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pgsql: Allow predicate_refuted_by() to deduce that NOT A refutes A.

From: tgl(at)postgresql(dot)org (Tom Lane)
To: pgsql-committers(at)postgresql(dot)org
Subject: pgsql: Allow predicate_refuted_by() to deduce that NOT A refutes A.
Date: 2010-02-25 21:00:15
Message-ID: 20100225210015.930447541D1@cvs.postgresql.org (view raw or flat)
Thread:
Lists: pgsql-committers
Log Message:
-----------
Allow predicate_refuted_by() to deduce that NOT A refutes A.

We had originally made the stronger assumption that NOT A refutes any B
if B implies A, but this fails in three-valued logic, because we need to
prove B is false not just that it's not true.  However the logic does
go through if B is equal to A.

Recognizing this limited case is enough to handle examples that arise when
we have simplified "bool_var = true" or "bool_var = false" to just "bool_var"
or "NOT bool_var".  If we had not done that simplification then the
btree-operator proof logic would have been able to prove that the expressions
were contradictory, but only for identical expressions being compared to the
constants; so handling identical A and B covers all the same cases.

The motivation for doing this is to avoid unexpected asymmetrical behavior
when a partitioned table uses a boolean partitioning column, as in today's
gripe from Dominik Sander.

Back-patch to 8.2, which is as far back as predicate_refuted_by attempts to
do anything at all with NOTs.

Tags:
----
REL8_2_STABLE

Modified Files:
--------------
    pgsql/src/backend/optimizer/util:
        predtest.c (r1.10.2.5 -> r1.10.2.6)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/util/predtest.c?r1=1.10.2.5&r2=1.10.2.6)

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