Fix plan created for inherited UPDATE/DELETE with all tables excluded.

In the case where inheritance_planner() finds that every table has
been excluded by constraints, it thought it could get away with
making a plan consisting of just a dummy Result node.  While certainly
there's no updating or deleting to be done, this had two user-visible
problems: the plan did not report the correct set of output columns
when a RETURNING clause was present, and if there were any
statement-level triggers that should be fired, it didn't fire them.

Hence, rather than only generating the dummy Result, we need to
stick a valid ModifyTable node on top, which requires a tad more
effort here.

It's been broken this way for as long as inheritance_planner() has
known about deleting excluded subplans at all (cf commit 635d42e9c),
so back-patch to all supported branches.

Amit Langote and Tom Lane, per a report from Petr Fedorov.

Discussion: https://postgr.es/m/5da6f0f0-1364-1876-6978-907678f89a3e@phystech.edu

src/backend/optimizer/plan/planner.c

@@ -1583,19 +1583,42 @@ inheritance_planner(PlannerInfo *root)
 	 * to get control here.
 	 */
 
-	/*
-	 * If we managed to exclude every child rel, return a dummy plan; it
-	 * doesn't even need a ModifyTable node.
-	 */
 	if (subpaths == NIL)
 	{
-		set_dummy_rel_pathlist(final_rel);
-		return;
-	}
-
 		/*
-	 * Put back the final adjusted rtable into the master copy of the Query.
-	 * (We mustn't do this if we found no non-excluded children.)
+		 * We managed to exclude every child rel, so generate a dummy path
+		 * representing the empty set.  Although it's clear that no data will
+		 * be updated or deleted, we will still need to have a ModifyTable
+		 * node so that any statement triggers are executed.  (This could be
+		 * cleaner if we fixed nodeModifyTable.c to support zero child nodes,
+		 * but that probably wouldn't be a net win.)
+		 */
+		List	   *tlist;
+		Path	   *dummy_path;
+
+		/* tlist processing never got done, either */
+		tlist = root->processed_tlist = preprocess_targetlist(root);
+		final_rel->reltarget = create_pathtarget(root, tlist);
+
+		/* Make a dummy path, cf set_dummy_rel_pathlist() */
+		dummy_path = (Path *) create_append_path(NULL, final_rel, NIL, NIL,
+												 NULL, 0, false, NIL, -1);
+
+		/* These lists must be nonempty to make a valid ModifyTable node */
+		subpaths = list_make1(dummy_path);
+		subroots = list_make1(root);
+		resultRelations = list_make1_int(parse->resultRelation);
+		if (parse->withCheckOptions)
+			withCheckOptionLists = list_make1(parse->withCheckOptions);
+		if (parse->returningList)
+			returningLists = list_make1(parse->returningList);
+	}
+	else
+	{
+		/*
+		 * Put back the final adjusted rtable into the master copy of the
+		 * Query.  (We mustn't do this if we found no non-excluded children,
+		 * since we never saved an adjusted rtable at all.)
 		 */
 		parse->rtable = final_rtable;
 		root->simple_rel_array_size = save_rel_array_size;
@@ -1612,6 +1635,7 @@ inheritance_planner(PlannerInfo *root)
 
 			root->simple_rte_array[rti++] = rte;
 		}
+	}
 
 	/*
 	 * If there was a FOR [KEY] UPDATE/SHARE clause, the LockRows node will

src/test/regress/expected/inherit.out

@@ -539,6 +539,47 @@ CREATE TEMP TABLE z (b TEXT, PRIMARY KEY(aa, b)) inherits (a);
 INSERT INTO z VALUES (NULL, 'text'); -- should fail
 ERROR:  null value in column "aa" violates not-null constraint
 DETAIL:  Failing row contains (null, text).
+-- Check inherited UPDATE with all children excluded
+create table some_tab (a int, b int);
+create table some_tab_child () inherits (some_tab);
+insert into some_tab_child values(1,2);
+explain (verbose, costs off)
+update some_tab set a = a + 1 where false;
+            QUERY PLAN            
+----------------------------------
+ Update on public.some_tab
+   Update on public.some_tab
+   ->  Result
+         Output: (a + 1), b, ctid
+         One-Time Filter: false
+(5 rows)
+
+update some_tab set a = a + 1 where false;
+explain (verbose, costs off)
+update some_tab set a = a + 1 where false returning b, a;
+            QUERY PLAN            
+----------------------------------
+ Update on public.some_tab
+   Output: b, a
+   Update on public.some_tab
+   ->  Result
+         Output: (a + 1), b, ctid
+         One-Time Filter: false
+(6 rows)
+
+update some_tab set a = a + 1 where false returning b, a;
+ b | a 
+---+---
+(0 rows)
+
+table some_tab;
+ a | b 
+---+---
+ 1 | 2
+(1 row)
+
+drop table some_tab cascade;
+NOTICE:  drop cascades to table some_tab_child
 -- Check UPDATE with inherited target and an inherited source table
 create temp table foo(f1 int, f2 int);
 create temp table foo2(f3 int) inherits (foo);

src/test/regress/expected/triggers.out

@@ -1716,6 +1716,40 @@ drop table self_ref_trigger;
 drop function self_ref_trigger_ins_func();
 drop function self_ref_trigger_del_func();
 --
+-- Check that statement triggers work correctly even with all children excluded
+--
+create table stmt_trig_on_empty_upd (a int);
+create table stmt_trig_on_empty_upd1 () inherits (stmt_trig_on_empty_upd);
+create function update_stmt_notice() returns trigger as $$
+begin
+	raise notice 'updating %', TG_TABLE_NAME;
+	return null;
+end;
+$$ language plpgsql;
+create trigger before_stmt_trigger
+	before update on stmt_trig_on_empty_upd
+	execute procedure update_stmt_notice();
+create trigger before_stmt_trigger
+	before update on stmt_trig_on_empty_upd1
+	execute procedure update_stmt_notice();
+-- inherited no-op update
+update stmt_trig_on_empty_upd set a = a where false returning a+1 as aa;
+NOTICE:  updating stmt_trig_on_empty_upd
+ aa 
+----
+(0 rows)
+
+-- simple no-op update
+update stmt_trig_on_empty_upd1 set a = a where false returning a+1 as aa;
+NOTICE:  updating stmt_trig_on_empty_upd1
+ aa 
+----
+(0 rows)
+
+drop table stmt_trig_on_empty_upd cascade;
+NOTICE:  drop cascades to table stmt_trig_on_empty_upd1
+drop function update_stmt_notice();
+--
 -- Check that index creation (or DDL in general) is prohibited in a trigger
 --
 create table trigger_ddl_table (

src/test/regress/sql/inherit.sql

@@ -97,6 +97,21 @@ SELECT relname, d.* FROM ONLY d, pg_class where d.tableoid = pg_class.oid;
 CREATE TEMP TABLE z (b TEXT, PRIMARY KEY(aa, b)) inherits (a);
 INSERT INTO z VALUES (NULL, 'text'); -- should fail
 
+-- Check inherited UPDATE with all children excluded
+create table some_tab (a int, b int);
+create table some_tab_child () inherits (some_tab);
+insert into some_tab_child values(1,2);
+
+explain (verbose, costs off)
+update some_tab set a = a + 1 where false;
+update some_tab set a = a + 1 where false;
+explain (verbose, costs off)
+update some_tab set a = a + 1 where false returning b, a;
+update some_tab set a = a + 1 where false returning b, a;
+table some_tab;
+
+drop table some_tab cascade;
+
 -- Check UPDATE with inherited target and an inherited source table
 create temp table foo(f1 int, f2 int);
 create temp table foo2(f3 int) inherits (foo);

src/test/regress/sql/triggers.sql

@@ -1177,6 +1177,33 @@ drop table self_ref_trigger;
 drop function self_ref_trigger_ins_func();
 drop function self_ref_trigger_del_func();
 
+--
+-- Check that statement triggers work correctly even with all children excluded
+--
+
+create table stmt_trig_on_empty_upd (a int);
+create table stmt_trig_on_empty_upd1 () inherits (stmt_trig_on_empty_upd);
+create function update_stmt_notice() returns trigger as $$
+begin
+	raise notice 'updating %', TG_TABLE_NAME;
+	return null;
+end;
+$$ language plpgsql;
+create trigger before_stmt_trigger
+	before update on stmt_trig_on_empty_upd
+	execute procedure update_stmt_notice();
+create trigger before_stmt_trigger
+	before update on stmt_trig_on_empty_upd1
+	execute procedure update_stmt_notice();
+
+-- inherited no-op update
+update stmt_trig_on_empty_upd set a = a where false returning a+1 as aa;
+-- simple no-op update
+update stmt_trig_on_empty_upd1 set a = a where false returning a+1 as aa;
+
+drop table stmt_trig_on_empty_upd cascade;
+drop function update_stmt_notice();
+
 --
 -- Check that index creation (or DDL in general) is prohibited in a trigger
 --