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 <[email protected]> Reviewed-by: Tender Wang <[email protected]> Discussion: https://postgr.es/m/cambws49bmaowvkdshxpudnniqjcecgq3_8dd_5wtr4xrqy8...@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(-)
