I made some quick test to show what I mean by "performance problem". Note that I'm not saying the the first and the second test cases should perform equally. But there is too much difference - see the conclusion.

1)

MariaDB [test]> SELECT BENCHMARK(50000000, (SELECT 1));
+---------------------------------+
| BENCHMARK(50000000, (SELECT 1)) |
+---------------------------------+
|                               0 |
+---------------------------------+
1 row in set (0.42 sec)

2)

DELIMITER ||
CREATE FUNCTION f()
        RETURNS TINYINT
BEGIN
        RETURN 1;
END ||
DELIMITER ;

MariaDB [test]> SELECT BENCHMARK(50000000, (SELECT f()));
+-----------------------------------+
| BENCHMARK(50000000, (SELECT f())) |
+-----------------------------------+
|                                 0 |
+-----------------------------------+
1 row in set (2 min 5.70 sec)

3)

DELIMITER ||
CREATE FUNCTION f(x TINYINT)
        RETURNS TINYINT
BEGIN
        RETURN x;
END ||
DELIMITER ;

MariaDB [test]> SELECT BENCHMARK(50000000, (SELECT f(1)));
+------------------------------------+
| BENCHMARK(50000000, (SELECT f(1))) |
+------------------------------------+
|                                  0 |
+------------------------------------+
1 row in set (3 min 35.20 sec)

-- Conclusions:

Times in seconds:

0.42
125.70
215.20

I don't know which ratio would be acceptable, but the difference betweem a trivial query and a trivial function is too high.
Also, the difference between 2) and 3) is that the function in 3) accepts and returns a parameter. Again, the performance difference seems to me too high.

Regards
Federico

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