Instability in partition_prune test?

Hi,

Apologies if this was already discussed, I didn't see it. One of my
animals elver had a one-off failure this morning:

https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=elver&dt=2018-04-12%2018%3A18%3A05

partition_prune ... FAILED

Subplans Removed: 6
-> Parallel Seq Scan on ab_a2_b1 (actual rows=0 loops=1)
Filter: ((a >= $1) AND (a <= $2) AND (b < 4))
! -> Parallel Seq Scan on ab_a2_b2 (actual rows=0 loops=1)
Filter: ((a >= $1) AND (a <= $2) AND (b < 4))
-> Parallel Seq Scan on ab_a2_b3 (actual rows=0 loops=1)
Filter: ((a >= $1) AND (a <= $2) AND (b < 4))
--- 1608,1614 ----
Subplans Removed: 6
-> Parallel Seq Scan on ab_a2_b1 (actual rows=0 loops=1)
Filter: ((a >= $1) AND (a <= $2) AND (b < 4))
! -> Parallel Seq Scan on ab_a2_b2 (actual rows=0 loops=2)
Filter: ((a >= $1) AND (a <= $2) AND (b < 4))
-> Parallel Seq Scan on ab_a2_b3 (actual rows=0 loops=1)
Filter: ((a >= $1) AND (a <= $2) AND (b < 4))

This is a Parallel Append with three processes working on three
subplans. It looks like one of the subplans got executed twice?

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