Fix mark-and-restore-skipping test case to not be a self-join.

There isn't any good reason for this test to be a self-join rather
than a join between separate tables, except that it saved a couple
of SQL commands for setup.  A proposed patch to optimize away
self-joins breaks the test, so adjust it to avoid that happening.

Discussion: https://postgr.es/m/64486b0b-0404-e39e-322d-0801154901f3@postgrespro.ru

src/test/regress/expected/join.out

@@ -5932,22 +5932,26 @@ left join j2 on j1.id1 = j2.id1 where j1.id2 = 1;
 set enable_nestloop to 0;
 set enable_hashjoin to 0;
 set enable_sort to 0;
--- create an index that will be preferred over the PK to perform the join
+-- create indexes that will be preferred over the PKs to perform the join
 create index j1_id1_idx on j1 (id1) where id1 % 1000 = 1;
-explain (costs off) select * from j1 j1
-inner join j1 j2 on j1.id1 = j2.id1 and j1.id2 = j2.id2
+create index j2_id1_idx on j2 (id1) where id1 % 1000 = 1;
+-- need an additional row in j2, if we want j2_id1_idx to be preferred
+insert into j2 values(1,2);
+analyze j2;
+explain (costs off) select * from j1
+inner join j2 on j1.id1 = j2.id1 and j1.id2 = j2.id2
 where j1.id1 % 1000 = 1 and j2.id1 % 1000 = 1;
                QUERY PLAN                
---------------------------------------------
+-----------------------------------------
  Merge Join
    Merge Cond: (j1.id1 = j2.id1)
    Join Filter: (j1.id2 = j2.id2)
    ->  Index Scan using j1_id1_idx on j1
-   ->  Index Scan using j1_id1_idx on j1 j2
+   ->  Index Scan using j2_id1_idx on j2
 (5 rows)
 
-select * from j1 j1
-inner join j1 j2 on j1.id1 = j2.id1 and j1.id2 = j2.id2
+select * from j1
+inner join j2 on j1.id1 = j2.id1 and j1.id2 = j2.id2
 where j1.id1 % 1000 = 1 and j2.id1 % 1000 = 1;
  id1 | id2 | id1 | id2 
 -----+-----+-----+-----

src/test/regress/sql/join.sql

@@ -2014,15 +2014,20 @@ set enable_nestloop to 0;
 set enable_hashjoin to 0;
 set enable_sort to 0;
 
--- create an index that will be preferred over the PK to perform the join
+-- create indexes that will be preferred over the PKs to perform the join
 create index j1_id1_idx on j1 (id1) where id1 % 1000 = 1;
+create index j2_id1_idx on j2 (id1) where id1 % 1000 = 1;
 
-explain (costs off) select * from j1 j1
-inner join j1 j2 on j1.id1 = j2.id1 and j1.id2 = j2.id2
+-- need an additional row in j2, if we want j2_id1_idx to be preferred
+insert into j2 values(1,2);
+analyze j2;
+
+explain (costs off) select * from j1
+inner join j2 on j1.id1 = j2.id1 and j1.id2 = j2.id2
 where j1.id1 % 1000 = 1 and j2.id1 % 1000 = 1;
 
-select * from j1 j1
-inner join j1 j2 on j1.id1 = j2.id1 and j1.id2 = j2.id2
+select * from j1
+inner join j2 on j1.id1 = j2.id1 and j1.id2 = j2.id2
 where j1.id1 % 1000 = 1 and j2.id1 % 1000 = 1;
 
 reset enable_nestloop;