From 2dc7da91054ee5135fae35fafe60bf2803d4d836 Mon Sep 17 00:00:00 2001
From: Bruce Momjian <bruce@momjian.us>
Date: Sat, 2 Jul 2016 11:22:36 -0400
Subject: [PATCH] doc: mention dependency on collation libraries

Document that index storage is dependent on the operating system's
collation library ordering, and any change in that ordering can create
invalid indexes.

Discussion: 20160617154311.GB19359@momjian.us

Backpatch-through: 9.1
---
 doc/src/sgml/runtime.sgml | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml
index b620ee76b50..310e2840119 100644
--- a/doc/src/sgml/runtime.sgml
+++ b/doc/src/sgml/runtime.sgml
@@ -183,6 +183,15 @@ postgres$ <userinput>initdb -D /usr/local/pgsql/data</userinput>
    locale setting.  For details see <xref linkend="multibyte">.
   </para>
 
+  <para>
+   Non<literal>C</> and and non-<literal>POSIX</> locales rely on the
+   operating system's collation library for character set ordering.
+   This controls the ordering of keys stored in indexes.  For this reason,
+   a cluster cannot switch to an incompatible collation library version,
+   either through snapshot restore, binary streaming replication, or
+   <application>pg_upgrade</> run.
+  </para>
+
   <sect2 id="creating-cluster-mount-points">
    <title>Use of Secondary File Systems</title>