Teach planner to transform "x IS [NOT] DISTINCT FROM NULL" to a NullTest
In the spirit of 8d19d0e13, this patch teaches the planner about the
principle that NullTest with !argisrow is fully equivalent to SQL's IS
[NOT] DISTINCT FROM NULL.
The parser already performs this transformation for literal NULLs.
However, a DistinctExpr expression with one input evaluating to NULL
during planning (e.g., via const-folding of "1 + NULL" or parameter
substitution in custom plans) currently remains as a DistinctExpr
node.
This patch closes the gap for const-folded NULLs. It specifically
targets the case where one input is a constant NULL and the other is a
nullable non-constant expression. (If the other input were otherwise,
the DistinctExpr node would have already been simplified to a constant
TRUE or FALSE.)
This transformation can be beneficial because NullTest is much more
amenable to optimization than DistinctExpr, since the planner knows a
good deal about the former and next to nothing about the latter.
Author: Richard Guo <guofenglinux@gmail.com>
Reviewed-by: Tender Wang <tndrwang@gmail.com>
Discussion: https://postgr.es/m/CAMbWs49BMAOWvkdSHxpUDnniqJcEcGq3_8dd_5wTR4xrQY8urA@mail.gmail.com
Branch
------
master
Details
-------
https://git.postgresql.org/pg/commitdiff/f41ab51573a4c3d11f906b32a068182a1fd8596d
Modified Files
--------------
src/backend/optimizer/util/clauses.c | 24 ++++++++++++
src/test/regress/expected/predicate.out | 69 ++++++++++++++++++++++++++++++++-
src/test/regress/sql/predicate.sql | 27 ++++++++++++-
3 files changed, 118 insertions(+), 2 deletions(-)
