public function describeColumn($tableName, $column) {
		
		// search in cached table structure
		if (isset($this->definitions[$tableName]['describe'])) {
			foreach ($this->definitions[$tableName]['describe'] as $d) {
				if ($column == $d->Field) {
					return $d;
				}
			}
		}
		
		$res = self::load('DESCRIBE `' . $tableName . '` `' . $column . '`', NULL, PAIR_DB_OBJECT);
		
		return ($res ? $res : NULL);
		
	}