On 21 July 2018 at 20:15, Didier Carlier <didier.carlier@xxxxxxxxxxx> wrote: > 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. You'd likely be better of writing the query as: select count(*) from measure where fromdateid >= (select calendarid from calendar where stddate = '2015-01-01') and fromdateid < (select calendarid from calendar where stddate = '2015-12-31'); The reason you get the poor nested loop plan is that nested loop is the only join method that supports non-equijoin. Unsure why you didn't get a parallel plan. Parallel in pg10 supports a few more plan shapes than 9.6 did. Unsure what version you're using. -- David Rowley http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services
