Improve the heuristic for ordering child paths of a parallel append.

Commit ab7271677 introduced code that attempts to order the child
scans of a Parallel Append node in a way that will minimize execution
time, based on total cost and startup cost.  However, it failed to
think hard about what to do when estimated costs are exactly equal;
a case that's particularly likely to occur when comparing on startup
cost.  In such a case the ordering of the child paths would be left
to the whims of qsort, an algorithm that isn't even stable.

We can improve matters by applying the rule used elsewhere in the
planner: if total costs are equal, sort on startup cost, and
vice versa.  When both cost estimates are exactly equal, rather
than letting qsort do something unpredictable, sort based on the
child paths' relids, which should typically result in sorting in
inheritance order.  (The latter provision requires inventing a
qsort-style comparator for bitmapsets, but maybe we'll have use
for that for other reasons in future.)

This results in a few plan changes in the select_parallel test,
but those all look more reasonable than before, when the actual
underlying cost numbers are taken into account.

Discussion: https://postgr.es/m/4944.1515446989@sss.pgh.pa.us

1 files changed, 7 insertions, 7 deletions

diff --git a/src/test/regress/expected/select_parallel.out b/src/test/regress/expected/select_parallel.out
index 7824ca52ca..452494fbfa 100644
--- a/src/test/regress/expected/select_parallel.out
+++ b/src/test/regress/expected/select_parallel.out
@@ -21,12 +21,12 @@ explain (costs off)
          Workers Planned: 3
          ->  Partial Aggregate
                ->  Parallel Append
-                     ->  Parallel Seq Scan on a_star
-                     ->  Parallel Seq Scan on b_star
-                     ->  Parallel Seq Scan on c_star
                      ->  Parallel Seq Scan on d_star
-                     ->  Parallel Seq Scan on e_star
                      ->  Parallel Seq Scan on f_star
+                     ->  Parallel Seq Scan on e_star
+                     ->  Parallel Seq Scan on b_star
+                     ->  Parallel Seq Scan on c_star
+                     ->  Parallel Seq Scan on a_star
 (11 rows)
 
 select round(avg(aa)), sum(aa) from a_star a1;
@@ -49,10 +49,10 @@ explain (costs off)
                ->  Parallel Append
                      ->  Seq Scan on d_star
                      ->  Seq Scan on c_star
-                     ->  Parallel Seq Scan on a_star
-                     ->  Parallel Seq Scan on b_star
-                     ->  Parallel Seq Scan on e_star
                      ->  Parallel Seq Scan on f_star
+                     ->  Parallel Seq Scan on e_star
+                     ->  Parallel Seq Scan on b_star
+                     ->  Parallel Seq Scan on a_star
 (11 rows)
 
 select round(avg(aa)), sum(aa) from a_star a2;