Date: Sat, 18 Jan 2025 23:20:25 -0800
From: Jeremy Schneider <schneider@ardentperf.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Kamen Kalchev <kalchev035@gmail.com>, pgsql-general@postgresql.org,
 abubakarm94@gmail.com
Subject: Re: glibc 2.35-2.39 upgrade requirements
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On Fri, 17 Jan 2025 10:27:04 -0500
Tom Lane <tgl@sss.pgh.pa.us> wrote:

> Kamen Kalchev <kalchev035@gmail.com> writes:
> > Hi everyone, we're planning to upgrade the OS running Postgres from
> > ubuntu jammy to ubuntu noble. As part of the OS change, the glibc
> > version will be changed from glibc 2.35 to glibc 2.39..
> > Can someone confirm if changing the glibc between those versions
> > will require a full reindex of the Postgres cluster?  
> 
> Maybe, maybe not.  According to [1], the last glibc collation change
> that the PG community really noticed was in glibc 2.28.  So maybe
> there weren't any significant changes between 2.35 and 2.39.  The
> conservative path would certainly be to reindex all textual columns
> (though you can skip any that have collation "C").

I haven't run 2.39 through the scan yet [1]; I should do that because
someone was asking the same question on postgres slack. But note that
every single ubuntu LTS and every single RHEL major release in the last
10 years has had collation changes, except for ubuntu 14.04 ... so it's
worth being cautious. Collations are a bit like time zones - small
changes are always happening, but you might not always notice.

Jeff Davis and I did a talk at the last pgconf about this, the
recording is online [2].

Personally I would recommend using the builtin C collation as database
default starting in pg17, and using ICU to do linguistic collation at
the table or query level when needed. With ICU there's at least the
option to rebuild old versions on new operating system majors, if
needed. (Though rebuilding objects - not just indexes, but anything
depending on the collation - is the best course.)

And be careful about hot standbys, FDWs, and other places where you can
get little surprises with different OS majors. The YouTube recording
has lots of info.

-Jeremy


1: https://github.com/ardentperf/glibc-unicode-sorting
2: https://www.youtube.com/watch?v=KTA6oau7tl8