Fix bogus Assert in find_var_for_subquery_tle

find_var_for_subquery_tle() searches a subquery rel's targetlist for a
Var referencing a given subquery output column, and asserted that any
Var it finds there belongs to that rel.  That does not hold for an
appendrel child.  set_append_rel_size() builds the child's targetlist
by translating the parent's, and as noted there the result can contain
arbitrary expressions.  In particular, when a member of a LATERAL UNION
ALL subquery is pulled up and its output is a lateral reference, the
translated expression is a Var of the laterally referenced rel.  If the
child is itself a subquery with sorted paths, convert_subquery_pathkeys()
would trip the Assert.

In non-assert builds, the function went on to compare such a Var's
varattno with the subquery column's resno, and so could match a Var of
an unrelated rel.

To fix, ignore Vars that do not belong to the subquery rel, the same
way set_rel_width() already does.  Also correct the adjacent comment,
which claimed that the only non-Var entries are placeholders.

Back-patch to all supported branches.

Author: Richard Guo <guofenglinux@gmail.com>
Reviewed-by: shihao zhong <zhong950419@gmail.com>
Discussion: https://postgr.es/m/CAMbWs48fFiPi7Q5yJCoM_nAs47zd5XgvPHd9nZyM_ewq+k_jpQ@mail.gmail.com
Backpatch-through: 14

src/backend/optimizer/path/pathkeys.c

@@ -1262,10 +1262,18 @@ find_var_for_subquery_tle(RelOptInfo *rel, TargetEntry *tle)
 	{
 		Var		   *var = (Var *) lfirst(lc);
 
-		/* Ignore placeholders */
+		/* Ignore placeholders and other non-Var expressions */
 		if (!IsA(var, Var))
 			continue;
-		Assert(var->varno == rel->relid);
+
+		/*
+		 * Ordinarily, a Var in a rel's targetlist must belong to that rel;
+		 * but there are corner cases involving LATERAL references where that
+		 * isn't so.  If the Var has the wrong varno, ignore it, since it
+		 * cannot be referencing the subquery's output.
+		 */
+		if (var->varno != rel->relid)
+			continue;
 
 		/* If we find a Var referencing this TLE, we're good */
 		if (var->varattno == tle->resno)

src/test/regress/expected/join.out

@@ -9591,6 +9591,24 @@ select * from generate_series(100,200) g,
  123 | 4567890123456789 |              123
 (3 rows)
 
+-- lateral with UNION ALL subselect, where a member is a subquery whose
+-- output contains a lateral reference
+explain (costs off)
+  select ss.v from int8_tbl x,
+    lateral (select x.q1 from (select distinct q2 from int8_tbl) s union all
+             select q2 from int8_tbl) ss(v);
+                       QUERY PLAN                        
+---------------------------------------------------------
+ Nested Loop
+   ->  Seq Scan on int8_tbl x
+   ->  Append
+         ->  Subquery Scan on s
+               ->  HashAggregate
+                     Group Key: int8_tbl_1.q2
+                     ->  Seq Scan on int8_tbl int8_tbl_1
+         ->  Seq Scan on int8_tbl
+(8 rows)
+
 -- lateral with VALUES
 explain (costs off)
   select count(*) from tenk1 a,

src/test/regress/sql/join.sql

@@ -3754,6 +3754,13 @@ select * from generate_series(100,200) g,
   lateral (select * from int8_tbl a where g = q1 union all
            select * from int8_tbl b where g = q2) ss;
 
+-- lateral with UNION ALL subselect, where a member is a subquery whose
+-- output contains a lateral reference
+explain (costs off)
+  select ss.v from int8_tbl x,
+    lateral (select x.q1 from (select distinct q2 from int8_tbl) s union all
+             select q2 from int8_tbl) ss(v);
+
 -- lateral with VALUES
 explain (costs off)
   select count(*) from tenk1 a,