Re: sub-select with aggregate

[ followup on a gripe from October ]

Tomasz Myrta <jasiek(at)klaster(dot)net> writes:
> I want to perform query looking like this:

> select
> user_id,
> a/sum_a as percent_a,
> b/sum_b as percent_b
> from
> users join
> (select
> group_id,
> sum(a) as sum_a,
> sum(b) as sum_b
> from users group by group_id) X using (group_id)
> where group_id=3;

> This query works, but very slow. Subquery with aggregate is performed
> for all table rows instead of group_id=3.

Just FYI, this problem is fixed as of CVS tip. Using an empty table
with an index on group_id, I get a plan like so:

Nested Loop (cost=17.10..34.21 rows=1 width=36)
-> Subquery Scan x (cost=17.10..17.11 rows=1 width=12)
-> HashAggregate (cost=17.10..17.11 rows=1 width=12)
-> Index Scan using users_group_id on users (cost=0.00..17.07 rows=5 width=12)
Index Cond: (3 = group_id)
-> Index Scan using users_group_id on users (cost=0.00..17.08 rows=1 width=16)
Index Cond: (users.group_id = "outer".group_id)
Filter: (group_id = 3)

Notice the condition on group_id has been propagated into both sides of
the join.

regards, tom lane