I’m trying to find out why parallel queries are sometimes not used.
For example, I have 2 tables, calendar (1 row per day, ~3K rows) and measure
(~300M rows) which includes a FK to calendar.
I.e knowing two day numbers, I can find out how many measures there are between
these two dates with a
select count(*) from measure m where m.fromdateid >=1462 and m.fromdateid <
1826;
(1462 and 1826 are the calendar ids corresponding to 2015-01-01 and 2015-12-31)
This uses parallel query:
explain select count(*) from measure m where m.fromdateid >=1462 and
m.fromdateid < 1826;
QUERY PLAN
--
Finalize Aggregate (cost=3894860.64..3894860.65 rows=1 width=8)
-> Gather (cost=3894860.61..3894860.62 rows=8 width=8)
Workers Planned: 8
-> Partial Aggregate (cost=3894860.61..3894860.62 rows=1 width=8)
-> Parallel Bitmap Heap Scan on measure m
(cost=11265.96..3881068.52 rows=5516835 width=0)
Recheck Cond: ((fromdateid >= 1462) AND (fromdateid < 1826))
-> Bitmap Index Scan on idx_measure_fromdate
(cost=0.00..232.29 rows=44134699 width=0)
Index Cond: ((fromdateid >= 1462) AND (fromdateid <
1826))
The “equivalent" query without hard coding the day numbers gives this query
plan:
explain select count(*) from calendar c1, calendar c2, measure m where
c1.stddate='2015-01-01' and c2.stddate='2015-12-31' and m.fromdateid
>=c1.calendarid and m.fromdateid < c2.calendarid;
QUERY PLAN
--
Aggregate (cost=5073362.73..5073362.74 rows=1 width=8)
-> Nested Loop (cost=8718.47..4988195.81 rows=34066770 width=0)
-> Index Scan using calendar_stddate_unique on calendar c2
(cost=0.28..2.30 rows=1 width=4)
Index Cond: (stddate = '2015-12-31 00:00:00+01'::timestamp with
time zone)
-> Nested Loop (cost=8718.19..4647525.81 rows=34066770 width=4)
-> Index Scan using calendar_stddate_unique on calendar c1
(cost=0.28..2.30 rows=1 width=4)
Index Cond: (stddate = '2015-01-01 00:00:00+01'::timestamp
with time zone)
-> Bitmap Heap Scan on measure m (cost=8717.91..4306855.81
rows=34066770 width=4)
Recheck Cond: ((fromdateid >= c1.calendarid) AND
(fromdateid < c2.calendarid))
-> Bitmap Index Scan on idx_measure_fromdate
(cost=0.00..201.22 rows=34072527 width=0)
Index Cond: ((fromdateid >= c1.calendarid) AND
(fromdateid < c2.calendarid))
Both queries return the same answers but I don't see why the second one doesn't
use parallel query.
I've tried a few different ways to express the same thing, e.g subselect, CTE
etc in order to try to ease the query planner work but it always avoids the
parallel query.
I also set the parallel_tuple_cost and parallel_setup_cost to 0 without success.
Any idea ? Or is there a way to ask the query planner more details about the
decisions it makes ?
Kind regards,
Didier
