Can i force mysql to perform subquery first?

Question

I have query like this:

SELECT `table_1`.* from `table_1`
  INNER JOIN `table_2` [...]
  INNER JOIN `table_3` [...]
WHERE `table_1`.`id` IN(
  SELECT `id` FROM [...]
)
AND [more conditions]

When I use EXPLAIN, there is 'DEPENDENT SUBQUERY' at the end, but I want this subquery to be performed first, before other conditions.

Is is possible?

Answer 1

SELECT  `table_1`.*
FROM    `table_1`
INNER JOIN
        `table_2` [...]
INNER JOIN
        `table_3` [...]
WHERE   `table_1`.`id` IN
        (
        SELECT  `id`
        FROM    [...]
        )
        AND [more conditions]

If the inner table is properly indexed, the subquery here is not being "performed" at all in a strict sense of word.

Since the subquery is a part of an IN expression, the condition is pushed into the subquery and it's transformed into an EXISTS.

In fact, this subquery is evaluated on each step:

EXISTS
(
SELECT  NULL
FROM    [...]
WHERE   id = table1.id
)

You can actually see it in the detailed description provided by EXPLAIN EXTENDED.

That's why it's called DEPENDENT SUBQUERY: the result of each evaluation depends on the value of table1.id. The subquery as such is not correlated, it's the optimized version that is correlated.

MySQL always evaluates the EXISTS clause after the more simple filters (since they are much easier to evaluate and there is a probability that the subquery won't be evaluated at all).

If you want the subquery to be evaluated all at once, rewrite the query as this:

SELECT  table_1.*
FROM    (
        SELECT  DISTINCT id
        FROM    [...]
        ) q
JOIN    table_1
ON      table_1.id = q.id
JOIN    table_2
ON      [...]
JOIN    table_3
ON      [...]
WHERE   [more conditions]

This forces the subquery to be leading in the join, which is more efficient if the subquery is small compared to table_1, and less efficient if the subquery is large compared to table_1.

If there is an index on [...].id used in the subquery, the subquery will be performed using an INDEX FOR GROUP-BY.

Answer 2

this is a known bug in mysql: http://bugs.mysql.com/bug.php?id=25926

a useful workaround is to push down the subquery into another select * from (subquery) as dt type subquery.