Jay Levitt <jay.lev...@gmail.com> writes:
> When I run the following query:
> select questions.id
> from questions
> join (
> select u.id as user_id
> from users as u
> left join scores as s
> on s.user_id = u.id
> ) as subquery
> on subquery.user_id = questions.user_id;
> the subquery is scanning my entire user table, even though it's restricted
> by the outer query. (My real subquery is much more complicated, of course,
> but this is the minimal fail case.)
> Is this just not a thing the optimizer can do?
Every release since 8.2 has been able to reorder joins in a query
written that way. Probably it just thinks it's cheaper than the
alternatives.
(Unless you've reduced the collapse_limit variables for some reason?)
regards, tom lane
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