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[email protected]

Backpatch-through: 9.1

diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml
index 4a0e35a5ebac3a8eacb3b51808edae47c175aabb..6d39ce7f22b10fed797e7c879336e302a5bd03b3 100644 (file)
--- a/doc/src/sgml/runtime.sgml
+++ b/doc/src/sgml/runtime.sgml
@@ -183,6 +183,15 @@ postgres$ initdb -D /usr/local/pgsql/data
    locale setting.  For details see .
   
 
+  
+   NonC and and non-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
+   pg_upgrade run.
+  
+
   
    Use of Secondary File Systems