Doc: Update PL/pgSQL sample function in plpgsql.sgml.

The example used to explain 'Looping Through Query Results' uses
pseudo-materialized views.  Replace it with a more up-to-date example
which does the same thing with actual materialized views, which have
been available since PostgreSQL 9.3.

In the passing, change '%' as format specifier instead of '%s' as is used
in other examples in plpgsql.sgml.

Reported-by: Ian Barwick
Author: Ian Barwick
Reviewed-by: Amit Kapila
Backpatch-through: 9.4
Discussion: https://postgr.es/m/9a70d393-7904-4918-c97c-649f6d114b6a@2ndquadrant.com

doc/src/sgml/plpgsql.sgml

@@ -2437,19 +2437,29 @@ END LOOP <optional> <replaceable>label</replaceable> </optional>;
      resulting from the <replaceable>query</replaceable> and the loop body is
      executed for each row. Here is an example:
 <programlisting>
-CREATE FUNCTION cs_refresh_mviews() RETURNS integer AS $$
+CREATE FUNCTION refresh_mviews() RETURNS integer AS $$
 DECLARE
     mviews RECORD;
 BEGIN
-    RAISE NOTICE 'Refreshing materialized views...';
-
-    FOR mviews IN SELECT * FROM cs_materialized_views ORDER BY sort_key LOOP
+    RAISE NOTICE 'Refreshing all materialized views...';
+
+    FOR mviews IN
+       SELECT n.nspname AS mv_schema,
+              c.relname AS mv_name,
+              pg_catalog.pg_get_userbyid(c.relowner) AS owner
+         FROM pg_catalog.pg_class c
+    LEFT JOIN pg_catalog.pg_namespace n ON (n.oid = c.relnamespace)
+        WHERE c.relkind = 'm'
+     ORDER BY 1
+    LOOP
 
-        -- Now "mviews" has one record from cs_materialized_views
+        -- Now "mviews" has one record with information about the materialized view
 
-        RAISE NOTICE 'Refreshing materialized view %s ...', quote_ident(mviews.mv_name);
-        EXECUTE format('TRUNCATE TABLE %I', mviews.mv_name);
-        EXECUTE format('INSERT INTO %I %s', mviews.mv_name, mviews.mv_query);
+        RAISE NOTICE 'Refreshing materialized view %.% (owner: %)...',
+                     quote_ident(mviews.mv_schema),
+                     quote_ident(mviews.mv_name),
+                     quote_ident(mviews.owner);
+        EXECUTE format('REFRESH MATERIALIZED VIEW %I.%I', mviews.mv_schema, mviews.mv_name);
     END LOOP;
 
     RAISE NOTICE 'Done refreshing materialized views.';