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Peter Geoghegan 1bd4bc85ca Optimize nbtree backwards scans. 
Make nbtree backwards scans optimistically access the next page to be
read to the left by following a prevPage block number that's now stashed
in currPos when the leaf page is first read.  This approach matches the
one taken during forward scans, which follow a symmetric nextPage block
number from currPos.  We stash both a prevPage and a nextPage, since the
scan direction might change (when fetching from a scrollable cursor).

Backwards scans will no longer need to lock the same page twice, except
in rare cases where the scan detects a concurrent page split (or page
deletion).  Testing has shown this optimization to be particularly
effective during parallel index-only backwards scans: ~12% reductions in
query execution time are quite possible.

We're much better off being optimistic; concurrent left sibling page
splits are rare in general.  It's possible that we'll need to lock more
pages than the pessimistic approach would have, but only when there are
_multiple_ concurrent splits of the left sibling page we now start at.
If there's just a single concurrent left sibling page split, the new
approach to scanning backwards will at least break even relative to the
old one (we'll acquire the same number of leaf page locks as before).

The optimization from this commit has long been contemplated by comments
added by commit 2ed5b87f96, which changed the rules for locking/pinning
during nbtree index scans.  The approach that that commit introduced to
leaf level link traversal when scanning forwards is now more or less
applied all the time, regardless of the direction we're scanning in.

Following uniform conventions around sibling link traversal is simpler.
The only real remaining difference between our forward and backwards
handling is that our backwards handling must still detect and recover
from any concurrent left sibling splits (and concurrent page deletions),
as documented in the nbtree README.  That is structured as a single,
isolated extra step that takes place in _bt_readnextpage.

Also use this opportunity to further simplify the functions that deal
with reading pages and traversing sibling links on the leaf level, and
to document their preconditions and postconditions (with respect to
things like buffer locks, buffer pins, and seizing the parallel scan).

This enhancement completely supersedes the one recently added by commit
3f44959f.

Author: Matthias van de Meent <boekewurm+postgres@gmail.com>
Author: Peter Geoghegan <pg@bowt.ie>
Discussion: https://postgr.es/m/CAEze2WgpBGRgTTxTWVPXc9+PB6fc1a7t+VyGXHzfnrFXcQVxnA@mail.gmail.com
Discussion: https://postgr.es/m/CAH2-WzkBTuFv7W2+84jJT8mWZLXVL0GHq2hMUTn6c9Vw=eYrCw@mail.gmail.com
2024-10-18 11:25:32 -04:00
.github Add CODE_OF_CONDUCT.md, CONTRIBUTING.md, and SECURITY.md. 2024-07-02 13:03:58 -05:00
config Simplify checking for xlocale.h 2024-10-01 07:23:45 -04:00
contrib Add contrib/pg_logicalinspect. 2024-10-14 17:22:02 -07:00
doc Adjust documentation for configuring Linux huge pages. 2024-10-18 10:20:15 -05:00
src Optimize nbtree backwards scans. 2024-10-18 11:25:32 -04:00
.cirrus.star Remove duplicate words in docs and code comments. 2023-10-09 09:18:47 +05:30
.cirrus.tasks.yml Add PG_TEST_PG_COMBINEBACKUP_MODE to CI tasks 2024-09-10 16:30:38 +02:00
.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 commit 7f7474a8e4 to .git-blame-ignore-revs. 2024-10-14 15:09:39 -05:00
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.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 Remove traces of BeOS. 2024-10-14 08:33:36 +02:00
configure.ac Remove traces of BeOS. 2024-10-14 08:33:36 +02:00
COPYRIGHT Update copyright for 2024 2024-01-03 20:49:05 -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 Remove AIX support 2024-02-28 15:17:23 +04:00
meson.build Simplify checking for xlocale.h 2024-10-01 07:23:45 -04:00
meson_options.txt Remove spinlocks and atomics from meson_options.txt. 2024-07-30 23:37:14 +12:00
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/.