On Tue, Jun 13, 2017 at 10:24 AM, Robert Haas <robertmh...@gmail.com> wrote:
> I think that's going to come as an unpleasant surprise to more than
> one user.  I'm not sure exactly how we need to restructure things here
> so that this works properly, and maybe modifying
> predicate_implied_by() isn't the right thing at all; for instance,
> there's also predicate_refuted_by(), which maybe could be used in some
> way (inject NOT?).  But I don't much like the idea that you copy and
> paste the partitioning constraint into a CHECK constraint and that
> doesn't work.  That's not cool.

OK, I think I see the problem here.  predicate_implied_by() and
predicate_refuted_by() differ in what they assume about the predicate
evaluating to NULL, but both of them assume that if the clause
evaluates to NULL, that's equivalent to false.  So there's actually no
option to get the behavior we want here, which is to treat both
operands using CHECK-semantics (null is true) rather than
WHERE-semantics (null is false).

Given that, Ashutosh's proposal of passing an additional flag to
predicate_implied_by() seems like the best option.  Here's a patch
implementing that.

Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Attachment: extend-predicate-implied-by-v1.patch
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