Unstable number of workers in select_parallel test on spurfowl

Hi,

https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=spurfowl&dt=2018-04-05%2003%3A22%3A05

*** /home/pgbuild/buildfarm/HEAD/pgsql.build/src/test/regress/expected/select_parallel.out
2018-04-04 23:22:07.297108347 -0400
--- /home/pgbuild/buildfarm/HEAD/pgsql.build/src/test/regress/results/select_parallel.out
2018-04-04 23:24:11.709955623 -0400
***************
*** 103,109 ****
  -----------------------------------------------------
   Finalize Aggregate
     ->  Gather
!          Workers Planned: 1
           ->  Partial Aggregate
                 ->  Append
                       ->  Parallel Seq Scan on a_star
--- 103,109 ----
  -----------------------------------------------------
   Finalize Aggregate
     ->  Gather
!          Workers Planned: 3
           ->  Partial Aggregate
                 ->  Append
                       ->  Parallel Seq Scan on a_star

This is the query like this:

-- Disable Parallel Append
alter table a_star reset (parallel_workers);
alter table b_star reset (parallel_workers);
alter table c_star reset (parallel_workers);
alter table d_star reset (parallel_workers);
alter table e_star reset (parallel_workers);
alter table f_star reset (parallel_workers);
set enable_parallel_append to off;
explain (costs off)
  select round(avg(aa)), sum(aa) from a_star;

I don't see why...

-- 
Thomas Munro
http://www.enterprisedb.com