pgsql: Reduce "X = X" to "X IS NOT NULL", if it's easy to do so.

Reduce "X = X" to "X IS NOT NULL", if it's easy to do so.

If the operator is a strict btree equality operator, and X isn't volatile,
then the clause must yield true for any non-null value of X, or null if X
is null. At top level of a WHERE clause, we can ignore the distinction
between false and null results, so it's valid to simplify the clause to
"X IS NOT NULL". This is a useful improvement mainly because we'll get
a far better selectivity estimate in most cases.

Because such cases seldom arise in well-written queries, it is unappetizing
to expend a lot of planner cycles looking for them ... but it turns out
that there's a place we can shoehorn this in practically for free, because
equivclass.c already has to detect and reject candidate equivalences of the
form X = X. That doesn't catch every place that it would be valid to
simplify to X IS NOT NULL, but it catches the typical case. Working harder
doesn't seem justified.

Patch by me, reviewed by Petr Jelinek

Discussion: https://postgr.es/m/CAMjNa7cC4X9YR-vAJS-jSYCajhRDvJQnN7m2sLH1wLh-_Z2bsw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/8ec5429e2f422f4d570d4909507db0d4ca83bbac

Modified Files
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src/backend/optimizer/path/equivclass.c | 66 +++++++++++++++++++++++++-------
src/backend/optimizer/plan/initsplan.c | 5 ++-
src/include/optimizer/paths.h | 3 +-
src/test/regress/expected/equivclass.out | 18 +++++++++
src/test/regress/sql/equivclass.sql | 8 ++++
5 files changed, 83 insertions(+), 17 deletions(-)