Allow subquery pullup to wrap a PlaceHolderVar in another one.

The code for wrapping subquery output expressions in PlaceHolderVars
believed that if the expression already was a PlaceHolderVar, it was
never necessary to wrap that in another one.  That's wrong if the
expression is underneath an outer join and involves a lateral
reference to outside that scope: failing to add an additional PHV
risks evaluating the expression at the wrong place and hence not
forcing it to null when the outer join should do so.  This is an
oversight in commit 9e7e29c75, which added logic to forcibly wrap
lateral-reference Vars in PlaceHolderVars, but didn't see that the
adjacent case for PlaceHolderVars needed the same treatment.

The test case we have for this doesn't fail before 4be058fe9, but now
that I see the problem I wonder if it is possible to demonstrate
related errors before that.  That's moot though, since all such
branches are out of support.

Per bug #18284 from Holger Reise.  Back-patch to all supported
branches.

Discussion: https://postgr.es/m/18284-47505a20c23647f8@postgresql.org

diff --git a/src/backend/optimizer/prep/prepjointree.c b/src/backend/optimizer/prep/prepjointree.c
index 73ff40721c974f1c33559250251992e10c405c91..57262f9c64ebfb23d9b774150469a40aa948fe43 100644 (file)
--- a/src/backend/optimizer/prep/prepjointree.c
+++ b/src/backend/optimizer/prep/prepjointree.c
@@ -2435,8 +2435,13 @@ pullup_replace_vars_callback(Var *var,
            else if (newnode && IsA(newnode, PlaceHolderVar) &&
                     ((PlaceHolderVar *) newnode)->phlevelsup == 0)
            {
-               /* No need to wrap a PlaceHolderVar with another one, either */
-               wrap = false;
+               /* The same rules apply for a PlaceHolderVar */
+               if (rcon->target_rte->lateral &&
+                   !bms_is_subset(((PlaceHolderVar *) newnode)->phrels,
+                                  rcon->relids))
+                   wrap = true;
+               else
+                   wrap = false;
            }
            else
            {

diff --git a/src/test/regress/expected/join.out b/src/test/regress/expected/join.out
index e3f29b1d16c8448570438c3cc9f8a57737a75f18..3715ae491d5fa3afd96fccc2bc13dbe697146e98 100644 (file)
--- a/src/test/regress/expected/join.out
+++ b/src/test/regress/expected/join.out
@@ -7013,6 +7013,33 @@ select * from
          Output: (COALESCE((COALESCE(b.q2, '42'::bigint)), d.q2))
 (24 rows)
 
+-- another case requiring nested PlaceHolderVars
+explain (verbose, costs off)
+select * from
+  (select 0 as val0) as ss0
+  left join (select 1 as val) as ss1 on true
+  left join lateral (select ss1.val as val_filtered where false) as ss2 on true;
+           QUERY PLAN           
+--------------------------------
+ Nested Loop Left Join
+   Output: 0, (1), ((1))
+   Join Filter: false
+   ->  Result
+         Output: 1
+   ->  Result
+         Output: (1)
+         One-Time Filter: false
+(8 rows)
+
+select * from
+  (select 0 as val0) as ss0
+  left join (select 1 as val) as ss1 on true
+  left join lateral (select ss1.val as val_filtered where false) as ss2 on true;
+ val0 | val | val_filtered 
+------+-----+--------------
+    0 |   1 |             
+(1 row)
+
 -- case that breaks the old ph_may_need optimization
 explain (verbose, costs off)
 select c.*,a.*,ss1.q1,ss2.q1,ss3.* from

diff --git a/src/test/regress/sql/join.sql b/src/test/regress/sql/join.sql
index 8bf18f11bc82c625636ab0602128a8c0946db6c0..3443b21e6b0dd1a505f8edf69b074833602f75f1 100644 (file)
--- a/src/test/regress/sql/join.sql
+++ b/src/test/regress/sql/join.sql
@@ -2522,6 +2522,18 @@ select * from
   ) on c.q2 = ss2.q1,
   lateral (select ss2.y offset 0) ss3;
 
+-- another case requiring nested PlaceHolderVars
+explain (verbose, costs off)
+select * from
+  (select 0 as val0) as ss0
+  left join (select 1 as val) as ss1 on true
+  left join lateral (select ss1.val as val_filtered where false) as ss2 on true;
+
+select * from
+  (select 0 as val0) as ss0
+  left join (select 1 as val) as ss1 on true
+  left join lateral (select ss1.val as val_filtered where false) as ss2 on true;
+
 -- case that breaks the old ph_may_need optimization
 explain (verbose, costs off)
 select c.*,a.*,ss1.q1,ss2.q1,ss3.* from