public function getConstraintCheckClause($tableName, $columnName)
    {
        // Check already processed table columns
        if (isset(self::$cached_checks[$tableName])) {
            if (!isset(self::$cached_checks[$tableName][$columnName])) {
                return null;
            }
            return self::$cached_checks[$tableName][$columnName];
        }

        // Regenerate cehcks for this table
        $checks = array();
        foreach ($this->preparedQuery("
			SELECT CAST(CHECK_CLAUSE AS TEXT) AS CHECK_CLAUSE, COLUMN_NAME
			FROM INFORMATION_SCHEMA.CHECK_CONSTRAINTS AS CC
			INNER JOIN INFORMATION_SCHEMA.CONSTRAINT_COLUMN_USAGE AS CCU ON CCU.CONSTRAINT_NAME = CC.CONSTRAINT_NAME
			WHERE TABLE_NAME = ?",
            array($tableName)
        ) as $record) {
            $checks[$record['COLUMN_NAME']] = $record['CHECK_CLAUSE'];
        }
        self::$cached_checks[$tableName] = $checks;

        // Return via cached records
        return $this->getConstraintCheckClause($tableName, $columnName);
    }