559f9e90db
When looking up statistical data about an expression, we failed to look through PlaceHolderVar nodes, treating them as opaque. This could prevent us from matching an expression to base columns, index expressions, or extended statistics, as examine_variable() relies on strict structural matching. As a result, queries involving PlaceHolderVar nodes often fell back to default selectivity estimates, potentially leading to poor plan choices. This patch updates examine_variable() to strip PlaceHolderVars before analysis. This is safe during estimation because PlaceHolderVars are transparent for the purpose of statistics lookup: they do not alter the value distribution of the underlying expression. To minimize performance overhead on this hot path, a lightweight walker first checks for the presence of PlaceHolderVars. The more expensive mutator is invoked only when necessary. There is one ensuing plan change in the regression tests, which is expected and demonstrates the fix: the rowcount estimate becomes much more accurate with this patch. Back-patch to v18. Although this issue exists before that, changes in this version made it common enough to notice. Given the lack of field reports for older versions, I am not back-patching further. Reported-by: Haowu Ge <gehaowu@bitmoe.com> Author: Richard Guo <guofenglinux@gmail.com> Discussion: https://postgr.es/m/62af586c-c270-44f3-9c5e-02c81d537e3d.gehaowu@bitmoe.com Backpatch-through: 18
PostgreSQL Database Management System
This directory contains the source code distribution of the PostgreSQL database management system.
PostgreSQL is an advanced object-relational database management system that supports an extended subset of the SQL standard, including transactions, foreign keys, subqueries, triggers, user-defined types and functions. This distribution also contains C language bindings.
Copyright and license information can be found in the file COPYRIGHT.
General documentation about this version of PostgreSQL can be found at https://www.postgresql.org/docs/devel/. In particular, information about building PostgreSQL from the source code can be found at https://www.postgresql.org/docs/devel/installation.html.
The latest version of this software, and related software, may be obtained at https://www.postgresql.org/download/. For more information look at our web site located at https://www.postgresql.org/.