Gregory Stark <stark@enterprisedb.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
froma ) 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..34.00rows=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
