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Tom Lane 3bbc1c4a73 Restore the ability to run pl/pgsql expression queries in parallel. 
pl/pgsql's notion of an "expression" is very broad, encompassing
any SQL SELECT query that returns a single column and no more than
one row.  So there are cases, for example evaluation of an aggregate
function, where the query involves significant work and it'd be useful
to run it with parallel workers.  This used to be possible, but
commits 3eea7a0c9 et al unintentionally disabled it.

The simplest fix is to make exec_eval_expr() pass maxtuples = 0
rather than 2 to exec_run_select().  This avoids the new rule that
we will never use parallelism when a nonzero "count" limit is passed
to ExecutorRun().  (Note that the pre-3eea7a0c9 behavior was indeed
unsafe, so reverting that rule is not in the cards.)  The reason
for passing 2 before was that exec_eval_expr() will throw an error
if it gets more than one returned row, so we figured that as soon
as we have two rows we know that will happen and we might as well
stop running the query.  That choice was cost-free when it was made;
but disabling parallelism is far from cost-free, so now passing 2
amounts to optimizing a failure case at the expense of useful cases.
An expression query that can return more than one row is certainly
broken.  People might now need to wait a bit longer to discover such
breakage; but hopefully few will use enormously expensive cases as
their first test of new pl/pgsql logic.

Author: Dipesh Dhameliya <dipeshdhameliya125@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CABgZEgdfbnq9t6xXJnmXbChNTcWFjeM_6nuig41tm327gYi2ig@mail.gmail.com
Backpatch-through: 13
2025-07-07 14:33:56 -04:00
config Log LLVM library version in configure output. 2023-10-22 14:23:11 +13:00
contrib Fix incompatibility with libxml2 >= 2.14 2025-07-07 08:54:39 +09:00
doc doc: Update outdated descriptions of wal_status in pg_replication_slots. 2025-07-03 23:09:21 +09:00
src Restore the ability to run pl/pgsql expression queries in parallel. 2025-07-07 14:33:56 -04:00
.cirrus.star ci: Prepare to make compute resources for CI configurable 2023-08-23 15:15:28 -07:00
.cirrus.tasks.yml ci: Upgrade FreeBSD image 2025-03-05 10:29:03 -05:00
.cirrus.yml ci: Make compute resources for CI configurable 2023-08-23 15:15:28 -07:00
.dir-locals.el Make Emacs perl-mode indent more like perltidy. 2019-01-13 11:32:31 -08:00
.editorconfig Add .editorconfig 2019-12-18 09:13:13 +01:00
.git-blame-ignore-revs Add b334612b8 to .git-blame-ignore-revs. 2023-06-20 09:52:52 -04:00
.gitattributes Exclude LLVM files from whitespace checks 2024-11-27 11:09:38 +01:00
.gitignore Update top-level .gitignore. 2022-12-04 15:23:00 -05:00
aclocal.m4 autoconf: Move export_dynamic determination to configure 2022-12-06 18:55:28 -08:00
configure Make our usage of memset_s() conform strictly to the C11 standard. 2025-05-18 12:45:55 -04:00
configure.ac Make our usage of memset_s() conform strictly to the C11 standard. 2025-05-18 12:45:55 -04:00
COPYRIGHT Align organization wording in copyright statement 2025-05-16 11:20:07 -04:00
GNUmakefile.in Integrate pg_bsd_indent into our build/test infrastructure. 2023-02-12 12:22:21 -05:00
HISTORY Canonicalize some URLs 2020-02-10 20:47:50 +01:00
Makefile Dynamically find correct installation docs in Makefile. 2022-01-19 14:48:25 +01:00
meson.build meson: Fix meson warning 2025-06-22 14:15:40 +02:00
meson_options.txt meson: Attach colon to keyword argument 2023-06-29 12:53:41 +02:00
README Canonicalize some URLs 2020-02-10 20:47:50 +01:00
README.git Canonicalize some URLs 2020-02-10 20:47:50 +01:00

README 

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.

PostgreSQL has many language interfaces, many of which are listed here:

	https://www.postgresql.org/download/

See the file INSTALL for instructions on how to build and install
PostgreSQL.  That file also lists supported operating systems and
hardware platforms and contains information regarding any other
software packages that are required to build or run the PostgreSQL
system.  Copyright and license information can be found in the
file COPYRIGHT.  A comprehensive documentation set is included in this
distribution; it can be read as described in the installation
instructions.

The latest version of this software may be obtained at
https://www.postgresql.org/download/.  For more information look at our
web site located at https://www.postgresql.org/.