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0a37961254
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
The PostgreSQL contrib tree
---------------------------
This subtree contains porting tools, analysis utilities, and plug-in
features that are not part of the core PostgreSQL system, mainly
because they address a limited audience or are too experimental to be
part of the main source tree. This does not preclude their
usefulness.
User documentation for each module appears in the main SGML
documentation.
When building from the source distribution, these modules are not
built automatically, unless you build the "world" target. You can
also build and install them all by running "make all" and "make
install" in this directory; or to build and install just one selected
module, do the same in that module's subdirectory.
Some directories supply new user-defined functions, operators, or
types. To make use of one of these modules, after you have installed
the code you need to register the new SQL objects in the database
system by executing a CREATE EXTENSION command. In a fresh database,
you can simply do
CREATE EXTENSION module_name;
See the PostgreSQL documentation for more information about this
procedure.