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pg_upgrade: Add --set-char-signedness to set the default char signedness of new cluster.

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This change adds a new option --set-char-signedness to pg_upgrade. It
enables user to set arbitrary signedness during pg_upgrade. This helps
cases where user who knew they copied the v17 source cluster from
x86 (signedness=true) to ARM (signedness=false) can pg_upgrade
properly without the prerequisite of acquiring an x86 VM.

Reviewed-by: Noah Misch <noah@leadboat.com>
Discussion: https://postgr.es/m/CB11ADBC-0C3F-4FE0-A678-666EE80CBB07%40amazon.com
This commit is contained in:
Masahiko Sawada
2025-02-21 10:23:39 -08:00
parent a8238f87f9
commit 1aab680591
6 changed files with 105 additions and 2 deletions
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53
doc/src/sgml/ref/pgupgrade.sgml
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@ -285,6 +285,59 @@ PostgreSQL documentation
</listitem>
</varlistentry>
<varlistentry>
<term><option>--set-char-signedness=</option><replaceable>option</replaceable></term>
<listitem>
<para>
Manually set the default char signedness of new clusters. Possible values
are <literal>signed</literal> and <literal>unsigned</literal>.
</para>
<para>
In the C language, the default signedness of the <type>char</type> type
(when not explicitly specified) varies across platforms. For example,
<type>char</type> defaults to <type>signed char</type> on x86 CPUs but
to <type>unsigned char</type> on ARM CPUs.
</para>
<para>
Starting from <productname>PostgreSQL</productname> 18, database clusters
maintain their own default char signedness setting, which can be used to
ensure consistent behavior across platforms with different default char
signedness. By default, <application>pg_upgrade</application> preserves
the char signedness setting when upgrading from an existing cluster.
However, when upgrading from <productname>PostgreSQL</productname> 17 or
earlier, <application>pg_upgrade</application> adopts the char signedness
of the platform on which it was built.
</para>
<para>
This option allows you to explicitly set the default char signedness for
the new cluster, overriding any inherited values. There are two specific
scenarios where this option is relevant:
<itemizedlist>
<listitem>
<para>
If you are planning to migrate to a different platform after the upgrade,
you should not use this option. The default behavior is right in this case.
Instead, perform the upgrade on the original platform without this flag,
and then migrate the cluster afterward. This is the recommended and safest
approach.
</para>
</listitem>
<listitem>
<para>
If you have already migrated the cluster to a platform with different
char signedness (for example, from an x86-based system to an ARM-based
system), you should use this option to specify the signedness matching
the original platform's default char signedness. Additionally, it's
essential not to modify any data files between migrating data files and
running <command>pg_upgrade</command>. <command>pg_upgrade</command>
should be the first operation that starts the cluster on the new platform.
</para>
</listitem>
</itemizedlist>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-?</option></term>
<term><option>--help</option></term>