Existing PostgreSQL databases need to be reindexed

Closes: #926627

en/issues.dbk

@@ -356,6 +356,34 @@ information mentioned in <xref linkend="morereading"/>.
     </para>
   </section>
 
+  <section id="postgresql-reindex">
+    <!-- stretch to buster-->
+    <title>Existing PostgreSQL databases need to be reindexed</title>
+    <para>
+      When upgrading from stretch to buster, the glibc locale data is upgraded.
+      Specifically, this changes how PostgreSQL sorts data in text indexes.
+      To avoid corruption, such indexes need to be <literal>REINDEX</literal>ed
+      immediately after upgrading the <literal>locales</literal> or
+      <literal>locales-all</literal> packages, before putting back the database
+      into production.
+    </para>
+    <para>
+      Suggested command: <literal>sudo -u postgres reindexdb --all</literal>
+    </para>
+    <para>
+      Alternatively, upgrade the databases to PostgreSQL 11 using
+      <literal>pg_upgradecluster</literal>.
+      (This uses <literal>pg_dump</literal> by default which will rebuild all
+      indexes. Using <literal>-m upgrade</literal> or <literal>pg_upgrade</literal>
+      is <emphasis>not</emphasis> safe because it preserves the now-wrong
+      index ordering.)
+    </para>
+    <para>
+      Refer to the <ulink url="https://wiki.postgresql.org/wiki/Locale_data_changes">PostgreSQL Wiki</ulink>
+      for more information.
+    </para>
+  </section>
+
 </section>
 
 </chapter>

en/release-notes.dbk

@@ -126,6 +126,11 @@
       <surname>Perrier</surname>
       <contrib>bubulle, Lenny installation</contrib>
     </author>,
+    <author>
+      <firstname>Christoph</firstname>
+      <surname>Berg</surname>
+      <contrib>PostgreSQL-specific issues</contrib>
+    </author>,
     <author>
       <firstname>Daniel</firstname>
       <surname>Baumann</surname>