From e45a1266a276665943417007c31770bd338ac995 Mon Sep 17 00:00:00 2001 From: Thomas Munro Date: Wed, 11 Mar 2020 15:01:23 +1300 Subject: [PATCH 7/7] doc: Add Collation Versions section. Supply a brief introduction to collation version concepts. --- doc/src/sgml/charset.sgml | 35 +++++++++++++++++++++++++++++++++++ 1 file changed, 35 insertions(+) diff --git a/doc/src/sgml/charset.sgml b/doc/src/sgml/charset.sgml index 20cdfabd7b..d4aa300c5a 100644 --- a/doc/src/sgml/charset.sgml +++ b/doc/src/sgml/charset.sgml @@ -936,6 +936,41 @@ CREATE COLLATION ignore_accents (provider = icu, locale = 'und-u-ks-level1-kc-tr + + + Collation Versions + + + The ordering defined by a collation is not necessarily fixed over time. + If a collation changes for any reason, persistent data structures such as + b-trees that depend on a stable ordering of text might be corrupted. + PostgreSQL defends against this by recording + the current version of each referenced collation for any index that + depends on it in the + pg_depend + catalog, if the collation provider makes it available. If the provider + later begins to report a different version, a warning will be reported + when the index is accessed, until either the + or the command is used to update the + version. + + + Version information is available for collations from the + icu provider on all operating systems. For the + libc provider, versions are currently only available + on systems using the GNU C library (most Linux systems). + + + + + When using the GNU C library for collations, the C library's version + is used as a proxy for the collation version. Many Linux distributions + change collation definitions only when upgrading the C library, but this + approach is imperfect as maintainers are free to back-port newer + collation definitions to older C library releases. + + + -- 2.20.1