Re: Parallel Inserts in CREATE TABLE AS

On Sun, Dec 27, 2020 at 2:28 PM Bharath Rupireddy <
bharath(dot)rupireddyforpostgres(at)gmail(dot)com> wrote:
>
> On Sat, Dec 26, 2020 at 9:20 PM vignesh C <vignesh21(at)gmail(dot)com> wrote:
> > +-- parallel inserts must occur
> > +select explain_pictas(
> > +'create table parallel_write as select length(stringu1) from tenk1;');
> > +select count(*) from parallel_write;
> > +drop table parallel_write;
> >
> > We can change comment "parallel inserts must occur" like "parallel
> > insert must be selected for CTAS on normal table"
> >
> > +-- parallel inserts must occur
> > +select explain_pictas(
> > +'create unlogged table parallel_write as select length(stringu1) from
tenk1;');
> > +select count(*) from parallel_write;
> > +drop table parallel_write;
> >
> > We can change comment "parallel inserts must occur" like "parallel
> > insert must be selected for CTAS on unlogged table"
> > Similar comment need to be handled in other places also.
>
> I think the existing comments look fine. The info like table type and
> the Query CTAS or CMV is visible by looking at the test case. What I
> wanted from the comments is whether we support parallel inserts or not
> and if not why so that it will be easy to read. I tried to keep it as
> succinctly as possible.
>

I saw few inconsistencies in the patch:

*+-- parallel inserts must occur*+select explain_pictas(
+'create table parallel_write as select length(stringu1) from tenk1;');
+ explain_pictas

*+-- parallel inserts must not occur as the table is temporary*+select
explain_pictas(
+'create temporary table parallel_write as select length(stringu1) from
tenk1;');
+ explain_pictas

*+-- parallel inserts must occur, as there is init plan that gets executed
by+-- each parallel worker*
+select explain_pictas(
+'create table parallel_write as select two col1,
+ (select two from (select * from tenk2) as tt limit 1) col2
+ from tenk1 where tenk1.four = 3;');
+ explain_pictas

*+-- the top node is Gather under which merge join happens, so parallel
inserts+-- must occur*
+set enable_nestloop to off;
+set enable_mergejoin to on;

*+-- parallel hash join happens under Gather node, so parallel inserts must
occur*+set enable_mergejoin to off;
+set enable_hashjoin to on;
+select explain_pictas(

Test comments are detailed in a few cases and in few others it is not
detailed for similar kinds of parallelism selected tests. I felt we could
make the test comments consistent across the file.

Regards,
Vignesh
EnterpriseDB: http://www.enterprisedb.com