Re: Parallel Inserts in CREATE TABLE AS

On Mon, Dec 7, 2020 at 3:44 PM Bharath Rupireddy
<bharath.rupireddyforpostgres@gmail.com> wrote:
>
> On Mon, Dec 7, 2020 at 2:55 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> > On Mon, Dec 7, 2020 at 11:32 AM Hou, Zhijie <houzj.fnst@cn.fujitsu.com> wrote:
> > >
> > > +       if (!(root->parse->isForCTAS &&
> > > +               root->query_level == 1))
> > > +               run_cost += parallel_tuple_cost * path->path.rows;
> > >
> > > I noticed that the parallel_tuple_cost will still be ignored,
> > > When Gather is not the top node.
> > >
> > > Example:
> > >         Create table test(i int);
> > >         insert into test values(generate_series(1,10000000,1));
> > >         explain create table ntest3 as select * from test where i < 200 limit 10000;
> > >
> > >                                   QUERY PLAN
> > > -------------------------------------------------------------------------------
> > >  Limit  (cost=1000.00..97331.33 rows=1000 width=4)
> > >    ->  Gather  (cost=1000.00..97331.33 rows=1000 width=4)
> > >          Workers Planned: 2
> > >          ->  Parallel Seq Scan on test  (cost=0.00..96331.33 rows=417 width=4)
> > >                Filter: (i < 200)
> > >
> > >
> > > The isForCTAS will be true because [create table as], the
> > > query_level is always 1 because there is no subquery.
> > > So even if gather is not the top node, parallel cost will still be ignored.
> > >
> > > Is that works as expected ?
> > >
> >
> > I don't think that is expected and is not the case without this patch.
> > The cost shouldn't be changed for existing cases where the write is
> > not pushed to workers.
> >
>
> Thanks for pointing that out. Yes it should not change for the cases
> where parallel inserts will not be picked later.
>
> Any better suggestions on how to make the planner consider that the
> CTAS might choose parallel inserts later at the same time avoiding the
> above issue in case it doesn't?
>

What is the need of checking query_level when 'isForCTAS' is set only
when Gather is a top-node?

-- 
With Regards,
Amit Kapila.