Re: very slow queries when max_parallel_workers_per_gather is higher than zero

2018-04-16 14:00 GMT+02:00 Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>:

>
>
> On 04/16/2018 11:34 AM, Pavel Stehule wrote:
> > 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
> > <http://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
> >
> >
>
> What happens when you disable sequential scans on pg10?
>

set enable_seqscan=off;
set max_parallel_workers_per_gather=2;

Query Performs nicely, but no parallel workers are used:
GroupAggregate (cost=2611148.87..2611152.89 rows=31 width=22) (actual
time=0.084..0.084 rows=0 loops=1)
Group Key: f_zendesktickets_aaeljtllr5at3el.cstm_custom_
38746665_primary_column
-> Sort (cost=2611148.87..2611149.11 rows=99 width=28) (actual
time=0.082..0.082 rows=0 loops=1)
Sort Key: f_zendesktickets_aaeljtllr5at3el.cstm_custom_
38746665_primary_column
Sort Method: quicksort Memory: 25kB
-> Nested Loop (cost=1639.25..2611145.59 rows=99 width=28)
(actual time=0.076..0.076 rows=0 loops=1)
Join Filter: (((f_ticketattributeshistory_
aajzjp98uraszb6.attrnewvalue_id = ANY ('{4757,4758,4759}'::integer[])) AND
(4754 = f_ticketattributeshistory_aajzjp98uraszb6.attroldvalue_id) AND
(4790 = f_ticketattributeshistory_aajzjp98uraszb6.ticketfield_id)) OR
(f_zendesktickets_aaeljtllr5at3el.dt_createda
t_id = f_ticketupdate_aad5jtwal0ayaax.dt_event_id))
-> Nested Loop (cost=1638.81..1809540.39 rows=350270
width=20) (actual time=0.075..0.075 rows=0 loops=1)
-> Nested Loop (cost=1638.24..1508474.08 rows=69140
width=8) (actual time=0.075..0.075 rows=0 loops=1)
-> Bitmap Heap Scan on dwh_dm_aabv5kk9rxac4lz_
aaonw7nchsan2n1_aad8xhr0m_aaewg8j61iagl1z dwh_dm_aabv5kk9rxac4lz_
aaonw7nchsan2n1_aad8xhr0m_aaewg8j61ia (cost=4.34..27.35 rows=7 width=4)
(actual time=0.026..0.038 rows=7 loops=1)
Recheck Cond: (6171 = id_euweek)
Heap Blocks: exact=7
-> Bitmap Index Scan on
dwh_dm_aabv5kk9rxac4lz_aaonw7nchsan2n1_aad8xhr0m_a_id_euweek_idx
(cost=0.00..4.33 rows=7 width=0) (actual time=0.019..0.019 rows=7 loops=1)
Index Cond: (6171 = id_euweek)
-> Bitmap Heap Scan on
f_ticketupdate_aad5jtwal0ayaax
(cost=1633.90..214617.67 rows=87472 width=8) (actual time=0.004..0.004
rows=0 loops=7)
Recheck Cond: (dt_event_id =
dwh_dm_aabv5kk9rxac4lz_aaonw7nchsan2n1_aad8xhr0m_aaewg8j61ia.id)
-> Bitmap Index Scan on f_ticketupdate_
aad5jtwal0ayaax_dt_event_id_idx (cost=0.00..1612.03 rows=87472 width=0)
(actual time=0.003..0.003 rows=0 loops=7)
Index Cond: (dt_event_id =
dwh_dm_aabv5kk9rxac4lz_aaonw7nchsan2n1_aad8xhr0m_aaewg8j61ia.id)
-> Index Scan using f_ticketattributeshistory_
aajzjp98uraszb6_ticketupdate_id_idx on
f_ticketattributeshistory_aajzjp98uraszb6
(cost=0.57..4.12 rows=23 width=20) (never executed)
Index Cond: (ticketupdate_id = f_ticketupdate_
aad5jtwal0ayaax.id)
-> Index Scan using f_zendesktickets_aaeljtllr5at3el_pkey
on f_zendesktickets_aaeljtllr5at3el (cost=0.43..2.27 rows=1 width=12)
(never executed)
Index Cond: (id = f_ticketattributeshistory_
aajzjp98uraszb6.zendesktickets_id)
Filter: ((4765 <> status_id) AND (group_id = 17429))
Planning time: 8.516 ms
Execution time: 1.895 ms

the speed is back

>
> regards
>
> --
> Tomas Vondra http://www.2ndQuadrant.com
> PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
>