Re: Parallel Inserts in CREATE TABLE AS

On Fri, Nov 27, 2020 at 11:57 AM Hou, Zhijie <houzj(dot)fnst(at)cn(dot)fujitsu(dot)com> wrote:
>
> > Thanks a lot for the use case. Yes with the current patch table will lose
> > data related to the subplan. On analyzing further, I think we can not allow
> > parallel inserts in the cases when the Gather node has some projections
> > to do. Because the workers can not perform that projection. So, having
> > ps_ProjInfo in the Gather node is an indication for us to disable parallel
> > inserts and only the leader can do the insertions after the Gather node
> > does the required projections.
> >
> > Thoughts?
> >
>
> Agreed.
>

Thanks! I will add/modify IsParallelInsertInCTASAllowed() to return
false in this case.

>
> 2.
> @@ -166,6 +228,16 @@ ExecGather(PlanState *pstate)
> {
> ParallelContext *pcxt;
>
> + /*
> + * Take the necessary information to be passed to workers for
> + * parallel inserts in CTAS.
> + */
> + if (ISCTAS(node->ps.intoclause))
> + {
> + node->ps.lefttree->intoclause = node->ps.intoclause;
> + node->ps.lefttree->objectid = node->ps.objectid;
> + }
> +
> /* Initialize, or re-initialize, shared state needed by workers. */
> if (!node->pei)
> node->pei = ExecInitParallelPlan(node->ps.lefttree,
>
> I found the code pass intoclause and objectid to Gather node's lefttree.
> Is it necessary? It seems only Gather node will use the information.
>

I am passing the required information from the up to here through
PlanState structure. Since the Gather node's leftree is also a
PlanState structure variable, here I just assigned them to pass that
information to ExecInitParallelPlan().

With Regards,
Bharath Rupireddy.
EnterpriseDB: http://www.enterprisedb.com