Gregory Stark <st...@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 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
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