Avoid inserting PlaceHolderVars in cases where pre-v16 PG did not.

Commit 2489d76c4 removed some logic from pullup_replace_vars()
that avoided wrapping a PlaceHolderVar around a pulled-up
subquery output expression if the expression could be proven
to go to NULL anyway (because it contained Vars or PHVs of the
pulled-up relation and did not contain non-strict constructs).
But removing that logic turns out to cause performance regressions
in some cases, because the extra PHV blocks subexpression folding,
and will do so even if outer-join reduction later turns it into a
no-op with no phnullingrels bits.  This can for example prevent
an expression from being matched to an index.

The reason for always adding a PHV was to ensure we had someplace
to put the varnullingrels marker bits of the Var being replaced.
However, it turns out we can optimize in exactly the same cases that
the previous code did, because we can instead attach the needed
varnullingrels bits to the contained Var(s)/PHV(s).

This is not a complete solution --- it would be even better if we
could remove PHVs after reducing them to no-ops.  It doesn't look
practical to back-patch such an improvement, but this change seems
safe and at least gets rid of the performance-regression cases.

Per complaint from Nikhil Raj.  Back-patch to v16 where the
problem appeared.

Discussion: https://postgr.es/m/CAG1ps1xvnTZceKK24OUfMKLPvDP2vjT-d+F2AOCWbw_v3KeEgg@mail.gmail.com

src/backend/optimizer/prep/prepjointree.c

@@ -2490,14 +2490,48 @@ pullup_replace_vars_callback(Var *var,
 				else
 					wrap = false;
 			}
+			else if (rcon->wrap_non_vars)
+			{
+				/* Caller told us to wrap all non-Vars in a PlaceHolderVar */
+				wrap = true;
+			}
 			else
 			{
 				/*
-				 * Must wrap, either because we need a place to insert
-				 * varnullingrels or because caller told us to wrap
-				 * everything.
+				 * If the node contains Var(s) or PlaceHolderVar(s) of the
+				 * subquery being pulled up, and does not contain any
+				 * non-strict constructs, then instead of adding a PHV on top
+				 * we can add the required nullingrels to those Vars/PHVs.
+				 * (This is fundamentally a generalization of the above cases
+				 * for bare Vars and PHVs.)
+				 *
+				 * This test is somewhat expensive, but it avoids pessimizing
+				 * the plan in cases where the nullingrels get removed again
+				 * later by outer join reduction.
+				 *
+				 * This analysis could be tighter: in particular, a non-strict
+				 * construct hidden within a lower-level PlaceHolderVar is not
+				 * reason to add another PHV.  But for now it doesn't seem
+				 * worth the code to be more exact.
+				 *
+				 * For a LATERAL subquery, we have to check the actual var
+				 * membership of the node, but if it's non-lateral then any
+				 * level-zero var must belong to the subquery.
 				 */
-				wrap = true;
+				if ((rcon->target_rte->lateral ?
+					 bms_overlap(pull_varnos(rcon->root, newnode),
+								 rcon->relids) :
+					 contain_vars_of_level(newnode, 0)) &&
+					!contain_nonstrict_functions(newnode))
+				{
+					/* No wrap needed */
+					wrap = false;
+				}
+				else
+				{
+					/* Else wrap it in a PlaceHolderVar */
+					wrap = true;
+				}
 			}
 
 			if (wrap)
@@ -2518,18 +2552,14 @@ pullup_replace_vars_callback(Var *var,
 		}
 	}
 
-	/* Must adjust varlevelsup if replaced Var is within a subquery */
-	if (var->varlevelsup > 0)
-		IncrementVarSublevelsUp(newnode, var->varlevelsup, 0);
-
-	/* Propagate any varnullingrels into the replacement Var or PHV */
+	/* Propagate any varnullingrels into the replacement expression */
 	if (var->varnullingrels != NULL)
 	{
 		if (IsA(newnode, Var))
 		{
 			Var		   *newvar = (Var *) newnode;
 
-			Assert(newvar->varlevelsup == var->varlevelsup);
+			Assert(newvar->varlevelsup == 0);
 			newvar->varnullingrels = bms_add_members(newvar->varnullingrels,
 													 var->varnullingrels);
 		}
@@ -2537,14 +2567,26 @@ pullup_replace_vars_callback(Var *var,
 		{
 			PlaceHolderVar *newphv = (PlaceHolderVar *) newnode;
 
-			Assert(newphv->phlevelsup == var->varlevelsup);
+			Assert(newphv->phlevelsup == 0);
 			newphv->phnullingrels = bms_add_members(newphv->phnullingrels,
 													var->varnullingrels);
 		}
 		else
-			elog(ERROR, "failed to wrap a non-Var");
+		{
+			/* There should be lower-level Vars/PHVs we can modify */
+			newnode = add_nulling_relids(newnode,
+										 NULL,	/* modify all Vars/PHVs */
+										 var->varnullingrels);
+			/* Assert we did put the varnullingrels into the expression */
+			Assert(bms_is_subset(var->varnullingrels,
+								 pull_varnos(rcon->root, newnode)));
+		}
 	}
 
+	/* Must adjust varlevelsup if replaced Var is within a subquery */
+	if (var->varlevelsup > 0)
+		IncrementVarSublevelsUp(newnode, var->varlevelsup, 0);
+
 	return newnode;
 }
 

src/backend/rewrite/rewriteManip.c

@@ -1141,7 +1141,8 @@ AddInvertedQual(Query *parsetree, Node *qual)
 /*
  * add_nulling_relids() finds Vars and PlaceHolderVars that belong to any
  * of the target_relids, and adds added_relids to their varnullingrels
- * and phnullingrels fields.
+ * and phnullingrels fields.  If target_relids is NULL, all level-zero
+ * Vars and PHVs are modified.
  */
 Node *
 add_nulling_relids(Node *node,
@@ -1170,7 +1171,8 @@ add_nulling_relids_mutator(Node *node,
 		Var		   *var = (Var *) node;
 
 		if (var->varlevelsup == context->sublevels_up &&
-			bms_is_member(var->varno, context->target_relids))
+			(context->target_relids == NULL ||
+			 bms_is_member(var->varno, context->target_relids)))
 		{
 			Relids		newnullingrels = bms_union(var->varnullingrels,
 												   context->added_relids);
@@ -1188,7 +1190,8 @@ add_nulling_relids_mutator(Node *node,
 		PlaceHolderVar *phv = (PlaceHolderVar *) node;
 
 		if (phv->phlevelsup == context->sublevels_up &&
-			bms_overlap(phv->phrels, context->target_relids))
+			(context->target_relids == NULL ||
+			 bms_overlap(phv->phrels, context->target_relids)))
 		{
 			Relids		newnullingrels = bms_union(phv->phnullingrels,
 												   context->added_relids);

src/test/regress/expected/subselect.out

@@ -1721,6 +1721,35 @@ fetch backward all in c1;
 (2 rows)
 
 commit;
+--
+-- Verify that we correctly flatten cases involving a subquery output
+-- expression that doesn't need to be wrapped in a PlaceHolderVar
+--
+explain (costs off)
+select tname, attname from (
+select relname::information_schema.sql_identifier as tname, * from
+  (select * from pg_class c) ss1) ss2
+  right join pg_attribute a on a.attrelid = ss2.oid
+where tname = 'tenk1' and attnum = 1;
+                                QUERY PLAN                                
+--------------------------------------------------------------------------
+ Nested Loop
+   ->  Index Scan using pg_class_relname_nsp_index on pg_class c
+         Index Cond: (relname = 'tenk1'::name)
+   ->  Index Scan using pg_attribute_relid_attnum_index on pg_attribute a
+         Index Cond: ((attrelid = c.oid) AND (attnum = 1))
+(5 rows)
+
+select tname, attname from (
+select relname::information_schema.sql_identifier as tname, * from
+  (select * from pg_class c) ss1) ss2
+  right join pg_attribute a on a.attrelid = ss2.oid
+where tname = 'tenk1' and attnum = 1;
+ tname | attname 
+-------+---------
+ tenk1 | unique1
+(1 row)
+
 --
 -- Tests for CTE inlining behavior
 --

src/test/regress/sql/subselect.sql

@@ -890,6 +890,24 @@ fetch backward all in c1;
 
 commit;
 
+--
+-- Verify that we correctly flatten cases involving a subquery output
+-- expression that doesn't need to be wrapped in a PlaceHolderVar
+--
+
+explain (costs off)
+select tname, attname from (
+select relname::information_schema.sql_identifier as tname, * from
+  (select * from pg_class c) ss1) ss2
+  right join pg_attribute a on a.attrelid = ss2.oid
+where tname = 'tenk1' and attnum = 1;
+
+select tname, attname from (
+select relname::information_schema.sql_identifier as tname, * from
+  (select * from pg_class c) ss1) ss2
+  right join pg_attribute a on a.attrelid = ss2.oid
+where tname = 'tenk1' and attnum = 1;
+
 --
 -- Tests for CTE inlining behavior
 --