In the planner, delete joinaliasvars lists after we're done with them.

Although joinaliasvars lists coming out of the parser are quite simple,
those lists can contain arbitrarily complex expressions after subquery
pullup.  We do not perform expression preprocessing on them, meaning that
expressions in those lists will not meet the expectations of later phases
of the planner (for example, that they do not contain SubLinks).  This had
been thought pretty harmless, since we don't intentionally touch those
lists in later phases --- but Andreas Seltenreich found a case in which
adjust_appendrel_attrs() could recurse into a joinaliasvars list and then
die on its assertion that it never sees a SubLink.  We considered a couple
of localized fixes to prevent that specific case from looking at the
joinaliasvars lists, but really this seems like a generic hazard for all
expression processing in the planner.  Therefore, probably the best answer
is to delete the joinaliasvars lists from the parsetree at the end of
expression preprocessing, so that there are no reachable expressions that
haven't been through preprocessing.

The case Andreas found seems to be harmless in non-Assert builds, and so
far there are no field reports suggesting that there are user-visible
effects in other cases.  I considered back-patching this anyway, but
it turns out that Andreas' test doesn't fail at all in 9.4-9.6, because
in those versions adjust_appendrel_attrs contains code (added in commit
842faa71 and removed again in commit 215b43cd) to process SubLinks
rather than complain about them.  Barring discovery of another path by
which unprocessed joinaliasvars lists can cause trouble, the most
prudent compromise seems to be to patch this into v10 but not further.

Patch by me, with thanks to Amit Langote for initial investigation
and review.

Discussion: https://postgr.es/m/87r2tvt9f1.fsf@ansel.ydns.eu

src/backend/optimizer/plan/planner.c

@@ -776,6 +776,27 @@ subquery_planner(PlannerGlobal *glob, Query *parse,
 		}
 	}
 
+	/*
+	 * Now that we are done preprocessing expressions, and in particular done
+	 * flattening join alias variables, get rid of the joinaliasvars lists.
+	 * They no longer match what expressions in the rest of the tree look
+	 * like, because we have not preprocessed expressions in those lists (and
+	 * do not want to; for example, expanding a SubLink there would result in
+	 * a useless unreferenced subplan).  Leaving them in place simply creates
+	 * a hazard for later scans of the tree.  We could try to prevent that by
+	 * using QTW_IGNORE_JOINALIASES in every tree scan done after this point,
+	 * but that doesn't sound very reliable.
+	 */
+	if (root->hasJoinRTEs)
+	{
+		foreach(l, parse->rtable)
+		{
+			RangeTblEntry *rte = lfirst_node(RangeTblEntry, l);
+
+			rte->joinaliasvars = NIL;
+		}
+	}
+
 	/*
 	 * In some cases we may want to transfer a HAVING clause into WHERE. We
 	 * cannot do so if the HAVING clause contains aggregates (obviously) or
@@ -902,11 +923,12 @@ preprocess_expression(PlannerInfo *root, Node *expr, int kind)
 
 	/*
 	 * If the query has any join RTEs, replace join alias variables with
-	 * base-relation variables.  We must do this before sublink processing,
-	 * else sublinks expanded out from join aliases would not get processed.
-	 * We can skip it in non-lateral RTE functions, VALUES lists, and
-	 * TABLESAMPLE clauses, however, since they can't contain any Vars of the
-	 * current query level.
+	 * base-relation variables.  We must do this first, since any expressions
+	 * we may extract from the joinaliasvars lists have not been preprocessed.
+	 * For example, if we did this after sublink processing, sublinks expanded
+	 * out from join aliases would not get processed.  But we can skip this in
+	 * non-lateral RTE functions, VALUES lists, and TABLESAMPLE clauses, since
+	 * they can't contain any Vars of the current query level.
 	 */
 	if (root->hasJoinRTEs &&
 		!(kind == EXPRKIND_RTFUNC ||

src/test/regress/expected/subselect.out

@@ -678,6 +678,20 @@ with q as (select max(f1) from int4_tbl group by f1 order by f1)
  (2147483647)
 (5 rows)
 
+--
+-- Test case for sublinks pulled up into joinaliasvars lists in an
+-- inherited update/delete query
+--
+begin;  --  this shouldn't delete anything, but be safe
+delete from road
+where exists (
+  select 1
+  from
+    int4_tbl cross join
+    ( select f1, array(select q1 from int8_tbl) as arr
+      from text_tbl ) ss
+  where road.name = ss.f1 );
+rollback;
 --
 -- Test case for sublinks pushed down into subselects via join alias expansion
 --

src/test/regress/sql/subselect.sql

@@ -376,6 +376,24 @@ select q from (select max(f1) from int4_tbl group by f1 order by f1) q;
 with q as (select max(f1) from int4_tbl group by f1 order by f1)
   select q from q;
 
+--
+-- Test case for sublinks pulled up into joinaliasvars lists in an
+-- inherited update/delete query
+--
+
+begin;  --  this shouldn't delete anything, but be safe
+
+delete from road
+where exists (
+  select 1
+  from
+    int4_tbl cross join
+    ( select f1, array(select q1 from int8_tbl) as arr
+      from text_tbl ) ss
+  where road.name = ss.f1 );
+
+rollback;
+
 --
 -- Test case for sublinks pushed down into subselects via join alias expansion
 --