Strange query plan with redundant aggregate nodes

This query surprised me. I expected us to do the Aggregate once for all the
aggregate functions in the select target which is what normally happens. If I
simplify the query further it actually does so.

I don't really understand what's going on here. It can't be the volatile
random() because in fact even if I make them depend on the random value the
subplans are executed with the same parameter values anyways and the sums end
up being the same.

postgres=# postgres=# explain 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=5676.06..5676.07 rows=1 width=0)
InitPlan
-> Aggregate (cost=2838.00..2838.01 rows=1 width=0)
-> Seq Scan on a (cost=0.00..2588.00 rows=100000 width=0)
-> Aggregate (cost=2838.00..2838.01 rows=1 width=0)
-> Seq Scan on a (cost=0.00..2588.00 rows=100000 width=0)
-> Result (cost=0.00..0.01 rows=1 width=0)
(7 rows)

--
Gregory Stark
EnterpriseDB http://www.enterprisedb.com
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