision'] . ', ' . $spec['scale'] . ')';
    }
    if (!empty($spec['unsigned'])) {
      $sql .= " CHECK ({$name} >= 0)";
    }
    if (isset($spec['not null'])) {
      if ($spec['not null']) {
        $sql .= ' NOT NULL';
      }
      else {
        $sql .= ' NULL';
      }
    }
    if (isset($spec['default'])) {
      $default = is_string($spec['default']) ? "'" . $spec['default'] . "'" : $spec['default'];
      $sql .= " default {$default}";
    }
    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 lowercase.
    if (isset($field['pgsql_type'])) {
      $field['pgsql_type'] = drupal_strtolower($field['pgsql_type']);
    }
    else {
      $map = $this
        ->getFieldTypeMap();
      $field['pgsql_type'] = $map[$field['type'] . ':' . $field['size']];
    }
    if (!empty($field['unsigned'])) {

      // Unsigned datatypes are not supported in PostgreSQL 8.3. In MySQL,
      // they are used to ensure a positive number is inserted and it also
      // doubles the maximum integer size that can be stored in a field.
      // The PostgreSQL schema in Drupal creates a check constraint
      // to ensure that a value inserted is >= 0. To provide the extra
      // integer capacity, here, we bump up the column field size.
      if (!isset($map)) {
        $map = $this
          ->getFieldTypeMap();
      }
      switch ($field['pgsql_type']) {
        case 'smallint':
          $field['pgsql_type'] = $map['int:medium'];
          break;
        case 'int':
          $field['pgsql_type'] = $map['int:big'];
          break;
      }
    }
    if (isset($field['type']) && $field['type'] == 'serial') {
      unset($field['not null']);
    }
    return $field;
  }

  /**
   * This maps a generic data type in combination with its data size
   * to the engine-specific data type.
   */
  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' => 'character',
      'text:tiny' => 'text',
      'text:small' => 'text',
      'text:medium' => 'text',
      'text:big' => 'text',
      'text:normal' => 'text',
      'int:tiny' => 'smallint',
      'int:small' => 'smallint',
      'int:medium' => 'int',
      'int:big' => 'bigint',
      'int:normal' => 'int',
      'float:tiny' => 'real',
      'float:small' => 'real',
      'float:medium' => 'real',
      'float:big' => 'double precision',
      'float:normal' => 'real',
      'numeric:normal' => 'numeric',
      'blob:big' => 'bytea',
      'blob:normal' => 'bytea',
      'serial:tiny' => 'serial',
      'serial:small' => 'serial',
      'serial:medium' => 'serial',
      'serial:big' => 'bigserial',
      'serial:normal' => 'serial',
    );
    return $map;
  }
  protected function _createKeySql($fields) {
    $return = array();
    foreach ($fields as $field) {
      if (is_array($field)) {
        $return[] = 'substr(' . $field[0] . ', 1, ' . $field[1] . ')';
      }
      else {
        $return[] = '"' . $field . '"';
      }
    }
    return implode(', ', $return);
  }
  function renameTable($table, $new_name) {
    if (!$this
      ->tableExists($table)) {
      throw new SchemaObjectDoesNotExistException(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 SchemaObjectExistsException(t("Cannot rename @table to @table_new: table @table_new already exists.", array(
        '@table' => $table,
        '@table_new' => $new_name,
      )));
    }

    // Get the schema and tablename for the old table.
    $old_full_name = $this->connection
      ->prefixTables('{' . $table . '}');
    list($old_schema, $old_table_name) = strpos($old_full_name, '.') ? explode('.', $old_full_name) : array(
      'public',
      $old_full_name,
    );

    // Index names and constraint names are global in PostgreSQL, so we need to
    // rename them when renaming the table.
    $indexes = $this->connection
      ->query('SELECT indexname FROM pg_indexes WHERE schemaname = :schema AND tablename = :table', array(
      ':schema' => $old_schema,
      ':table' => $old_table_name,
    ));
    foreach ($indexes as $index) {
      if (preg_match('/^' . preg_quote($old_full_name) . '_(.*)$/', $index->indexname, $matches)) {
        $index_name = $matches[1];
        $this->connection
          ->query('ALTER INDEX ' . $index->indexname . ' RENAME TO {' . $new_name . '}_' . $index_name);
      }
    }

    // Now rename the table.
    // Ensure the new table name does not include schema syntax.
    $prefixInfo = $this
      ->getPrefixInfo($new_name);
    $this->connection
      ->query('ALTER TABLE {' . $table . '} RENAME TO ' . $prefixInfo['table']);
  }
  public function dropTable($table) {
    if (!$this
      ->tableExists($table)) {
      return FALSE;
    }
    $this->connection
      ->query('DROP TABLE {' . $table . '}');
    return TRUE;
  }
  public function addField($table, $field, $spec, $new_keys = array()) {
    if (!$this
      ->tableExists($table)) {
      throw new SchemaObjectDoesNotExistException(t("Cannot add field @table.@field: table doesn't exist.", array(
        '@field' => $field,
        '@table' => $table,
      )));
    }
    if ($this
      ->fieldExists($table, $field)) {
      throw new SchemaObjectExistsException(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 COLUMN ';
    $query .= $this
      ->createFieldSql($field, $this
      ->processField($spec));
    $this->connection
      ->query($query);
    if (isset($spec['initial'])) {
      $this->connection
        ->update($table)
        ->fields(array(
        $field => $spec['initial'],
      ))
        ->execute();
    }
    if ($fixnull) {
      $this->connection
        ->query("ALTER TABLE {" . $table . "} ALTER {$field} SET NOT NULL");
    }
    if (isset($new_keys)) {
      $this
        ->_createKeys($table, $new_keys);
    }

    // Add column comment.
    if (!empty($spec['description'])) {
      $this->connection
        ->query('COMMENT ON COLUMN {' . $table . '}.' . $field . ' IS ' . $this
        ->prepareComment($spec['description']));
    }
  }
  public function dropField($table, $field) {
    if (!$this
      ->fieldExists($table, $field)) {
      return FALSE;
    }
    $this->connection
      ->query('ALTER TABLE {' . $table . '} DROP COLUMN "' . $field . '"');
    return TRUE;
  }
  public function fieldSetDefault($table, $field, $default) {
    if (!$this
      ->fieldExists($table, $field)) {
      throw new SchemaObjectDoesNotExistException(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 SchemaObjectDoesNotExistException(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) {

    // Details http://www.postgresql.org/docs/8.3/interactive/view-pg-indexes.html
    $index_name = '{' . $table . '}_' . $name . '_idx';
    return (bool) $this->connection
      ->query("SELECT 1 FROM pg_indexes WHERE indexname = '{$index_name}'")
      ->fetchField();
  }

  /**
   * Helper function: check if a constraint (PK, FK, UK) exists.
   *
   * @param $table
   *   The name of the table.
   * @param $name
   *   The name of the constraint (typically 'pkey' or '[constraint]_key').
   */
  protected function constraintExists($table, $name) {
    $constraint_name = '{' . $table . '}_' . $name;
    return (bool) $this->connection
      ->query("SELECT 1 FROM pg_constraint WHERE conname = '{$constraint_name}'")
      ->fetchField();
  }
  public function addPrimaryKey($table, $fields) {
    if (!$this
      ->tableExists($table)) {
      throw new SchemaObjectDoesNotExistException(t("Cannot add primary key to table @table: table doesn't exist.", array(
        '@table' => $table,
      )));
    }
    if ($this
      ->constraintExists($table, 'pkey')) {
      throw new SchemaObjectExistsException(t("Cannot add primary key to table @table: primary key already exists.", array(
        '@table' => $table,
      )));
    }
    $this->connection
      ->query('ALTER TABLE {' . $table . '} ADD PRIMARY KEY (' . implode(',', $fields) . ')');
  }
  public function dropPrimaryKey($table) {
    if (!$this
      ->constraintExists($table, 'pkey')) {
      return FALSE;
    }
    $this->connection
      ->query('ALTER TABLE {' . $table . '} DROP CONSTRAINT ' . $this
      ->prefixNonTable($table, 'pkey'));
    return TRUE;
  }
  function addUniqueKey($table, $name, $fields) {
    if (!$this
      ->tableExists($table)) {
      throw new SchemaObjectDoesNotExistException(t("Cannot add unique key @name to table @table: table doesn't exist.", array(
        '@table' => $table,
        '@name' => $name,
      )));
    }
    if ($this
      ->constraintExists($table, $name . '_key')) {
      throw new SchemaObjectExistsException(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 CONSTRAINT "' . $this
      ->prefixNonTable($table, $name, 'key') . '" UNIQUE (' . implode(',', $fields) . ')');
  }
  public function dropUniqueKey($table, $name) {
    if (!$this
      ->constraintExists($table, $name . '_key')) {
      return FALSE;
    }
    $this->connection
      ->query('ALTER TABLE {' . $table . '} DROP CONSTRAINT "' . $this
      ->prefixNonTable($table, $name, 'key') . '"');
    return TRUE;
  }
  public function addIndex($table, $name, $fields) {
    if (!$this
      ->tableExists($table)) {
      throw new SchemaObjectDoesNotExistException(t("Cannot add index @name to table @table: table doesn't exist.", array(
        '@table' => $table,
        '@name' => $name,
      )));
    }
    if ($this
      ->indexExists($table, $name)) {
      throw new SchemaObjectExistsException(t("Cannot add index @name to table @table: index already exists.", array(
        '@table' => $table,
        '@name' => $name,
      )));
    }
    $this->connection
      ->query($this
      ->_createIndexSql($table, $name, $fields));
  }
  public function dropIndex($table, $name) {
    if (!$this
      ->indexExists($table, $name)) {
      return FALSE;
    }
    $this->connection
      ->query('DROP INDEX ' . $this
      ->prefixNonTable($table, $name, 'idx'));
    return TRUE;
  }
  public function changeField($table, $field, $field_new, $spec, $new_keys = array()) {
    if (!$this
      ->fieldExists($table, $field)) {
      throw new SchemaObjectDoesNotExistException(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 SchemaObjectExistsException(t("Cannot rename field @table.@name to @name_new: target field already exists.", array(
        '@table' => $table,
        '@name' => $field,
        '@name_new' => $field_new,
      )));
    }
    $spec = $this
      ->processField($spec);

    // We need to typecast the new column to best be able to transfer the data
    // Schema_pgsql::getFieldTypeMap() will return possibilities that are not
    // 'cast-able' such as 'serial' - so they need to be casted int instead.
    if (in_array($spec['pgsql_type'], array(
      'serial',
      'bigserial',
      'numeric',
    ))) {
      $typecast = 'int';
    }
    else {
      $typecast = $spec['pgsql_type'];
    }
    if (in_array($spec['pgsql_type'], array(
      'varchar',
      'character',
      'text',
    )) && isset($spec['length'])) {
      $typecast .= '(' . $spec['length'] . ')';
    }
    elseif (isset($spec['precision']) && isset($spec['scale'])) {
      $typecast .= '(' . $spec['precision'] . ', ' . $spec['scale'] . ')';
    }

    // Remove old check constraints.
    $field_info = $this
      ->queryFieldInformation($table, $field);
    foreach ($field_info as $check) {
      $this->connection
        ->query('ALTER TABLE {' . $table . '} DROP CONSTRAINT "' . $check . '"');
    }

    // Remove old default.
    $this
      ->fieldSetNoDefault($table, $field);

    // Convert field type.
    // Usually, we do this via a simple typecast 'USING fieldname::type'. But
    // the typecast does not work for conversions to bytea.
    // @see http://www.postgresql.org/docs/current/static/datatype-binary.html
    if ($spec['pgsql_type'] != 'bytea') {
      $this->connection
        ->query('ALTER TABLE {' . $table . '} ALTER "' . $field . '" TYPE ' . $typecast . ' USING "' . $field . '"::' . $typecast);
    }
    else {

      // Do not attempt to convert a field that is bytea already.
      $table_information = $this
        ->queryTableInformation($table);
      if (!in_array($field, $table_information->blob_fields)) {

        // Convert to a bytea type by using the SQL replace() function to
        // convert any single backslashes in the field content to double
        // backslashes ('\' to '\\').
        $this->connection
          ->query('ALTER TABLE {' . $table . '} ALTER "' . $field . '" TYPE ' . $typecast . ' USING decode(replace("' . $field . '"' . ", '\\', '\\\\'), 'escape');");
      }
    }
    if (isset($spec['not null'])) {
      if ($spec['not null']) {
        $nullaction = 'SET NOT NULL';
      }
      else {
        $nullaction = 'DROP NOT NULL';
      }
      $this->connection
        ->query('ALTER TABLE {' . $table . '} ALTER "' . $field . '" ' . $nullaction);
    }
    if (in_array($spec['pgsql_type'], array(
      'serial',
      'bigserial',
    ))) {

      // Type "serial" is known to PostgreSQL, but *only* during table creation,
      // not when altering. Because of that, the sequence needs to be created
      // and initialized by hand.
      $seq = "{" . $table . "}_" . $field_new . "_seq";
      $this->connection
        ->query("CREATE SEQUENCE " . $seq);

      // Set sequence to maximal field value to not conflict with existing
      // entries.
      $this->connection
        ->query("SELECT setval('" . $seq . "', MAX(\"" . $field . '")) FROM {' . $table . "}");
      $this->connection
        ->query('ALTER TABLE {' . $table . '} ALTER "' . $field . '" SET DEFAULT nextval(\'' . $seq . '\')');
    }

    // Rename the column if necessary.
    if ($field != $field_new) {
      $this->connection
        ->query('ALTER TABLE {' . $table . '} RENAME "' . $field . '" TO "' . $field_new . '"');
    }

    // Add unsigned check if necessary.
    if (!empty($spec['unsigned'])) {
      $this->connection
        ->query('ALTER TABLE {' . $table . '} ADD CHECK ("' . $field_new . '" >= 0)');
    }

    // Add default if necessary.
    if (isset($spec['default'])) {
      $this
        ->fieldSetDefault($table, $field_new, $spec['default']);
    }

    // Change description if necessary.
    if (!empty($spec['description'])) {
      $this->connection
        ->query('COMMENT ON COLUMN {' . $table . '}."' . $field_new . '" IS ' . $this
        ->prepareComment($spec['description']));
    }
    if (isset($new_keys)) {
      $this
        ->_createKeys($table, $new_keys);
    }
  }
  protected function _createIndexSql($table, $name, $fields) {
    $query = 'CREATE INDEX "' . $this
      ->prefixNonTable($table, $name, 'idx') . '" ON {' . $table . '} (';
    $query .= $this
      ->_createKeySql($fields) . ')';
    return $query;
  }
  protected function _createKeys($table, $new_keys) {
    if (isset($new_keys['primary key'])) {
      $this
        ->addPrimaryKey($table, $new_keys['primary key']);
    }
    if (isset($new_keys['unique keys'])) {
      foreach ($new_keys['unique keys'] as $name => $fields) {
        $this
          ->addUniqueKey($table, $name, $fields);
      }
    }
    if (isset($new_keys['indexes'])) {
      foreach ($new_keys['indexes'] as $name => $fields) {
        $this
          ->addIndex($table, $name, $fields);
      }
    }
  }

  /**
   * Retrieve a table or column comment.
   */
  public function getComment($table, $column = NULL) {
    $info = $this
      ->getPrefixInfo($table);

    // Don't use {} around pg_class, pg_attribute tables.
    if (isset($column)) {
      return $this->connection
        ->query('SELECT col_description(oid, attnum) FROM pg_class, pg_attribute WHERE attrelid = oid AND relname = ? AND attname = ?', array(
        $info['table'],
        $column,
      ))
        ->fetchField();
    }
    else {
      return $this->connection
        ->query('SELECT obj_description(oid, ?) FROM pg_class WHERE relname = ?', array(
        'pg_class',
        $info['table'],
      ))
        ->fetchField();
    }
  }

}