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/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
 --