Re: Parallel INSERT (INTO ... SELECT ...)

On Wed, Dec 23, 2020 at 1:45 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> On Wed, Dec 23, 2020 at 7:52 AM Hou, Zhijie <houzj.fnst@cn.fujitsu.com> wrote:
> >
> > Hi
> >
> > > > I may be wrong, and if I miss sth in previous mails, please give me some
> > > hints.
> > > > IMO, serial insertion with underlying parallel SELECT can be
> > > > considered for foreign table or temporary table, as the insertions only
> > > happened in the leader process.
> > > >
> > >
> > > I don't think we support parallel scan for temporary tables. Can you please
> > > try once both of these operations without Insert being involved? If you
> > > are able to produce a parallel plan without Insert then we can see why it
> > > is not supported with Insert.
> >
> > Sorry, may be I did not express it clearly, I actually means the case when insert's target(not in select part)
tableis temporary.
 
> > And you are right that parallel select is not enabled when temporary table is in select part.
> >
>
> I think Select can be parallel for this case and we should support this case.
>

So I think we're saying that if the target table is a foreign table or
temporary table, it can be regarded as PARALLEL_RESTRICTED, right?

i.e. code-wise:

        /*
-        * We can't support table modification in parallel-mode if
it's a foreign
-        * table/partition (no FDW API for supporting parallel access) or a
+        * We can't support table modification in a parallel worker if it's a
+        * foreign table/partition (no FDW API for supporting parallel
access) or a
         * temporary table.
         */
        if (rel->rd_rel->relkind == RELKIND_FOREIGN_TABLE ||
                RelationUsesLocalBuffers(rel))
        {
-               table_close(rel, lockmode);
-               context->max_hazard = PROPARALLEL_UNSAFE;
-               return true;
+               if (max_parallel_hazard_test(PROPARALLEL_RESTRICTED, context))
+               {
+                       table_close(rel, lockmode);
+                       return true;
+               }
        }


Regards,
Greg Nancarrow
Fujitsu Australia