Partition prune with stable Expr

Hi:

I find we can't prune partitions in the planner if the qual is a stable
function.

CREATE TABLE measurement (
city_id int not null,
logdate date not null,
peaktemp int,
unitsales int
) PARTITION BY RANGE (logdate);

CREATE TABLE measurement_y2006m02 PARTITION OF measurement
FOR VALUES FROM ('2006-02-01') TO ('2006-03-01');

CREATE TABLE measurement_y2006m03 PARTITION OF measurement
FOR VALUES FROM ('2006-03-01') TO ('2006-04-01');

postgres=# explain (costs off) select * from measurement
postgres-# where logdate = to_date('2006-03-02', 'yyyy-mm-dd');
QUERY PLAN
-----------------------------------------------------------------------------
Append
Subplans Removed: 1 <-- Here
-> Seq Scan on measurement_y2006m03 measurement_1
Filter: (logdate = to_date('2006-03-02'::text, 'yyyy-mm-dd'::text))
(4 rows)

IMO, we should do it. Why not? The attached is used to show the things
in my mind.

Btw, why the to_date function is declared as stable rather than immutable
since it always delivers the same result for the same inputs.

--
Best Regards
Andy Fan