MDEV-36106 New-style hints: [NO_]DERIVED_CONDITION_PUSHDOWN, [NO_]MERGE

[DaveGosselin-MariaDB]

Implements and tests the following optimizer hints:

• DERIVED_CONDITION_PUSHDOWN
• NO_DERIVED_CONDITION_PUSHDOWN
• MERGE
• NO_MERGE

Each of these are table-level hints. The first pair enable and disable, respectively, the condition pushdown for derived tables which is typically controlled by the condition_pushdown_for_derived optimizer switch. The second pair enable and disable, respectively, the derived table merge optimization which is typically controlled by the derived_merge optimizer switch.

[spetrunia]

Note: this adds

          !this->opt_hints_table)        // Table hints are not adjusted yet
        select_lex->opt_hints_qb->fix_hints_for_table(this);

code in TABLE_LIST::init_derived() which is called from open_and_lock_tables/mysql_handle_derived.

I guess the need to make a decision whether to do merging in open_and_lock_tables() is a given (we can't change it within the scope of this project). But at least the docs in opt_hints.h need to be updated.

[spetrunia]

MariaDB [test]>  explain extended select /*+ NO_MERGE(T) */ * from (select * from t1) T;
+------+-------------+------------+------+---------------+------+---------+------+------+----------+-------+
| id   | select_type | table      | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra |
+------+-------------+------------+------+---------------+------+---------+------+------+----------+-------+
|    1 | PRIMARY     | <derived2> | ALL  | NULL          | NULL | NULL    | NULL | 100  |   100.00 |       |
|    2 | DERIVED     | t1         | ALL  | NULL          | NULL | NULL    | NULL | 100  |   100.00 |       |
+------+-------------+------------+------+---------------+------+---------+------+------+----------+-------+
2 rows in set, 1 warning (2.764 sec)

Without the hint, the above query would merge the derived table. It didn't, so the hint had an effect. But EXPLAIN text or optimizer trace do not have anything about the hint:

MariaDB [test]> show warnings\G
*************************** 1. row ***************************
  Level: Note
   Code: 1003
Message: /* select#1 */ select `T`.`a` AS `a`,`T`.`b` AS `b` from (/* select#2 */ select `test`.`t1`.`a` AS `a`,`test`.`t1`.`b` AS `b` from `test`.`t1`) `T`
1 row in set (0.000 sec)

      "join_optimization": {
        "select_id": 1,
        "steps": [
          {
            "hints": ""
          },
          {
            "join_optimization": {
              "select_id": 2,
              "steps": [
                {

[DaveGosselin-MariaDB]

MariaDB [test]>  explain extended select /*+ NO_MERGE(T) */ * from (select * from t1) T;
+------+-------------+------------+------+---------------+------+---------+------+------+----------+-------+
| id   | select_type | table      | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra |
+------+-------------+------------+------+---------------+------+---------+------+------+----------+-------+
|    1 | PRIMARY     | <derived2> | ALL  | NULL          | NULL | NULL    | NULL | 100  |   100.00 |       |
|    2 | DERIVED     | t1         | ALL  | NULL          | NULL | NULL    | NULL | 100  |   100.00 |       |
+------+-------------+------------+------+---------------+------+---------+------+------+----------+-------+
2 rows in set, 1 warning (2.764 sec)

Without the hint, the above query would merge the derived table. It didn't, so the hint had an effect. But EXPLAIN text or optimizer trace do not have anything about the hint:

MariaDB [test]> show warnings\G
*************************** 1. row ***************************
  Level: Note
   Code: 1003
Message: /* select#1 */ select `T`.`a` AS `a`,`T`.`b` AS `b` from (/* select#2 */ select `test`.`t1`.`a` AS `a`,`test`.`t1`.`b` AS `b` from `test`.`t1`) `T`
1 row in set (0.000 sec)

     "join_optimization": {
       "select_id": 1,
       "steps": [
         {
           "hints": ""
         },
         {
           "join_optimization": {
             "select_id": 2,
             "steps": [
               {

@spetrunia If I’m understanding the code correctly, it is working as expected. We use a bitmap to remember which hints are specified. But this bitmap conflates “is specified as false” with “not specified”. So in the Opt_hints::print method, is_specified will return false if the hint is given but it’s hint value is false, and it won’t be printed in the optimizer trace output.

[spetrunia]

@DaveGosselin-MariaDB,

Trying on MySQL (I am using 8.4 but I suppose earlier versions would work the same):

create table t1 (a int);
insert into t1 values (1),(2);
explain  select /*+ NO_MERGE(T) */ * from (select * from t1) T;

+----+-------------+------------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table      | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra |
+----+-------------+------------+------------+------+---------------+------+---------+------+------+----------+-------+
|  1 | PRIMARY     | <derived2> | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    2 |   100.00 | NULL  |
|  2 | DERIVED     | t1         | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    2 |   100.00 | NULL  |
+----+-------------+------------+------------+------+---------------+------+---------+------+------+----------+-------+
2 rows in set, 1 warning (0,00 sec)

mysql> show warnings\G
*************************** 1. row ***************************
  Level: Note
   Code: 1003
Message: /* select#1 */ select /*+ NO_MERGE(`T`@`select#1`) */ `T`.`a` AS `a` from (/* select#2 */ select `j21`.`t1`.`a` AS `a` from `j21`.`t1`) `T`
1 row in set (0,00 sec)

Note the NO_MERGE in the warning...

[DaveGosselin-MariaDB]

@spetrunia and @Olernov, I fixed the issue with the hint name not appearing in the warnings. I'll update the PR as soon as I finish adding the new tests.

[DaveGosselin-MariaDB]

This PR is in a logical place until I pick it back up in the next development sprint. I have more test cases to add, specifically for query block hints.