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The IS DISTINCT FROM construct compares values acting as though NULL were a normal data value, rather than "unknown". Semantically, "x IS DISTINCT FROM y" yields true if the values differ or if exactly one is NULL, and false if they are equal or both NULL. Unlike ordinary comparison operators, it never returns NULL. Previously, the planner only simplified this construct if all inputs were constants, folding it to a constant boolean result. This patch extends the optimization to cases where inputs are non-constant but proven to be non-nullable. Specifically, "x IS DISTINCT FROM NULL" folds to constant TRUE if "x" is known to be non-nullable. For cases where both inputs are guaranteed not to be NULL, the expression becomes semantically equivalent to "x <> y", and the DistinctExpr is converted into an inequality OpExpr. This transformation provides several benefits. It converts the comparison into a standard operator, allowing the use of partial indexes and constraint exclusion. Furthermore, if the clause is negated (i.e., "IS NOT DISTINCT FROM"), it simplifies to an equality operator. This enables the planner to generate better plans using index scans, merge joins, hash joins, and EC-based qual deduction. Author: Richard Guo <guofenglinux@gmail.com> Reviewed-by: Tender Wang <tndrwang@gmail.com> Discussion: https://postgr.es/m/CAMbWs49BMAOWvkdSHxpUDnniqJcEcGq3_8dd_5wTR4xrQY8urA@mail.gmail.com |
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