Make contain_volatile_functions/contain_mutable_functions look into SubLinks.

This change prevents us from doing inappropriate subquery flattening in
cases such as dangerous functions hidden inside a sub-SELECT in the
targetlist of another sub-SELECT.  That could result in unexpected behavior
due to multiple evaluations of a volatile function, as in a recent
complaint from Etienne Dube.  It's been questionable from the very
beginning whether these functions should look into subqueries (as noted in
their comments), and this case seems to provide proof that they should.

Because the new code only descends into SubLinks, not SubPlans or
InitPlans, the change only affects the planner's behavior during
prepjointree processing and not later on --- for example, you can still get
it to use a volatile function in an indexqual if you wrap the function in
(SELECT ...).  That's a historical behavior, for sure, but it's reasonable
given that the executor's evaluation rules for subplans don't depend on
whether there are volatile functions inside them.  In any case, we need to
constrain the behavioral change as narrowly as we can to make this
reasonable to back-patch.

src/backend/optimizer/util/clauses.c

@@ -822,8 +822,8 @@ contain_subplans_walker(Node *node, void *context)
  * mistakenly think that something like "WHERE random() < 0.5" can be treated
  * as a constant qualification.
  *
- * XXX we do not examine sub-selects to see if they contain uses of
- * mutable functions.  It's not real clear if that is correct or not...
+ * We will recursively look into Query nodes (i.e., SubLink sub-selects)
+ * but not into SubPlans.  See comments for contain_volatile_functions().
  */
 bool
 contain_mutable_functions(Node *clause)
@@ -920,6 +920,13 @@ contain_mutable_functions_walker(Node *node, void *context)
 		}
 		/* else fall through to check args */
 	}
+	else if (IsA(node, Query))
+	{
+		/* Recurse into subselects */
+		return query_tree_walker((Query *) node,
+								 contain_mutable_functions_walker,
+								 context, 0);
+	}
 	return expression_tree_walker(node, contain_mutable_functions_walker,
 								  context);
 }
@@ -934,11 +941,18 @@ contain_mutable_functions_walker(Node *node, void *context)
  *	  Recursively search for volatile functions within a clause.
  *
  * Returns true if any volatile function (or operator implemented by a
- * volatile function) is found. This test prevents invalid conversions
- * of volatile expressions into indexscan quals.
+ * volatile function) is found. This test prevents, for example,
+ * invalid conversions of volatile expressions into indexscan quals.
  *
- * XXX we do not examine sub-selects to see if they contain uses of
- * volatile functions.	It's not real clear if that is correct or not...
+ * We will recursively look into Query nodes (i.e., SubLink sub-selects)
+ * but not into SubPlans.  This is a bit odd, but intentional.	If we are
+ * looking at a SubLink, we are probably deciding whether a query tree
+ * transformation is safe, and a contained sub-select should affect that;
+ * for example, duplicating a sub-select containing a volatile function
+ * would be bad.  However, once we've got to the stage of having SubPlans,
+ * subsequent planning need not consider volatility within those, since
+ * the executor won't change its evaluation rules for a SubPlan based on
+ * volatility.
  */
 bool
 contain_volatile_functions(Node *clause)
@@ -1036,6 +1050,13 @@ contain_volatile_functions_walker(Node *node, void *context)
 		}
 		/* else fall through to check args */
 	}
+	else if (IsA(node, Query))
+	{
+		/* Recurse into subselects */
+		return query_tree_walker((Query *) node,
+								 contain_volatile_functions_walker,
+								 context, 0);
+	}
 	return expression_tree_walker(node, contain_volatile_functions_walker,
 								  context);
 }

src/test/regress/expected/subselect.out

@@ -636,3 +636,67 @@ where a.thousand = b.thousand
 ----------
 (0 rows)
 
+--
+-- Check that nested sub-selects are not pulled up if they contain volatiles
+--
+explain (verbose, costs off)
+  select x, x from
+    (select (select now()) as x from (values(1),(2)) v(y)) ss;
+        QUERY PLAN         
+---------------------------
+ Values Scan on "*VALUES*"
+   Output: $0, $1
+   InitPlan 1 (returns $0)
+     ->  Result
+           Output: now()
+   InitPlan 2 (returns $1)
+     ->  Result
+           Output: now()
+(8 rows)
+
+explain (verbose, costs off)
+  select x, x from
+    (select (select random()) as x from (values(1),(2)) v(y)) ss;
+            QUERY PLAN            
+----------------------------------
+ Subquery Scan on ss
+   Output: ss.x, ss.x
+   ->  Values Scan on "*VALUES*"
+         Output: $0
+         InitPlan 1 (returns $0)
+           ->  Result
+                 Output: random()
+(7 rows)
+
+explain (verbose, costs off)
+  select x, x from
+    (select (select now() where y=y) as x from (values(1),(2)) v(y)) ss;
+                              QUERY PLAN                              
+----------------------------------------------------------------------
+ Values Scan on "*VALUES*"
+   Output: (SubPlan 1), (SubPlan 2)
+   SubPlan 1
+     ->  Result
+           Output: now()
+           One-Time Filter: ("*VALUES*".column1 = "*VALUES*".column1)
+   SubPlan 2
+     ->  Result
+           Output: now()
+           One-Time Filter: ("*VALUES*".column1 = "*VALUES*".column1)
+(10 rows)
+
+explain (verbose, costs off)
+  select x, x from
+    (select (select random() where y=y) as x from (values(1),(2)) v(y)) ss;
+                                 QUERY PLAN                                 
+----------------------------------------------------------------------------
+ Subquery Scan on ss
+   Output: ss.x, ss.x
+   ->  Values Scan on "*VALUES*"
+         Output: (SubPlan 1)
+         SubPlan 1
+           ->  Result
+                 Output: random()
+                 One-Time Filter: ("*VALUES*".column1 = "*VALUES*".column1)
+(8 rows)
+

src/test/regress/sql/subselect.sql

@@ -389,3 +389,19 @@ where a.thousand = b.thousand
   and exists ( select 1 from tenk1 c where b.hundred = c.hundred
                    and not exists ( select 1 from tenk1 d
                                     where a.thousand = d.thousand ) );
+
+--
+-- Check that nested sub-selects are not pulled up if they contain volatiles
+--
+explain (verbose, costs off)
+  select x, x from
+    (select (select now()) as x from (values(1),(2)) v(y)) ss;
+explain (verbose, costs off)
+  select x, x from
+    (select (select random()) as x from (values(1),(2)) v(y)) ss;
+explain (verbose, costs off)
+  select x, x from
+    (select (select now() where y=y) as x from (values(1),(2)) v(y)) ss;
+explain (verbose, costs off)
+  select x, x from
+    (select (select random() where y=y) as x from (values(1),(2)) v(y)) ss;