advisor: fix included column order in suggested indexes

If an index optimizes multiple quals, the previous code wasn't deterministic
and could append the columns from the included quals in any order, which could
lead to a non optimal index suggestion.

To fix, the include quals are not emitted in ascending order of column they
contains, so there's a guarantee that the first columns are in the correct
order.  The result is still not fully deterministic in case of a single qual
having multiple columns not included in a child qual, but this doesn't really
matter as the order is not significant there (at least from the advisor point
of view).

Thanks to disqus user Sivan for the report.

pg_qualstats--2.1.0.sql

@@ -546,7 +546,7 @@ BEGIN
         filtered AS (
           SELECT (qual).relid, amname, coalesce(qualid, qualnodeid) AS parent,
             count(*) AS weight,
-            (array_agg(qualnodeid),
+            (array_agg(DISTINCT qualnodeid),
              array_agg(queryid)
             )::@[email protected]_quals AS quals
           FROM pgqs
@@ -628,10 +628,13 @@ BEGIN
             v_cur json;
         BEGIN
             IF rec.included IS NOT NULL THEN
-              FOREACH v_cur IN ARRAY rec.included LOOP
+              FOR v_cur IN SELECT v->'qualnodeids'
+                    FROM (SELECT * FROM unnest(rec.included)) AS r(v)
+                    ORDER BY pg_catalog.json_array_length(v->'qualnodeids') ASC
+              LOOP
                 -- Direct cast from json to bigint is only possible since pg10
                 FOR v_qualnodeid IN
-                    SELECT pg_catalog.json_array_elements(v_cur->'qualnodeids')::text::bigint
+                    SELECT pg_catalog.json_array_elements(v_cur)::text::bigint
                 LOOP
                   v_quals_todo := v_quals_todo || v_qualnodeid;
                 END LOOP;