Document cross-version compatibility issues for contrib/postgres_fdw.

One of the use-cases for postgres_fdw is extracting data from older PG
servers, so cross-version compatibility is important.  Document what we
can do here, and further annotate some of the coding choices that create
compatibility constraints.  In passing, remove one unnecessary
incompatibility with old servers, namely assuming that we didn't need to
quote the timezone name 'UTC'.

contrib/postgres_fdw/connection.c

@@ -326,9 +326,10 @@ configure_remote_session(PGconn *conn)
 	 * anyway.	However it makes the regression test outputs more predictable.
 	 *
 	 * We don't risk setting remote zone equal to ours, since the remote
-	 * server might use a different timezone database.
+	 * server might use a different timezone database.  Instead, use UTC
+	 * (quoted, because very old servers are picky about case).
 	 */
-	do_sql_command(conn, "SET timezone = UTC");
+	do_sql_command(conn, "SET timezone = 'UTC'");
 
 	/*
 	 * Set values needed to ensure unambiguous data output from remote.  (This
@@ -542,8 +543,14 @@ pgfdw_xact_callback(XactEvent event, void *arg)
 				 * prepared statements, do a DEALLOCATE ALL to make sure we
 				 * get rid of all prepared statements.	This is annoying and
 				 * not terribly bulletproof, but it's probably not worth
-				 * trying harder.  We intentionally ignore any errors in the
-				 * DEALLOCATE.
+				 * trying harder.
+				 *
+				 * DEALLOCATE ALL only exists in 8.3 and later, so this
+				 * constrains how old a server postgres_fdw can communicate
+				 * with.  We intentionally ignore errors in the DEALLOCATE, so
+				 * that we can hobble along to some extent with older servers
+				 * (leaking prepared statements as we go; but we don't really
+				 * support update operations pre-8.3 anyway).
 				 */
 				if (entry->have_prep_stmt && entry->have_error)
 				{

contrib/postgres_fdw/postgres_fdw.c

@@ -801,6 +801,9 @@ postgresGetForeignPlan(PlannerInfo *root,
 	 * The extra roundtrips involved in trying to duplicate the local
 	 * semantics exactly don't seem worthwhile (see also comments for
 	 * RowMarkType).
+	 *
+	 * Note: because we actually run the query as a cursor, this assumes that
+	 * DECLARE CURSOR ... FOR UPDATE is supported, which it isn't before 8.3.
 	 */
 	if (baserel->relid == root->parse->resultRelation &&
 		(root->parse->commandType == CMD_UPDATE ||

doc/src/sgml/postgres-fdw.sgml

@@ -318,6 +318,26 @@
   </para>
  </sect2>
 
+ <sect2>
+  <title>Cross-Version Compatibility</title>
+
+  <para>
+   <filename>postgres_fdw</> can be used with remote servers dating back
+   to <productname>PostgreSQL</> 8.3.  Read-only capability is available
+   back to 8.1.  A limitation however is that <filename>postgres_fdw</>
+   generally assumes that immutable built-in functions and operators are
+   safe to send to the remote server for execution, if they appear in a
+   <literal>WHERE</> clause for a foreign table.  Thus, a built-in
+   function that was added since the remote server's release might be sent
+   to it for execution, resulting in <quote>function does not exist</> or
+   a similar error.  This type of failure can be worked around by
+   rewriting the query, for example by embedding the foreign table
+   reference in a sub-<literal>SELECT</> with <literal>OFFSET 0</> as an
+   optimization fence, and placing the problematic function or operator
+   outside the sub-<literal>SELECT</>.
+  </para>
+ </sect2>
+
  <sect2>
   <title>Author</title>
   <para>