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Andres Freund f63ca33790 read_stream: Only increase read-ahead distance when waiting for IO 
This avoids increasing the distance to the maximum in cases where the I/O
subsystem is already keeping up. This turns out to be important for
performance for two reasons:

- Pinning a lot of buffers is not cheap. If additional pins allow us to avoid
  IO waits, it's definitely worth it, but if we can already do all the
  necessary readahead at a distance of 16, reading ahead 512 buffers can
  increase the CPU overhead substantially.  This is particularly noticeable
  when the to-be-read blocks are already in the kernel page cache.

- If the read stream is read to completion, reading in data earlier than
  needed is of limited consequences, leaving aside the CPU costs mentioned
  above. But if the read stream will not be fully consumed, e.g. because it is
  on the inner side of a nested loop join, the additional IO can be a serious
  performance issue. This is not that commonly a problem for current read
  stream users, but the upcoming work, to use a read stream to fetch table
  pages as part of an index scan, frequently encounters this.

Note that this commit would have substantial performance downsides without
earlier commits:

- Commit 6e36930f9a, which avoids decreasing the readahead distance when
  there was recent IO, is crucial, as otherwise we very often would end up not
  reading ahead aggressively enough anymore with this commit, due to
  increasing the distance less often.

- "read stream: Split decision about look ahead for AIO and combining" is
  important as we would otherwise not perform IO combining when the IO
  subsystem can keep up.

- "aio: io_uring: Trigger async processing for large IOs" is important to
  continue to benefit from memory copy parallelism when using fewer IOs.

Reviewed-by: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Nazir Bilal Yavuz <byavuz81@gmail.com>
Tested-by: Tomas Vondra <tomas@vondra.me>
Discussion: https://postgr.es/m/f3xxfrkafjxpyqxywcxricxgyizjirfceychyxsgn7bwjp5eda@kwbduhy7tfmu
Discussion: https://postgr.es/m/CA+hUKGL2PhFyDoqrHefqasOnaXhSg48t1phs3VM8BAdrZqKZkw@mail.gmail.com
2026-04-05 00:43:54 -04:00
.github Add CODE_OF_CONDUCT.md, CONTRIBUTING.md, and SECURITY.md. 2024-07-02 13:03:58 -05:00
config Compute CRC32C on ARM using the Crypto Extension where available 2026-04-04 20:47:01 +07:00
contrib Merge init and max size options on shmem hash tables 2026-04-04 02:40:20 +03:00
doc oauth: Let validators provide failure DETAILs 2026-04-03 16:05:33 -07:00
src read_stream: Only increase read-ahead distance when waiting for IO 2026-04-05 00:43:54 -04:00
.cirrus.star ci: Simplify ci-os-only handling 2025-08-14 12:09:34 -04:00
.cirrus.tasks.yml Enable test_cplusplusext with MSVC 2026-04-01 07:48:47 +02:00
.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 commit 874da8b1f6 to .git-blame-ignore-revs. 2026-03-30 14:35:24 -05:00
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aclocal.m4 autoconf: Move export_dynamic determination to configure 2022-12-06 18:55:28 -08:00
configure Compute CRC32C on ARM using the Crypto Extension where available 2026-04-04 20:47:01 +07:00
configure.ac Compute CRC32C on ARM using the Crypto Extension where available 2026-04-04 20:47:01 +07:00
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 Compute CRC32C on ARM using the Crypto Extension where available 2026-04-04 20:47:01 +07:00
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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/.