Use context with correct lifetime in hypothetical_dense_rank_final.

The query lifetime expression context created in
hypothetical_dense_rank_final() was buggily allocated in the calling
memory context. I (Andres) broke that in bf6c614a2f.

Reported-By: Rajkumar Raghuwanshi
Author: Amit Langote
Discussion:  https://postgr.es/m/CAKcux6kmzWmur5HhA_aU6gYVFu0RLQdgJJ+aC9SLdcOvBSrpfA@mail.gmail.com
Backpatch: 11-

src/backend/utils/adt/orderedsetaggs.c

@@ -1310,7 +1310,15 @@ hypothetical_dense_rank_final(PG_FUNCTION_ARGS)
 	osastate = (OSAPerGroupState *) PG_GETARG_POINTER(0);
 	econtext = osastate->qstate->econtext;
 	if (!econtext)
-		osastate->qstate->econtext = econtext = CreateStandaloneExprContext();
+	{
+		MemoryContext oldcontext;
+
+		/* Make sure to we create econtext under correct parent context. */
+		oldcontext = MemoryContextSwitchTo(osastate->qstate->qcontext);
+		osastate->qstate->econtext = CreateStandaloneExprContext();
+		econtext = osastate->qstate->econtext;
+		MemoryContextSwitchTo(oldcontext);
+	}
 
 	/* Adjust nargs to be the number of direct (or aggregated) args */
 	if (nargs % 2 != 0)

src/test/regress/expected/aggregates.out

@@ -2092,3 +2092,12 @@ SELECT variance(unique1::int4), sum(unique1::int8) FROM tenk1;
 (1 row)
 
 ROLLBACK;
+-- test coverage for dense_rank
+SELECT dense_rank(x) WITHIN GROUP (ORDER BY x) FROM (VALUES (1),(1),(2),(2),(3),(3)) v(x) GROUP BY (x) ORDER BY 1;
+ dense_rank 
+------------
+          1
+          1
+          1
+(3 rows)
+

src/test/regress/sql/aggregates.sql

@@ -925,3 +925,6 @@ EXPLAIN (COSTS OFF)
 SELECT variance(unique1::int4), sum(unique1::int8) FROM tenk1;
 
 ROLLBACK;
+
+-- test coverage for dense_rank
+SELECT dense_rank(x) WITHIN GROUP (ORDER BY x) FROM (VALUES (1),(1),(2),(2),(3),(3)) v(x) GROUP BY (x) ORDER BY 1;