MDEV-37713/MDEV-37714 Fold boolean literals in SELECT-list

[jaeheonshim]

The order of evaluation of the expressions that appear in a SELECT-list is undefined. This change exploits this fact by recursively folding TRUE/FALSE literals in OR/AND expressions which may allow for skipping evaluation of some parts of the expression or even the whole expression.

Example

Setup

CREATE TABLE t1(c0 INT8);
CREATE TABLE t2(c0 INT8);
INSERT INTO t1 VALUES(1);
INSERT INTO t2 SELECT seq FROM seq_1_to_10000000;

Before

SELECT (SELECT MIN(c0) FROM t2)<0 OR true;
+------------------------------------+
| (SELECT MIN(c0) FROM t2)<0 OR true |
+------------------------------------+
|                                  1 |
+------------------------------------+
1 row in set (1.335 sec)


SELECT ((SELECT MIN(c0) FROM t2)<0 AND false) OR (SELECT MAX(c0) FROM t1)>0;
+----------------------------------------------------------------------+
| ((SELECT MIN(c0) FROM t2)<0 AND false) OR (SELECT MAX(c0) FROM t1)>0 |
+----------------------------------------------------------------------+
|                                                                    1 |
+----------------------------------------------------------------------+
1 row in set (1.413 sec)

After

SELECT (SELECT MIN(c0) FROM t2)<0 OR true;
+------------------------------------+
| (SELECT MIN(c0) FROM t2)<0 OR true |
+------------------------------------+
|                                  1 |
+------------------------------------+
1 row in set (0.000 sec)


SELECT ((SELECT MIN(c0) FROM t2)<0 AND false) OR (SELECT MAX(c0) FROM t1)>0;
+----------------------------------------------------------------------+
| ((SELECT MIN(c0) FROM t2)<0 AND false) OR (SELECT MAX(c0) FROM t1)>0 |
+----------------------------------------------------------------------+
|                                                                    1 |
+----------------------------------------------------------------------+
1 row in set (0.000 sec)

[gkodinov]

Thank you for your contribution! This is a preliminary review.

Please also add a test case.

And, the jira mentions one optimization you're not really doing:

SELECT (a > 0 AND a < 0) FROM t1;
SELECT (a > 0 OR a <=0) FROM t1;

Would you consider doing this too? Or, if you wont, we'll need to open a sub-task Jira instead of using the current one.

[gkodinov]

Can the list ever be empty, given that you're removing one of the branches only if another branch exists?

I'd say no. And then this is dead code. And should be turned into an assert.

[gkodinov]

Any specific reason why you're doing this only for the SELECT list expressions?
Why not do this during fix_fields for all Item_conds for example?

[gkodinov]

Forgot to mention one thing: When doing the optimization you are doing you need to check for NULL-ness!

SELECT true AND NULL FROM t1

should return NULL and not true.