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.

[jaeheonshim]

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.

Unfortunately, I don't think something like that can be added. We would risk breaking the semantics of the sql query. For example, if a is NULL, we cannot fold the expression (a > 0 AND a < 0) to FALSE, as the correct answer should be NULL.

[gkodinov]

There hasn't been any activity for more than 3 weeks now. Closing due to inactivity. If you would like to keep working on that please ping me or anybody else of the MariaDB committers to re-open the PR for you.

[grooverdan]

my only comment is that before these ANALYZE statement is a comment explaining how the MDEV implementation affects results. E.g Here the OR true expression will eliminate the need for the subselect resulting in No tables used

Before you start comment on those, I'll get another reviewer to see if they agree with this approach.

[Olernov]

@jaeheonshim , generally we try to avoid test files names like mdev_xxxx.test as not descriptive enough. I'd suggest something like boolean_cond_folding.test

[Olernov]

remove_eq_conds() protects against deeply nested expressions with:

if (check_stack_overrun(thd, STACK_MIN_SIZE, NULL))
{
  *cond_value= Item::COND_FALSE;
  return (COND*) 0;                           // Fatal error flag is set!
}

The recursive simplify_cond() has no such guard. For deeply nested AND/OR trees, this could potentially overflow the stack.

[jaeheonshim]

@Olernov , thanks for the suggestions! I added the check_stack_overrun in simplify_cond as well, and also changed the name of the tests.