Fix mark_placeholder_maybe_needed to handle LATERAL references.

If a PlaceHolderVar contains a pulled-up LATERAL reference, its minimum
possible evaluation level might be higher in the join tree than its
original syntactic location.  That in turn affects the ph_needed level for
any contained PlaceHolderVars (that is, those PHVs had better propagate up
the join tree at least to the evaluation level of the outer PHV).  We got
this mostly right, but mark_placeholder_maybe_needed() failed to account
for the effect, and in consequence could leave the inner PHVs with
ph_may_need less than what their ultimate ph_needed value will be.  That's
bad because it could lead to failure to select a join order that will allow
evaluation of the inner PHV at a valid location.  Fix that, and add an
Assert that checks that we don't ever set ph_needed to more than
ph_may_need.

src/backend/optimizer/plan/initsplan.c

@@ -199,10 +199,13 @@ add_vars_to_targetlist(PlannerInfo *root, List *vars,
 			/*
 			 * If we are creating PlaceHolderInfos, mark them with the correct
 			 * maybe-needed locations.	Otherwise, it's too late to change
-			 * that.
+			 * that, so we'd better not have set ph_needed to more than
+			 * ph_may_need.
 			 */
 			if (create_new_ph)
 				mark_placeholder_maybe_needed(root, phinfo, where_needed);
+			else
+				Assert(bms_is_subset(phinfo->ph_needed, phinfo->ph_may_need));
 		}
 		else
 			elog(ERROR, "unrecognized node type: %d", (int) nodeTag(node));

src/backend/optimizer/util/placeholder.c

@@ -250,6 +250,8 @@ void
 mark_placeholder_maybe_needed(PlannerInfo *root, PlaceHolderInfo *phinfo,
 							  Relids relids)
 {
+	Relids		est_eval_level;
+
 	/* Mark the PHV as possibly needed at the given syntactic level */
 	phinfo->ph_may_need = bms_add_members(phinfo->ph_may_need, relids);
 
@@ -258,11 +260,18 @@ mark_placeholder_maybe_needed(PlannerInfo *root, PlaceHolderInfo *phinfo,
 	 * lower-level PHVs.  We need to get those into the PlaceHolderInfo list,
 	 * but they aren't going to be needed where the outer PHV is referenced.
 	 * Rather, they'll be needed where the outer PHV is evaluated.  We can
-	 * estimate that (conservatively) as the syntactic location of the PHV's
-	 * expression.	Recurse to take care of any such PHVs.
+	 * estimate that conservatively as the syntactic location of the PHV's
+	 * expression, but not less than the level of any Vars it contains.
+	 * (Normally the Vars would come from below the syntactic location anyway,
+	 * but this might not be true if the PHV contains any LATERAL references.)
 	 */
+	est_eval_level = bms_union(phinfo->ph_var->phrels, phinfo->ph_eval_at);
+
+	/* Now recurse to take care of any such PHVs */
 	mark_placeholders_in_expr(root, (Node *) phinfo->ph_var->phexpr,
-							  phinfo->ph_var->phrels);
+							  est_eval_level);
+
+	bms_free(est_eval_level);
 }
 
 /*

src/test/regress/expected/join.out

@@ -3465,6 +3465,46 @@ select v.* from
  -4567890123456789 |                  
 (20 rows)
 
+-- case requiring nested PlaceHolderVars
+explain (verbose, costs off)
+select * from
+  int8_tbl c left join (
+    int8_tbl a left join (select q1, coalesce(q2,42) as x from int8_tbl b) ss1
+      on a.q2 = ss1.q1
+    cross join
+    lateral (select q1, coalesce(ss1.x,q2) as y from int8_tbl d) ss2
+  ) on c.q2 = ss2.q1,
+  lateral (select ss2.y) ss3;
+                                                                                  QUERY PLAN                                                                                  
+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
+ Nested Loop
+   Output: c.q1, c.q2, a.q1, a.q2, b.q1, (COALESCE(b.q2, 42::bigint)), d.q1, (COALESCE((COALESCE(b.q2, 42::bigint)), d.q2)), ((COALESCE((COALESCE(b.q2, 42::bigint)), d.q2)))
+   ->  Hash Right Join
+         Output: c.q1, c.q2, a.q1, a.q2, b.q1, d.q1, (COALESCE(b.q2, 42::bigint)), (COALESCE((COALESCE(b.q2, 42::bigint)), d.q2))
+         Hash Cond: (d.q1 = c.q2)
+         ->  Nested Loop
+               Output: a.q1, a.q2, b.q1, d.q1, (COALESCE(b.q2, 42::bigint)), COALESCE((COALESCE(b.q2, 42::bigint)), d.q2)
+               ->  Hash Left Join
+                     Output: a.q1, a.q2, b.q1, (COALESCE(b.q2, 42::bigint))
+                     Hash Cond: (a.q2 = b.q1)
+                     ->  Seq Scan on public.int8_tbl a
+                           Output: a.q1, a.q2
+                     ->  Hash
+                           Output: b.q1, (COALESCE(b.q2, 42::bigint))
+                           ->  Seq Scan on public.int8_tbl b
+                                 Output: b.q1, COALESCE(b.q2, 42::bigint)
+               ->  Materialize
+                     Output: d.q1, d.q2
+                     ->  Seq Scan on public.int8_tbl d
+                           Output: d.q1, d.q2
+         ->  Hash
+               Output: c.q1, c.q2
+               ->  Seq Scan on public.int8_tbl c
+                     Output: c.q1, c.q2
+   ->  Result
+         Output: (COALESCE((COALESCE(b.q2, 42::bigint)), d.q2))
+(26 rows)
+
 -- test some error cases where LATERAL should have been used but wasn't
 select f1,g from int4_tbl a, generate_series(0, f1) g;
 ERROR:  column "f1" does not exist

src/test/regress/sql/join.sql

@@ -942,6 +942,17 @@ select v.* from
   left join int4_tbl z on z.f1 = x.q2,
   lateral (select x.q1,y.q1 from dual union all select x.q2,y.q2 from dual) v(vx,vy);
 
+-- case requiring nested PlaceHolderVars
+explain (verbose, costs off)
+select * from
+  int8_tbl c left join (
+    int8_tbl a left join (select q1, coalesce(q2,42) as x from int8_tbl b) ss1
+      on a.q2 = ss1.q1
+    cross join
+    lateral (select q1, coalesce(ss1.x,q2) as y from int8_tbl d) ss2
+  ) on c.q2 = ss2.q1,
+  lateral (select ss2.y) ss3;
+
 -- test some error cases where LATERAL should have been used but wasn't
 select f1,g from int4_tbl a, generate_series(0, f1) g;
 select f1,g from int4_tbl a, generate_series(0, a.f1) g;