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.

2 files changed, 80 insertions, 0 deletions

diff --git a/src/test/regress/expected/subselect.out b/src/test/regress/expected/subselect.out
index 8bda3039d68..1baf3d3e23e 100644
--- a/src/test/regress/expected/subselect.out
+++ b/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)
+
diff --git a/src/test/regress/sql/subselect.sql b/src/test/regress/sql/subselect.sql
index 8a55474b54a..0795d435346 100644
--- a/src/test/regress/sql/subselect.sql
+++ b/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;