pgsql: Arrange to cache the results of looking up a btree predicate

Log Message:
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Arrange to cache the results of looking up a btree predicate proof comparison
operator. The result depends only on the two input operators and the proof
direction (imply or refute), so it's easy to cache. This provides a very
large savings in cases such as Sergey Konoplev's long NOT-IN-list example,
where predtest spends all its time repeatedly figuring out that the same pair
of operators cannot be used to prove anything. (But of course the O(N^2)
behavior still catches up with you eventually.) I'm not convinced it buys
a whole lot when constraint_exclusion isn't turned on, but it's not a lot
of added code so we might as well cache all the time.

Modified Files:
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pgsql/src/backend/optimizer/util:
predtest.c (r1.21 -> r1.22)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/util/predtest.c?r1=1.21&r2=1.22)