Re: Slow query with backwards index scan

[Jeremy Harris <jgh(at)wizmail(dot)org>]

Tilmann Singer wrote:
> * andrew(at)pillette(dot)com <andrew(at)pillette(dot)com> [20070728 21:05]:
> > 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;
>
> It's not possible to use ORDER BY or LIMIT within unioned queries.
>
> http://www.postgresql.org/docs/8.2/static/sql-select.html#SQL-UNION

"ORDER BY and LIMIT can be attached to a subexpression if it is enclosed in parentheses"