findPrimaryKeyColumns example

if (!$this->fieldExists($table, $field)) {
      return FALSE;
    }

    // When dropping a field that is part of a composite primary key MySQL     // automatically removes the field from the primary key, which can leave the     // table in an invalid state. MariaDB 10.2.8 requires explicitly dropping     // the primary key first for this reason. We perform this deletion     // explicitly which also makes the behavior on both MySQL and MariaDB     // consistent with PostgreSQL.     // @see https://mariadb.com/kb/en/library/alter-table     $primary_key = $this->findPrimaryKeyColumns($table);
    if ((count($primary_key) > 1) && in_array($field, $primary_key, TRUE)) {
      $this->dropPrimaryKey($table);
    }

    $this->connection->query('ALTER TABLE {' . $table . '} DROP [' . $field . ']');
    return TRUE;
  }

  /** * {@inheritdoc} */