Fix planning of btree index scans using ScalarArrayOpExpr quals.

In commit 9e8da0f75731aaa7605cf4656c21ea09e84d2eb1, I improved btree
to handle ScalarArrayOpExpr quals natively, so that constructs like
"indexedcol IN (list)" could be supported by index-only scans.  Using
such a qual results in multiple scans of the index, under-the-hood.
I went to some lengths to ensure that this still produces rows in index
order ... but I failed to recognize that if a higher-order index column
is lacking an equality constraint, rescans can produce out-of-order
data from that column.  Tweak the planner to not expect sorted output
in that case.  Per trouble report from Robert McGehee.

src/backend/optimizer/path/indxpath.c

@@ -756,6 +756,7 @@ build_index_paths(PlannerInfo *root, RelOptInfo *rel,
 	List	   *index_pathkeys;
 	List	   *useful_pathkeys;
 	bool		found_clause;
+	bool		found_lower_saop_clause;
 	bool		pathkeys_possibly_useful;
 	bool		index_is_ordered;
 	bool		index_only_scan;
@@ -793,12 +794,20 @@ build_index_paths(PlannerInfo *root, RelOptInfo *rel,
 	 * if saop_control is SAOP_REQUIRE, it has to be a ScalarArrayOpExpr
 	 * clause.
 	 *
+	 * found_lower_saop_clause is set true if there's a ScalarArrayOpExpr
+	 * index clause for a non-first index column.  This prevents us from
+	 * assuming that the scan result is ordered.  (Actually, the result is
+	 * still ordered if there are equality constraints for all earlier
+	 * columns, but it seems too expensive and non-modular for this code to be
+	 * aware of that refinement.)
+	 *
 	 * We also build a Relids set showing which outer rels are required by the
 	 * selected clauses.
 	 */
 	index_clauses = NIL;
 	clause_columns = NIL;
 	found_clause = false;
+	found_lower_saop_clause = false;
 	outer_relids = NULL;
 	for (indexcol = 0; indexcol < index->ncolumns; indexcol++)
 	{
@@ -814,6 +823,8 @@ build_index_paths(PlannerInfo *root, RelOptInfo *rel,
 				if (saop_control == SAOP_PER_AM && !index->amsearcharray)
 					continue;
 				found_clause = true;
+				if (indexcol > 0)
+					found_lower_saop_clause = true;
 			}
 			else
 			{
@@ -850,9 +861,11 @@ build_index_paths(PlannerInfo *root, RelOptInfo *rel,
 	/*
 	 * 2. Compute pathkeys describing index's ordering, if any, then see how
 	 * many of them are actually useful for this query.  This is not relevant
-	 * if we are only trying to build bitmap indexscans.
+	 * if we are only trying to build bitmap indexscans, nor if we have to
+	 * assume the scan is unordered.
 	 */
 	pathkeys_possibly_useful = (scantype != ST_BITMAPSCAN &&
+								!found_lower_saop_clause &&
 								has_useful_pathkeys(root, rel));
 	index_is_ordered = (index->sortopfamily != NULL);
 	if (index_is_ordered && pathkeys_possibly_useful)

src/test/regress/expected/create_index.out

@@ -2676,3 +2676,48 @@ SELECT count(*) FROM dupindexcols
     97
 (1 row)
 
+--
+-- Check ordering of =ANY indexqual results (bug in 9.2.0)
+--
+vacuum analyze tenk1;		-- ensure we get consistent plans here
+explain (costs off)
+SELECT unique1 FROM tenk1
+WHERE unique1 IN (1,42,7)
+ORDER BY unique1;
+                      QUERY PLAN                       
+-------------------------------------------------------
+ Index Only Scan using tenk1_unique1 on tenk1
+   Index Cond: (unique1 = ANY ('{1,42,7}'::integer[]))
+(2 rows)
+
+SELECT unique1 FROM tenk1
+WHERE unique1 IN (1,42,7)
+ORDER BY unique1;
+ unique1 
+---------
+       1
+       7
+      42
+(3 rows)
+
+explain (costs off)
+SELECT thousand, tenthous FROM tenk1
+WHERE thousand < 2 AND tenthous IN (1001,3000)
+ORDER BY thousand;
+                                      QUERY PLAN                                      
+--------------------------------------------------------------------------------------
+ Sort
+   Sort Key: thousand
+   ->  Index Only Scan using tenk1_thous_tenthous on tenk1
+         Index Cond: ((thousand < 2) AND (tenthous = ANY ('{1001,3000}'::integer[])))
+(4 rows)
+
+SELECT thousand, tenthous FROM tenk1
+WHERE thousand < 2 AND tenthous IN (1001,3000)
+ORDER BY thousand;
+ thousand | tenthous 
+----------+----------
+        0 |     3000
+        1 |     1001
+(2 rows)
+

src/test/regress/sql/create_index.sql

@@ -888,3 +888,27 @@ EXPLAIN (COSTS OFF)
     WHERE f1 > 'WA' and id < 1000 and f1 ~<~ 'YX';
 SELECT count(*) FROM dupindexcols
   WHERE f1 > 'WA' and id < 1000 and f1 ~<~ 'YX';
+
+--
+-- Check ordering of =ANY indexqual results (bug in 9.2.0)
+--
+
+vacuum analyze tenk1;		-- ensure we get consistent plans here
+
+explain (costs off)
+SELECT unique1 FROM tenk1
+WHERE unique1 IN (1,42,7)
+ORDER BY unique1;
+
+SELECT unique1 FROM tenk1
+WHERE unique1 IN (1,42,7)
+ORDER BY unique1;
+
+explain (costs off)
+SELECT thousand, tenthous FROM tenk1
+WHERE thousand < 2 AND tenthous IN (1001,3000)
+ORDER BY thousand;
+
+SELECT thousand, tenthous FROM tenk1
+WHERE thousand < 2 AND tenthous IN (1001,3000)
+ORDER BY thousand;