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Optimizer in 8.0 in some cases not materializing subquery
Posted by: Benjamin Peikes
Date: January 24, 2022 03:37PM

We have a query that went from taking 2 seconds in MySQL 5.7 to 50+ seconds in 8.0. It appears it has to do with how the optimizer is handling some subqueries.

Example of query:

SELECT, e.status_id, COUNT( AS cnt
FROM Events e
WHERE e.ent_id IN (1)
SELECT eam.account
FROM EventAccountMapping eam
eam.event_id = AND
eam.account NOT IN
SELECT uapt.account
FROM UserAccountPermissions uapt
WHERE uapt.user_id = 1
) /* Subquery 2 */

In 5.7, it was treating Subquery 2 as a subquery, and in 5.8, it's be treated as a dependent subquery, even though it is not dependent on the other tables

If I break this into 2 queries where the select from UserAccountPermissions saves the results in a temp table, and then use that temp table, it goes from 50 seconds to 2. If I actually generate the results fo subquery 2, and inline the results ( 'aaa', 'bbbb', 'cccc', etc), the query goes down to less than one second.

In 5.8, I moved the subquery 2 to a CTE, and that doesn't resolve the issue either. Is there a way to force a CTE to get materialized?

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Optimizer in 8.0 in some cases not materializing subquery
January 24, 2022 03:37PM

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