Re: Query plan, nested EXISTS

Matt Daw <matt@shotgunsoftware.com> writes:
> Howdy, I've been debugging a client's slow query today and I'm curious
> about the query plan. It's picking a plan that hashes lots of rows from the
> versions table (on v9.0.10)...

> EXPLAIN ANALYZE
> SELECT COUNT(*) FROM notes a WHERE
> a.project_id = 114 AND
> EXISTS (
>     SELECT 1 FROM note_links b
>     WHERE
>     b.note_id = a.id AND
>     b.entity_type = 'Version' AND
>     EXISTS (
>         SELECT 1 FROM versions c
>         WHERE
>         c.id = b.entity_id AND
>         c.code ILIKE '%comp%' AND
>         c.retirement_date IS NULL
>     ) AND
>     b.retirement_date IS NULL
> )

I think the real problem here is that 9.0 is incapable of avoiding a
full table scan on "note_links", which means it doesn't really have any
better option than to do the inner EXISTS as a full-table semijoin.
This is because it can't push a.id down through two levels of join, and
because the semijoins don't commute, there's no way to get a.id into the
scan of note_links to pull out only the useful rows.  The hack with
LIMIT avoids this problem by preventing the inner EXISTS from being
treated as a full-fledged semijoin; but of course that hack leaves you
vulnerable to very bad plans if the statistics are such that a nestloop
join isn't the best bet for the inner EXISTS.

The work I did for parameterized paths in 9.2 was intended to address
exactly this type of scenario.  I would be interested to know if 9.2
does this any better for you.

                        regards, tom lane