Add a simple test case covering a join against an inheritance tree,

since we're evidently not testing that at all right now :-(

src/test/regress/sql/join.sql

@@ -389,6 +389,14 @@ SELECT * FROM t3;
 DELETE FROM t3 USING t3 t3_other WHERE t3.x = t3_other.x AND t3.y = t3_other.y;
 SELECT * FROM t3;
 
+-- Test join against inheritance tree
+
+create temp table t2a () inherits (t2);
+
+insert into t2a values (200, 2001);
+
+select * from t1 left join t2 on (t1.a = t2.a);
+
 --
 -- regression test for 8.1 merge right join bug
 --