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Tom Lane d80b022501 Correctly calculate "MCV frequency" for a unique column. 
In commit bd3e3e9e5, I over-hastily used 1 / rel->rows as the assumed
frequency of entries in a column that ANALYZE has found to be unique.
However, rel->rows is the number of table rows that are estimated to
pass the query's restriction conditions, so that we got a too-large
result if the query has selective restrictions.  What I should have
used is 1 / rel->tuples, since that is the estimated total number of
table rows.  The pre-existing code path that digs a frequency out of
the histogram produces a frequency relative to the whole table, so
surely this new alternative code path must do so as well.  Any
correction needed on the basis of selectivity must be done by the
user of the mcv_freq value.

Fixing this causes all the regression test plans changed by bd3e3e9e5
to revert to what they had been, except for the first change in
join.out.  As I correctly argued in bd3e3e9e5, in that test case we
have no stats and should not risk a hash join.  Evidently I was less
correct to argue that the other changes were improvements.

Reported-by: Joel Jacobson <joel@compiler.org>
Diagnosed-by: Tender Wang <tndrwang@gmail.com>
Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/341b723c-da45-4058-9446-1514dedb17c1@app.fastmail.com
2026-03-01 12:56:55 -05:00
.github Add CODE_OF_CONDUCT.md, CONTRIBUTING.md, and SECURITY.md. 2024-07-02 13:03:58 -05:00
config Restore AIX support. 2026-02-23 13:34:22 -05:00
contrib Use pg_malloc_object() and pg_alloc_array() variants in frontend code 2026-02-27 18:59:41 +09:00
doc psql: Show comments in \dRp+, \dRs+, and \dX+ psql meta-commands. 2026-02-28 23:56:46 +09:00
src Correctly calculate "MCV frequency" for a unique column. 2026-03-01 12:56:55 -05:00
.cirrus.star ci: Simplify ci-os-only handling 2025-08-14 12:09:34 -04:00
.cirrus.tasks.yml meson: allow disabling building/installation of static libraries. 2026-02-23 16:45:40 +01:00
.cirrus.yml ci: Per-repo configuration for manually trigger tasks 2025-08-14 11:54:03 -04:00
.dir-locals.el Make Emacs perl-mode indent more like perltidy. 2019-01-13 11:32:31 -08:00
.editorconfig Update .editorconfig and .gitattributes for postgresql.conf.sample. 2025-11-18 10:28:36 -06:00
.git-blame-ignore-revs Add a couple of recent commits to .git-blame-ignore-revs. 2026-01-28 15:56:48 -06:00
.gitattributes Update .editorconfig and .gitattributes for postgresql.conf.sample. 2025-11-18 10:28:36 -06:00
.gitignore Update top-level .gitignore. 2022-12-04 15:23:00 -05:00
.mailmap Add a Git .mailmap file 2024-11-05 13:56:02 +01:00
aclocal.m4 autoconf: Move export_dynamic determination to configure 2022-12-06 18:55:28 -08:00
configure Restore AIX support. 2026-02-23 13:34:22 -05:00
configure.ac Restore AIX support. 2026-02-23 13:34:22 -05:00
COPYRIGHT Update copyright for 2026 2026-01-01 13:24:10 -05:00
GNUmakefile.in Allow selecting the git revision to be packaged by "make dist". 2024-05-03 11:08:50 -04:00
HISTORY Canonicalize some URLs 2020-02-10 20:47:50 +01:00
Makefile Restore AIX support. 2026-02-23 13:34:22 -05:00
meson.build Restore AIX support. 2026-02-23 13:34:22 -05:00
meson_options.txt Update copyright for 2026 2026-01-01 13:24:10 -05:00
README.md Revise the style of a paragraph in README.md. 2024-03-21 10:16:41 -05:00

README.md

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/.