Re: Slow query with backwards index scan

Tilmann Singer <tils-pgsql(at)tils(dot)net> wrote ..
> * Nis Jrgensen <nis(at)superlativ(dot)dk> [20070727 20:31]:
> > How does the "obvious" UNION query do - ie:
> >
> > SELECT * FROM (
> > SELECT * FROM large_table lt
> > WHERE lt.user_id = 12345
> >
> > UNION
> >
> > SELECT * FROM large_table lt
> > WHERE user_id IN (SELECT contact_id FROM relationships WHERE user_id=12345)
> > ) q
> >
> > ORDER BY created_at DESC LIMIT 10;

Let's try putting the sort/limit in each piece of the UNION to speed them up separately.

SELECT * FROM (
(SELECT * FROM large_table lt
WHERE lt.user_id = 12345
ORDER BY created_at DESC LIMIT 10) AS q1
UNION
(SELECT * FROM large_table lt
WHERE user_id IN (SELECT contact_id FROM relationships WHERE user_id=12345)
ORDER BY created_at DESC LIMIT 10) AS q2
ORDER BY created_at DESC LIMIT 10;