Re: Slow query with backwards index scan - Mailing list pgsql-performance

* andrew@pillette.com <andrew@pillette.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

Would that make sense at all given the way the postgresql planner
works?  Does that work in other DB's?


Til