RE: Parallel Inserts in CREATE TABLE AS

From: Bharath Rupireddy <bharath(dot)rupireddyforpostgres(at)gmail(dot)com>
Sent: Thursday, May 27, 2021 3:41 PM
>
> On Thu, May 27, 2021 at 1:03 PM tsunakawa(dot)takay(at)fujitsu(dot)com
> <tsunakawa(dot)takay(at)fujitsu(dot)com> wrote:
> >
> > From: houzj(dot)fnst(at)fujitsu(dot)com <houzj(dot)fnst(at)fujitsu(dot)com>
> > > Although, the 4 workers case still has performance degradation
> > > compared to serial case.
> > >
> > > SERIAL: 58759.213 ms
> > > PARALLEL 2 WORKER [NOT SKIP FSM]: 68390.221 ms [SKIP FSM]:
> > > 58633.924 ms
> > > PARALLEL 4 WORKER [NOT SKIP FSM]: 67448.142 ms [SKIP FSM]:
> > > 66,960.305 ms
> >
> > Can you see any difference in table sizes?
>
> Also, the number of pages the table occupies in each case along with table size
> would give more insights.
>
> I do as follows to get the number of pages a relation occupies:
> CREATE EXTENSION pgstattuple;
> SELECT pg_relpages('test');

It seems the difference between SKIP FSM and NOT SKIP FSM is not big.
I tried serval times and the average result is almost the same.

pg_relpages
-------------
1428575

pg_relation_size
-------------
11702976512(11G)

Best regards,
houzj