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Tom Lane f4c7c410ee Revert "Optimize order of GROUP BY keys". 
This reverts commit db0d67db24 and
several follow-on fixes.  The idea of making a cost-based choice
of the order of the sorting columns is not fundamentally unsound,
but it requires cost information and data statistics that we don't
really have.  For example, relying on procost to distinguish the
relative costs of different sort comparators is pretty pointless
so long as most such comparator functions are labeled with cost 1.0.
Moreover, estimating the number of comparisons done by Quicksort
requires more than just an estimate of the number of distinct values
in the input: you also need some idea of the sizes of the larger
groups, if you want an estimate that's good to better than a factor of
three or so.  That's data that's often unknown or not very reliable.
Worse, to arrive at estimates of the number of calls made to the
lower-order-column comparison functions, the code needs to make
estimates of the numbers of distinct values of multiple columns,
which are necessarily even less trustworthy than per-column stats.
Even if all the inputs are perfectly reliable, the cost algorithm
as-implemented cannot offer useful information about how to order
sorting columns beyond the point at which the average group size
is estimated to drop to 1.

Close inspection of the code added by db0d67db2 shows that there
are also multiple small bugs.  These could have been fixed, but
there's not much point if we don't trust the estimates to be
accurate in-principle.

Finally, the changes in cost_sort's behavior made for very large
changes (often a factor of 2 or so) in the cost estimates for all
sorting operations, not only those for multi-column GROUP BY.
That naturally changes plan choices in many situations, and there's
precious little evidence to show that the changes are for the better.
Given the above doubts about whether the new estimates are really
trustworthy, it's hard to summon much confidence that these changes
are better on the average.

Since we're hard up against the release deadline for v15, let's
revert these changes for now.  We can always try again later.

Note: in v15, I left T_PathKeyInfo in place in nodes.h even though
it's unreferenced.  Removing it would be an ABI break, and it seems
a bit late in the release cycle for that.

Discussion: https://postgr.es/m/TYAPR01MB586665EB5FB2C3807E893941F5579@TYAPR01MB5866.jpnprd01.prod.outlook.com
2022-10-03 10:56:16 -04:00
..
authentication Add authentication TAP test for peer authentication 2022-10-03 16:42:25 +09:00
examples Remove configure probe for sys/select.h. 2022-08-14 00:09:47 +12:00
icu meson: Add initial version of meson based build system 2022-09-21 22:37:17 -07:00
isolation Harmonize more lexer function parameter names. 2022-09-22 13:27:16 -07:00
kerberos Introduce SYSTEM_USER 2022-09-29 15:05:40 +09:00
ldap meson: Add initial version of meson based build system 2022-09-21 22:37:17 -07:00
locale pycodestyle (PEP 8) cleanup in Python scripts 2022-03-09 10:54:20 +01:00
mb
modules Remove PQsendQuery support in pipeline mode 2022-09-23 18:21:22 +02:00
perl meson: Add initial version of meson based build system 2022-09-21 22:37:17 -07:00
recovery Improve wording of log messages triggered by max_slot_wal_keep_size. 2022-09-29 13:27:48 -04:00
regress Revert "Optimize order of GROUP BY keys". 2022-10-03 10:56:16 -04:00
ssl meson: Add initial version of meson based build system 2022-09-21 22:37:17 -07:00
subscription Fix tab-completion after commit 790bf615dd 2022-09-30 12:53:31 +02:00
Makefile Refactor PG_TEST_EXTRA logic in autoconf build 2022-09-20 11:24:16 -07:00
meson.build meson: Add initial version of meson based build system 2022-09-21 22:37:17 -07:00
README Remove the option to build thread_test.c outside configure. 2020-10-21 12:08:48 -04:00

README 

PostgreSQL tests
================

This directory contains a variety of test infrastructure as well as some of the
tests in PostgreSQL. Not all tests are here -- in particular, there are more in
individual contrib/ modules and in src/bin.

Not all these tests get run by "make check". Check src/test/Makefile to see
which tests get run automatically.

authentication/
  Tests for authentication (but see also below)

examples/
  Demonstration programs for libpq that double as regression tests via
  "make check"

isolation/
  Tests for concurrent behavior at the SQL level

kerberos/
  Tests for Kerberos/GSSAPI authentication and encryption

ldap/
  Tests for LDAP-based authentication

locale/
  Sanity checks for locale data, encodings, etc

mb/
  Tests for multibyte encoding (UTF-8) support

modules/
  Extensions used only or mainly for test purposes, generally not suitable
  for installing in production databases

perl/
  Infrastructure for Perl-based TAP tests

recovery/
  Test suite for recovery and replication

regress/
  PostgreSQL's main regression test suite, pg_regress

ssl/
  Tests to exercise and verify SSL certificate handling

subscription/
  Tests for logical replication