[MDEV-19429] Wrong query result with EXISTS and LIMIT 0 - Jira

"LIMIT 0" is a degenerate case.

EXISTS (SELECT ... LIMIT 0) should evaluate to FALSE, but it evaluates to TRUE.

create table t10 (a int);

insert into t10 values (1),(2),(3);

create table t12 (a int);

insert into t12 values (1),(2),(3);

LIMIT 0 query:

select * from t12 order by a limit 0;

Empty set (0.00 sec)

Make it a subquery:

select * from t10 where exists (select * from t12 order by a limit 0);

+------+

| a    |

+------+

|    1 |

|    2 |

|    3 |

+------+

3 rows in set (0.00 sec)

mysql> explain select * from t10 where exists (select * from t12 order by a limit 0);

+------+-------------+-------+------+---------------+------+---------+------+------+----------------+

| id   | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra          |

+------+-------------+-------+------+---------------+------+---------+------+------+----------------+

|    1 | PRIMARY     | t10   | ALL  | NULL          | NULL | NULL    | NULL | 3    |                |

|    2 | SUBQUERY    | t12   | ALL  | NULL          | NULL | NULL    | NULL | 3    | Using filesort |

+------+-------------+-------+------+---------------+------+---------+------+------+----------------+

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