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Tom Lane 65a3ff8f1b Doc: improve user docs and code comments about EXISTS(SELECT * ...). 
Point out that Postgres automatically optimizes away the target list
of an EXISTS' subquery, except in weird cases such as target lists
containing set-returning functions.  Thus, both common conventions
EXISTS(SELECT * FROM ...) and EXISTS(SELECT 1 FROM ...) are
overhead-free and there's little reason to prefer one over the other.

In the code comments, mention that the SQL spec says that
EXISTS(SELECT * FROM ...) should be interpreted as EXISTS(SELECT
some-literal FROM ...), but we don't choose to do it exactly that way.

Author: Peter Eisentraut <peter@eisentraut.org>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/9b301c70-3909-4f0f-98ca-9e3c4d142f3e@eisentraut.org
2026-02-27 15:20:16 -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 Doc: improve user docs and code comments about EXISTS(SELECT * ...). 2026-02-27 15:20:16 -05:00
src Doc: improve user docs and code comments about EXISTS(SELECT * ...). 2026-02-27 15:20:16 -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
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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/.