optimizer not eliminating redundant sorts

Another odd thing. This type of query seems to be causing redundant sorts.
>From what I've read in the source it tries quite hard to keep track of the
sort order of subplans to avoid unnecessary sorts, so I think these are
supposed to get optimized out. I'm not sure if the group nodes can get
optimized out, but surely the sorts can.

slo=> explain select x from (select x,y from (select x,y,z from ttt group by x,y,z) as x group by x,y) as x group by x;
QUERY PLAN
----------------------------------------------------------------------------------------------------
Group (cost=0.05..0.06 rows=1 width=16)
-> Sort (cost=0.05..0.06 rows=1 width=16)
Sort Key: x
-> Subquery Scan x (cost=0.03..0.04 rows=1 width=16)
-> Group (cost=0.03..0.04 rows=1 width=16)
-> Sort (cost=0.03..0.04 rows=1 width=16)
Sort Key: x, y
-> Subquery Scan x (cost=0.01..0.02 rows=1 width=16)
-> Group (cost=0.01..0.02 rows=1 width=16)
-> Sort (cost=0.01..0.02 rows=1 width=16)
Sort Key: x, y, z
-> Seq Scan on ttt (cost=0.00..0.00 rows=1 width=16)

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greg