Work around implementation restriction in adjust_appendrel_attrs.

adjust_appendrel_attrs can't transfer nullingrel labeling to a non-Var
translation expression (mainly because it's too late to wrap such an
expression in a PlaceHolderVar).  I'd supposed in commit 2489d76c4
that that restriction was unreachable because we'd not attempt to push
problematic clauses down to an appendrel child relation.  I forgot that
set_append_rel_size blindly converts all the parent rel's joininfo
clauses to child clauses, and that list could well contain clauses
from above a nulling outer join.

We might eventually have to devise a direct fix for this implementation
restriction, but for now it seems enough to filter out troublesome
clauses while constructing the child's joininfo list.  Such clauses
are certainly not useful while constructing paths for the child rel;
they'll have to be applied later when we join the completed appendrel
to something else.  So we don't need them here, and omitting them from
the list should save a few cycles while processing the child rel.

Per bug #17832 from Marko Tiikkaja.

Discussion: https://postgr.es/m/17832-d0a8106cdf1b722e@postgresql.org

src/backend/optimizer/path/allpaths.c

@@ -979,8 +979,10 @@ set_append_rel_size(PlannerInfo *root, RelOptInfo *rel,
 		int			childRTindex;
 		RangeTblEntry *childRTE;
 		RelOptInfo *childrel;
+		List	   *childrinfos;
 		ListCell   *parentvars;
 		ListCell   *childvars;
+		ListCell   *lc;
 
 		/* append_rel_list contains all append rels; ignore others */
 		if (appinfo->parent_relid != parentRTindex)
@@ -1021,6 +1023,28 @@ set_append_rel_size(PlannerInfo *root, RelOptInfo *rel,
 		 * Constraint exclusion failed, so copy the parent's join quals and
 		 * targetlist to the child, with appropriate variable substitutions.
 		 *
+		 * We skip join quals that came from above outer joins that can null
+		 * this rel, since they would be of no value while generating paths
+		 * for the child.  This saves some effort while processing the child
+		 * rel, and it also avoids an implementation restriction in
+		 * adjust_appendrel_attrs (it can't apply nullingrels to a non-Var).
+		 */
+		childrinfos = NIL;
+		foreach(lc, rel->joininfo)
+		{
+			RestrictInfo *rinfo = (RestrictInfo *) lfirst(lc);
+
+			if (!bms_overlap(rinfo->clause_relids, rel->nulling_relids))
+				childrinfos = lappend(childrinfos,
+									  adjust_appendrel_attrs(root,
+															 (Node *) rinfo,
+															 1, &appinfo));
+		}
+		childrel->joininfo = childrinfos;
+
+		/*
+		 * Now for the child's targetlist.
+		 *
 		 * NB: the resulting childrel->reltarget->exprs may contain arbitrary
 		 * expressions, which otherwise would not occur in a rel's targetlist.
 		 * Code that might be looking at an appendrel child must cope with
@@ -1028,10 +1052,6 @@ set_append_rel_size(PlannerInfo *root, RelOptInfo *rel,
 		 * PlaceHolderVars.)  XXX we do not bother to update the cost or width
 		 * fields of childrel->reltarget; not clear if that would be useful.
 		 */
-		childrel->joininfo = (List *)
-			adjust_appendrel_attrs(root,
-								   (Node *) rel->joininfo,
-								   1, &appinfo);
 		childrel->reltarget->exprs = (List *)
 			adjust_appendrel_attrs(root,
 								   (Node *) rel->reltarget->exprs,

src/test/regress/expected/join.out

@@ -3613,6 +3613,28 @@ from int4_tbl t1
    One-Time Filter: false
 (3 rows)
 
+-- Test handling of qual pushdown to appendrel members with non-Var outputs
+explain (verbose, costs off)
+select * from int4_tbl left join (
+  select text 'foo' union all select text 'bar'
+) ss(x) on true
+where ss.x is null;
+               QUERY PLAN                
+-----------------------------------------
+ Nested Loop Left Join
+   Output: int4_tbl.f1, ('foo'::text)
+   Filter: (('foo'::text) IS NULL)
+   ->  Seq Scan on public.int4_tbl
+         Output: int4_tbl.f1
+   ->  Materialize
+         Output: ('foo'::text)
+         ->  Append
+               ->  Result
+                     Output: 'foo'::text
+               ->  Result
+                     Output: 'bar'::text
+(12 rows)
+
 --
 -- test inlining of immutable functions
 --

src/test/regress/sql/join.sql

@@ -1189,6 +1189,13 @@ from int4_tbl t1
               left join information_schema.column_udt_usage on null)
   on null;
 
+-- Test handling of qual pushdown to appendrel members with non-Var outputs
+explain (verbose, costs off)
+select * from int4_tbl left join (
+  select text 'foo' union all select text 'bar'
+) ss(x) on true
+where ss.x is null;
+
 --
 -- test inlining of immutable functions
 --