Re: Strange query plan with redundant aggregate nodes

Gregory Stark <stark(at)enterprisedb(dot)com> writes:
> I don't really understand what's going on here.

It's flattening the sub-select, converting

select sum(n),sum(n)
from (select (select count(*) as n from a ) as n
from (select random() as s) as xyzzy) as xyzzy ;

to

select sum((select count(*) from a)), sum((select count(*) from a))
from (select random() as s) as xyzzy;

Maybe we could stop it from doing that when there are sub-selects in the
sub-select's targetlist, but I'm afraid that would make other cases
worse.

BTW, in CVS HEAD it looks like this

regression=# explain verbose select sum(n),sum(n) from (select (select count(*) as n from a ) as n
from (select random() as s) as xyzzy) as xyzzy ;
QUERY PLAN
-------------------------------------------------------------------
Aggregate (cost=80.06..80.07 rows=1 width=0)
Output: sum($0), sum($1)
InitPlan 1 (returns $0)
-> Aggregate (cost=40.00..40.01 rows=1 width=0)
Output: count(*)
-> Seq Scan on a (cost=0.00..34.00 rows=2400 width=0)
Output: public.a.f1
InitPlan 2 (returns $1)
-> Aggregate (cost=40.00..40.01 rows=1 width=0)
Output: count(*)
-> Seq Scan on a (cost=0.00..34.00 rows=2400 width=0)
Output: public.a.f1
-> Result (cost=0.00..0.01 rows=1 width=0)
Output: random()
(14 rows)

which makes it at least a little clearer where the subplans are
connected to ...

regards, tom lane