public function cacheTableDefinition($table, $definition, $which_connection='')
    {
        if ($which_connection == '') {
            $which_connection=$this->default_db;
        }
        if (!isset($this->structure_cache[$which_connection])) {
            $this->structure_cache[$which_connection] = array();
        }
        $this->structure_cache[$which_connection][$table] = $definition;

        return true;
    }