Sequential parallel scan / agg

James Sewell <james.sewell@xxxxxxxxxxxx> · Mon, 14 Nov 2016 10:04:44 +1100

Hi all,
I have a customer who is making use of the new parallel features on a 64 core box - this is working really well for them. However one issue (which I've since realised I've never seen an example of) is getting the data from these queries into a table.

I can't seem to create a plan which does this and still uses multiple workers, probably best to just show by example.

Below you can see:
INSERT ... SELECT doesn't work
WITH a AS (SELECT ...) INSERT FROM a doesn't work
SELECT .. INTO doesn't work
SELECT with no insert does work as expected

test=# explain
insert into jirotech.test
(select count(*)  FROM  meter_read mr

where TIMESTAMP_ROUNDED >= (timestamp'2016-03-30 00:00:00' - interval '2' day)

and TIMESTAMP_ROUNDED < timestamp'2016-03-30 00:00:00' )
;
                                                                                         QUERY PLAN                        
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Insert on test  (cost=13336866.41..13336866.44 rows=1 width=438)
   ->  Subquery Scan on "*SELECT*"  (cost=13336866.41..13336866.44 rows=1 width=438)
         ->  Aggregate  (cost=13336866.41..13336866.42 rows=1 width=8)
               ->  Append  (cost=0.00..12405302.12 rows=372625715 width=0)
                     ->  Seq Scan on meter_read mr  (cost=0.00..0.00 rows=1 width=0)
                           Filter: ((timestamp_rounded >= '2016-03-28 00:00:00'::timestamp without time zone) AND (timestamp_rounded < '2016-03-30 00:00:00'::timestamp without time zone))
                     ->  Seq Scan on meter_read_2016_03_29 mr_1  (cost=0.00..6201295.24 rows=186240748 width=0)
                           Filter: ((timestamp_rounded >= '2016-03-28 00:00:00'::timestamp without time zone) AND (timestamp_rounded < '2016-03-30 00:00:00'::timestamp without time zone))
                     ->  Seq Scan on meter_read_2016_03_28 mr_2  (cost=0.00..6204006.88 rows=186384966 width=0)
                           Filter: ((timestamp_rounded >= '2016-03-28 00:00:00'::timestamp without time zone) AND (timestamp_rounded < '2016-03-30 00:00:00'::timestamp without time zone))
(10 rows)

test=# explain
with a as
(select count(*)  FROM  meter_read mr

where TIMESTAMP_ROUNDED >= (timestamp'2016-03-30 00:00:00' - interval '2' day)

and TIMESTAMP_ROUNDED < timestamp'2016-03-30 00:00:00'  )
INSERT INTO jirotech.test SELECT * FROM A
;

                                                                                QUERY PLAN                          
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Insert on test  (cost=13336866.42..13336866.45 rows=1 width=438)
   CTE a
     ->  Aggregate  (cost=13336866.41..13336866.42 rows=1 width=8)
           ->  Append  (cost=0.00..12405302.12 rows=372625715 width=0)
                 ->  Seq Scan on meter_read mr  (cost=0.00..0.00 rows=1 width=0)
                       Filter: ((timestamp_rounded >= '2016-03-28 00:00:00'::timestamp without time zone) AND (timestamp_rounded < '2016-03-30 00:00:00'::timestamp without time zone))
                 ->  Seq Scan on meter_read_2016_03_29 mr_1  (cost=0.00..6201295.24 rows=186240748 width=0)
                       Filter: ((timestamp_rounded >= '2016-03-28 00:00:00'::timestamp without time zone) AND (timestamp_rounded < '2016-03-30 00:00:00'::timestamp without time zone))
                 ->  Seq Scan on meter_read_2016_03_28 mr_2  (cost=0.00..6204006.88 rows=186384966 width=0)
                       Filter: ((timestamp_rounded >= '2016-03-28 00:00:00'::timestamp without time zone) AND (timestamp_rounded < '2016-03-30 00:00:00'::timestamp without time zone))
   ->  CTE Scan on a  (cost=0.00..0.03 rows=1 width=438)
(11 rows)

test=# explain
select count(*) into jirotech.jamestest FROM  meter_read mr

where TIMESTAMP_ROUNDED >= (timestamp'2016-03-30 00:00:00' - interval '2' day)

and TIMESTAMP_ROUNDED < timestamp'2016-03-30 00:00:00'
;
                                                                                   QUERY PLAN                              
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Aggregate  (cost=13336866.41..13336866.42 rows=1 width=8)
   ->  Append  (cost=0.00..12405302.12 rows=372625715 width=0)
         ->  Seq Scan on meter_read mr  (cost=0.00..0.00 rows=1 width=0)
               Filter: ((timestamp_rounded >= '2016-03-28 00:00:00'::timestamp without time zone) AND (timestamp_rounded < '2016-03-30 00:00:00'::timestamp without time zone))
         ->  Seq Scan on meter_read_2016_03_29 mr_1  (cost=0.00..6201295.24 rows=186240748 width=0)
               Filter: ((timestamp_rounded >= '2016-03-28 00:00:00'::timestamp without time zone) AND (timestamp_rounded < '2016-03-30 00:00:00'::timestamp without time zone))
         ->  Seq Scan on meter_read_2016_03_28 mr_2  (cost=0.00..6204006.88 rows=186384966 width=0)
               Filter: ((timestamp_rounded >= '2016-03-28 00:00:00'::timestamp without time zone) AND (timestamp_rounded < '2016-03-30 00:00:00'::timestamp without time zone))
(8 rows)

Time: 29.229 ms

test=# explain
select count(*)  FROM  meter_read mr

where TIMESTAMP_ROUNDED >= (timestamp'2016-03-30 00:00:00' - interval '2' day)

and TIMESTAMP_ROUNDED < timestamp'2016-03-30 00:00:00'
;
                                                                                         QUERY PLAN                        
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Finalize Aggregate  (cost=7631566.71..7631566.72 rows=1 width=8)
   ->  Gather  (cost=7631566.68..7631566.69 rows=8 width=8)
         Workers Planned: 8
         ->  Partial Aggregate  (cost=7630566.68..7630566.69 rows=1 width=8)
               ->  Append  (cost=0.00..7514121.14 rows=46578216 width=0)
                     ->  Parallel Seq Scan on meter_read mr  (cost=0.00..0.00 rows=1 width=0)
                           Filter: ((timestamp_rounded >= '2016-03-28 00:00:00'::timestamp without time zone) AND (timestamp_rounded < '2016-03-30 00:00:00'::timestamp without time zone))
                     ->  Parallel Seq Scan on meter_read_2016_03_29 mr_1  (cost=0.00..3756417.28 rows=23280094 width=0)
                           Filter: ((timestamp_rounded >= '2016-03-28 00:00:00'::timestamp without time zone) AND (timestamp_rounded < '2016-03-30 00:00:00'::timestamp without time zone))
                     ->  Parallel Seq Scan on meter_read_2016_03_28 mr_2  (cost=0.00..3757703.86 rows=23298121 width=0)
                           Filter: ((timestamp_rounded >= '2016-03-28 00:00:00'::timestamp without time zone) AND (timestamp_rounded < '2016-03-30 00:00:00'::timestamp without time zone))
(11 rows)

Time: 29.819 ms

Cheers,

James Sewell,
PostgreSQL Team Lead / Solutions Architect 

Suite 112, Jones Bay Wharf, 26-32 Pirrama Road, Pyrmont NSW 2009
P (+61) 2 8099 9000  W www.jirotech.com  F (+61) 2 8099 9099

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