very slow queries when max_parallel_workers_per_gather is higher than zero

Hi,

my customer does performance checks of PostgreSQL 9.5 and 10. Almost all queries on 10 are faster, but there are few queries (40 from 1000) where PostgreSQL 9.5 is significantly faster than. Unfortunately - pretty fast queries (about 20ms) are too slow now (5 sec).

attached execution plans

It looks like some cost issue, slow queries prefers Seq scan against bitmap heap scan

Hash Cond: (f_ticketupdate_aad5jtwal0ayaax.dt_event_id = dwh_dm_aabv5kk9rxac4lz_aaonw7nchsan2n1_aad8xhr0m_aaewg8j61ia.id)
   ->  Parallel Seq Scan on f_ticketupdate_aad5jtwal0ayaax  (cost=0.00..1185867.47 rows=24054847 width=8) (actual time=0.020..3741.409 rows=19243863 loops=3)
   ->  Hash  (cost=27.35..27.35 rows=7 width=4) (actual time=0.089..0.089 rows=7 loops=3)
        Buckets: 1024  Batches: 1  Memory Usage: 9kB

Regards

Pavel