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MDEV-19134: EXISTS() slower if ORDER BY is defined

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Step 1: Removal of ORDER BY [LIMIT] from the subquery should be done
earlier and for broader class of subqueries.

The rewrite was done in Item_in_subselect::select_in_like_transformer(),
but this had problems:
- It didn't cover EXISTS subqueries
- It covered IN-subqueries, but was done after the semi-join transformation
  was considered inapplicable, because ORDER BY was present.

Remaining issue:
- EXISTS->IN transformation happens before
  check_and_do_in_subquery_rewrites() is called, so it is still prevented
  by the present ORDER BY.
This commit is contained in:
Sergei Petrunia
2019-05-04 21:33:39 +03:00
parent 366bf10475
commit b1f828a82a
9 changed files with 82 additions and 32 deletions
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4
mysql-test/main/subselect_innodb.result
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@ -458,7 +458,7 @@ EXPLAIN
SELECT * FROM t1 WHERE EXISTS ( SELECT b FROM t2, t3 GROUP BY b HAVING b != 3 );
id select_type table type possible_keys key key_len ref rows Extra
1 PRIMARY NULL NULL NULL NULL NULL NULL NULL Impossible WHERE
2 SUBQUERY t2 index PRIMARY PRIMARY 4 NULL 1 Using where; Using index; Using temporary; Using filesort
2 SUBQUERY t2 index PRIMARY PRIMARY 4 NULL 1 Using where; Using index; Using temporary
2 SUBQUERY t3 ALL NULL NULL NULL NULL 1 Using join buffer (flat, BNL join)
SELECT * FROM t1 WHERE EXISTS ( SELECT b FROM t2, t3 GROUP BY b HAVING b != 3 );
a
@ -480,7 +480,7 @@ HAVING SQ2_alias1 . col_int_key >= 7
);
id select_type table type possible_keys key key_len ref rows Extra
1 PRIMARY NULL NULL NULL NULL NULL NULL NULL Impossible WHERE
2 SUBQUERY SQ2_alias2 index NULL col_int_key 5 NULL 1 Using index; Using temporary; Using filesort
2 SUBQUERY SQ2_alias2 index NULL col_int_key 5 NULL 1 Using index; Using temporary
2 SUBQUERY SQ2_alias1 ref col_int_key col_int_key 5 test.SQ2_alias2.col_int_key 1 Using where; Using index
SELECT 1 FROM t1 AS alias1
WHERE EXISTS ( SELECT SQ2_alias1 . col_int_key AS SQ2_field1