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Doc: Update notes about libc collation versions.

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The per-index collation version tracking feature was reverted, but we
still have the ability to ask Windows (352f6f2d) and FreeBSD
(ca051d8b) for collation versions to store in pg_collation.collversion.
So, from the reverted patch, take a few words of documentation about
libc on all three supported OSes to replace the pre-existing note that
mentioned only glibc.

Discussion: https://postgr.es/m/CA%2BhUKGLhj5t1fcjqAu8iD9B3ixJtsTNqyCCD4V0aTO9kAKAjjA%40mail.gmail.com
This commit is contained in:
Thomas Munro 2021-05-07 21:47:08 +12:00
parent ec48314708
commit b65431ca5e
1 changed files with 17 additions and 5 deletions
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22
doc/src/sgml/ref/alter_collation.sgml
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@ -129,12 +129,24 @@ HINT: Rebuild all objects affected by this collation and run ALTER COLLATION pg
correctly.
</para>
<para>
When using collations provided by <literal>libc</literal> and
<productname>PostgreSQL</productname> was built with the GNU C library, the
C library's version is used as a collation version. Since collation
definitions typically change only with GNU C library releases, this provides
some defense against corruption, but it is not completely reliable.
When using collations provided by <literal>libc</literal>, version
information is recorded on systems using the GNU C library (most Linux
systems), FreeBSD and Windows.
</para>
<note>
<para>
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.
</para>
<para>
When using Windows for collations, version information is only available
for collations defined with BCP 47 language tags such as
<literal>en-US</literal>.
</para>
</note>
<para>
Currently, there is no version tracking for the database default collation.
</para>