> https://www.postgresql.org/docs/14/using-explain.html > The doc says >
> "the loops value reports the total number of executions of the node, and the > > actual time and rows values shown are averages per-execution. This is done > > to make the numbers comparable with the way that the cost estimates are > > shown." > > But I found for parallel operation, the above description maybe not > > correct. > > For example > > postgres=# create table c(id int); > > CREATE TABLE > > postgres=# insert into c select generate_series(1,1000000); > > INSERT 0 1000000 > > postgres=# explain analyze select count(*) from c; > > QUERY PLAN > -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- > > Finalize Aggregate (cost=10633.55..10633.56 rows=1 width=8) (actual > > time=290.460..290.508 rows=1 loops=1) > > -> Gather (cost=10633.33..10633.54 rows=2 width=8) (actual > > time=289.605..290.484 rows=3 loops=1) > > Workers Planned: 2 > > Workers Launched: 2 > > -> Partial Aggregate (cost=9633.33..9633.34 rows=1 width=8) > > (actual time=188.336..188.337 rows=1 loops=3) > > -> Parallel Seq Scan on c (cost=0.00..8591.67 rows=416667 > > width=0) (actual time=0.030..140.036 rows=333333 loops=3) > > Planning Time: 0.331 ms > > Execution Time: 290.607 ms > > (8 rows) > > postgres=# > > According to PG-doc, the "Parallel Seq Scan" node cost 140.036*3=420ms, but > > the total cost for this SQL is only 290ms. > > Is the output of this explain correct? Parallel query is explained in https://www.postgresql.org/docs/14/how-parallel-query-works.html and https://www.postgresql.org/docs/14/parallel-plans.html The docs seem clear to me that as the nodes are executed in parallel then the time execution time is not 140.036*3. The 140.036 value is actual time the Parallel Seq Scan nodes ran for but there were up to 2 running in parallel.
publickey - nikolai.berkoff@pm.me - 0xD189792D.asc
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