Re: sub-select with aggregate

[ followup on a gripe from October ]

Tomasz Myrta <jasiek@klaster.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..17.07rows=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