[Maria-developers] On the question of whether select_id allow to reconstruct the query tree
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Hello Igor, On the question of whether the set of used select_id allow to reconstruct the query tree: Please find below two examples: Example #1: select(t1) has two children, select(t2), select(t3): MariaDB [test]> explain select * from t1 where a in (select a from t2) or b in (select a from t3); +------+--------------+-------+------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +------+--------------+-------+------+---------------+------+---------+------+------+-------------+ | 1 | PRIMARY | t1 | ALL | NULL | NULL | NULL | NULL | 2 | Using where | | 3 | MATERIALIZED | t3 | ALL | NULL | NULL | NULL | NULL | 2 | | | 2 | MATERIALIZED | t2 | ALL | NULL | NULL | NULL | NULL | 2 | | +------+--------------+-------+------+---------------+------+---------+------+------+-------------+ 3 rows in set (0.00 sec) Example #2: select(t1) has a child select(t2), which has a child, select(t3): MariaDB [test]> explain select * from t1 where a in (select a from t2 where b in (select a from t3) or b <3) or b < 3; +------+--------------+-------+------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +------+--------------+-------+------+---------------+------+---------+------+------+-------------+ | 1 | PRIMARY | t1 | ALL | NULL | NULL | NULL | NULL | 2 | Using where | | 2 | MATERIALIZED | t2 | ALL | NULL | NULL | NULL | NULL | 2 | Using where | | 3 | MATERIALIZED | t3 | ALL | NULL | NULL | NULL | NULL | 2 | | +------+--------------+-------+------+---------------+------+---------+------+------+-------------+ 3 rows in set (0.01 sec) The order of selects in EXPLAIN output is different, but I don't believe it allows to reconstruct the structure of the tree in all cases. (if it does, is it practically useful? the function is complicated and fragile) BR Sergei -- Sergei Petrunia, Software Developer Monty Program AB, http://askmonty.org Blog: http://s.petrunia.net/blog
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Sergei Petrunia