Re: Parallel Inserts in CREATE TABLE AS

On Sun, Dec 27, 2020 at 2:28 PM Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com> wrote:
>
> On Sat, Dec 26, 2020 at 9:20 PM vignesh C <vignesh21@gmail.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