Re: Incorrect cost for MergeAppend

Hi,

> Here is a small patch that reproduces the problem on two tables with
> inheritance, and fixes it. I'll add it to the Commitfest.

Thanks for the patch.

I can confirm that it changes the plan from Sort + Append to MergeAppend.

Before:

```
explain (costs off) select * from ma0 where a < 1000 order by a;
                       QUERY PLAN
---------------------------------------------------------
 Sort
   Sort Key: ma0.a
   ->  Append
         ->  Index Only Scan using ma0_pkey on ma0 ma0_1
               Index Cond: (a < 1000)
         ->  Seq Scan on ma1 ma0_2
               Filter: (a < 1000)
```

After:

```
=# explain (costs off) select * from ma0 where a < 1000 order by a;
                    QUERY PLAN
---------------------------------------------------
 Merge Append
   Sort Key: ma0.a
   ->  Index Only Scan using ma0_pkey on ma0 ma0_1
         Index Cond: (a < 1000)
   ->  Sort
         Sort Key: ma0_2.a
         ->  Seq Scan on ma1 ma0_2
               Filter: (a < 1000)
```

The rest of the tests pass.

-- 
Best regards,
Aleksander Alekseev