Peter Geoghegan 92fe23d93a Add nbtree skip scan optimization. Teach nbtree multi-column index scans to opportunistically skip over irrelevant sections of the index given a query with no "=" conditions on one or more prefix index columns. When nbtree is passed input scan keys derived from a predicate "WHERE b = 5", new nbtree preprocessing steps output "WHERE a = ANY(<every possible 'a' value>) AND b = 5" scan keys. That is, preprocessing generates a "skip array" (and an output scan key) for the omitted prefix column "a", which makes it safe to mark the scan key on "b" as required to continue the scan. The scan is therefore able to repeatedly reposition itself by applying both the "a" and "b" keys. A skip array has "elements" that are generated procedurally and on demand, but otherwise works just like a regular ScalarArrayOp array. Preprocessing can freely add a skip array before or after any input ScalarArrayOp arrays. Index scans with a skip array decide when and where to reposition the scan using the same approach as any other scan with array keys. This design builds on the design for array advancement and primitive scan scheduling added to Postgres 17 by commit `5bf748b8`. Testing has shown that skip scans of an index with a low cardinality skipped prefix column can be multiple orders of magnitude faster than an equivalent full index scan (or sequential scan). In general, the cardinality of the scan's skipped column(s) limits the number of leaf pages that can be skipped over. The core B-Tree operator classes on most discrete types generate their array elements with the help of their own custom skip support routine. This infrastructure gives nbtree a way to generate the next required array element by incrementing (or decrementing) the current array value. It can reduce the number of index descents in cases where the next possible indexable value frequently turns out to be the next value stored in the index. Opclasses that lack a skip support routine fall back on having nbtree "increment" (or "decrement") a skip array's current element by setting the NEXT (or PRIOR) scan key flag, without directly changing the scan key's sk_argument. These sentinel values behave just like any other value from an array -- though they can never locate equal index tuples (they can only locate the next group of index tuples containing the next set of non-sentinel values that the scan's arrays need to advance to). A skip array's range is constrained by "contradictory" inequality keys. For example, a skip array on "x" will only generate the values 1 and 2 given a qual such as "WHERE x BETWEEN 1 AND 2 AND y = 66". Such a skip array qual usually has near-identical performance characteristics to a comparable SAOP qual "WHERE x = ANY('{1, 2}') AND y = 66". However, improved performance isn't guaranteed. Much depends on physical index characteristics. B-Tree preprocessing is optimistic about skipping working out: it applies static, generic rules when determining where to generate skip arrays, which assumes that the runtime overhead of maintaining skip arrays will pay for itself -- or lead to only a modest performance loss. As things stand, these assumptions are much too optimistic: skip array maintenance will lead to unacceptable regressions with unsympathetic queries (queries whose scan can't skip over many irrelevant leaf pages). An upcoming commit will address the problems in this area by enhancing _bt_readpage's approach to saving cycles on scan key evaluation, making it work in a way that directly considers the needs of = array keys (particularly = skip array keys). Author: Peter Geoghegan <pg@bowt.ie> Reviewed-By: Masahiro Ikeda <masahiro.ikeda@nttdata.com> Reviewed-By: Heikki Linnakangas <heikki.linnakangas@iki.fi> Reviewed-By: Matthias van de Meent <boekewurm+postgres@gmail.com> Reviewed-By: Tomas Vondra <tomas@vondra.me> Reviewed-By: Aleksander Alekseev <aleksander@timescale.com> Reviewed-By: Alena Rybakina <a.rybakina@postgrespro.ru> Discussion: https://postgr.es/m/CAH2-Wzmn1YsLzOGgjAQZdn1STSG_y8qP__vggTaPAYXJP+G4bw@mail.gmail.com		9 months ago
.github	Add CODE_OF_CONDUCT.md, CONTRIBUTING.md, and SECURITY.md.	2 years ago
config	Optimize popcount functions with ARM SVE intrinsics.	9 months ago
contrib	Fix autoprewarm neglect of tablespaces	9 months ago
doc	Add nbtree skip scan optimization.	9 months ago
src	Add nbtree skip scan optimization.	9 months ago
.cirrus.star	Remove duplicate words in docs and code comments.	2 years ago
.cirrus.tasks.yml	aio: Add io_method=io_uring	9 months ago
.cirrus.yml	ci: Test NetBSD and OpenBSD	11 months ago
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.editorconfig	Add script to keep .editorconfig in sync with .gitattributes	11 months ago
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.gitattributes	Add script to keep .editorconfig in sync with .gitattributes	11 months ago
.gitignore	Update top-level .gitignore.	3 years ago
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COPYRIGHT	Update copyright for 2025	12 months ago
GNUmakefile.in	Allow selecting the git revision to be packaged by "make dist".	2 years ago
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Makefile	Remove AIX support	2 years ago
README.md	Revise the style of a paragraph in README.md.	2 years ago
aclocal.m4	autoconf: Move export_dynamic determination to configure	3 years ago
configure	libpq: Add support for dumping SSL key material to file	9 months ago
configure.ac	libpq: Add support for dumping SSL key material to file	9 months ago
meson.build	libpq: Add support for dumping SSL key material to file	9 months ago
meson_options.txt	aio: Add liburing dependency	9 months ago

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.

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/.