Re: First draft of PG 19 release notes

I will review this and hopefully apply it on Friday.

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On Sat, Oct 31, 2026 at 11:24:33PM -0400, shihao zhong wrote:
> Hi Bruce,
>
> One more item for the migration section. After the BUG #19749 fix
> (18740523a), older btree indexes on character columns must be rebuilt.
> The same goes for oidvector. pg_upgrade does not do it. Until then,
> index-only scans can return wrong values.
>
> The attached patch adds a short entry that tells people to REINDEX them
> after upgrading. It is for REL_19_STABLE only. Would you add it?
>
> The fix was also back-patched to 14 through 18 (882fd416d6e on 18).
> The next minor release notes for those branches need the same REINDEX
> advice for character indexes. oidvector keeps deduplication there, so
> only 19 needs it.
>
> Thanks,
> Shihao

> From 83088d79d6d7ecb998070afa06bcce30ebe33a4d Mon Sep 17 00:00:00 2001
> From: Shihao Zhong <zhong950419(at)gmail(dot)com>
> Date: Sat, 31 Oct 2026 22:52:18 -0400
> Subject: [PATCH v1] doc: Add PG 19 migration note about bpchar index
> deduplication.
>
> Commit 18740523a4a stopped nbtree deduplication for bpchar indexes, but
> indexes built by earlier major versions keep the old allequalimage flag.
> pg_upgrade does not rebuild them, and pg_upgrade --check does not warn
> about them, so amcheck reports them until they are rebuilt, and an
> index-only scan can return a value that differs from the heap in
> trailing spaces. Tell users to REINDEX them after upgrading.
>
> Discussion: https://postgr.es/m/CAGRkXqQQ311Lg=62x5UdZtPuRFgpT6XpPMw4X9E3tgbTu162Mw@mail.gmail.com
> ---
> doc/src/sgml/release-19.sgml | 32 ++++++++++++++++++++++++++++++++
> 1 file changed, 32 insertions(+)
>
> diff --git a/doc/src/sgml/release-19.sgml b/doc/src/sgml/release-19.sgml
> index 253fa07179a..91eb2a3aae3 100644
> --- a/doc/src/sgml/release-19.sgml
> +++ b/doc/src/sgml/release-19.sgml
> @@ -431,6 +431,38 @@ recommended to reindex any indexes that use these functions after the upgrade.
> </para>
> </listitem>
>
> +<!--
> +Author: Peter Geoghegan <pg(at)bowt(dot)ie>
> +2026-10-06 [18740523a] Rescind unsafe deduplication support.
> +-->
> +
> +<listitem>
> +<para>
> +Disable <link linkend="btree-deduplication">B-tree deduplication</link> for
> +indexes on <type>character</type> (<type>bpchar</type>) columns
> +(Shihao Zhong, Peter Geoghegan)
> +<ulink url="&commit_baseurl;18740523a">&sect;</ulink>
> +</para>
> +
> +<para>
> +Values of <type>character</type> that differ only in trailing spaces compare
> +as equal but are stored differently, so deduplication could make an index-only
> +scan return a value other than the one in the table. Indexes built by earlier
> +major versions still claim to support deduplication, and
> +<link linkend="pgupgrade"><application>pg_upgrade</application></link> does
> +not change that. After upgrading, run <command>REINDEX</command> on every
> +B-tree index on a <type>character</type> column, including
> +<literal>character(<replaceable>n</replaceable>)</literal> columns and indexes
> +that use <literal>bpchar_pattern_ops</literal>.
> +<xref linkend="amcheck"/> reports such indexes until they are rebuilt.
> +</para>
> +
> +<para>
> +The same applies to B-tree indexes on <type>oidvector</type> columns, which
> +are rarely used outside the system catalogs.
> +</para>
> +</listitem>
> +
> <!--
> Author: Thomas Munro <tmunro(at)postgresql(dot)org>
> 2026-04-08 [77645d44e] Remove MULE_INTERNAL encoding.
> --
> 2.37.1 (Apple Git-137.1)
>

--
Bruce Momjian <bruce(at)momjian(dot)us> https://momjian.us
EDB https://enterprisedb.com

Do not let urgent matters crowd out time for investment in the future.