Re: Parallel Inserts in CREATE TABLE AS

On Tue, Dec 8, 2020 at 6:24 PM Hou, Zhijie <houzj.fnst@cn.fujitsu.com> wrote:
>
> > > I'm not quite sure how to address this. Can we not allow the planner
> > > to consider that the select is for CTAS and check only after the
> > > planning is done for the Gather node and other checks?
> > >
> >
> > IIUC, you are saying that we should not influence the cost of gather node
> > even when the insertion would be done by workers? I think that should be
> > our fallback option anyway but that might miss some paths to be considered
> > parallel where the cost becomes more due to parallel_tuple_cost (aka tuple
> > transfer cost). I think the idea is we can avoid the tuple transfer cost
> > only when Gather is the top node because only at that time we can push
> > insertion down, right? How about if we have some way to detect the same
> > before calling generate_useful_gather_paths()? I think when we are calling
> > apply_scanjoin_target_to_paths() in grouping_planner(), if the
> > query_level is 1, it is for CTAS, and it doesn't have a chance to create
> > UPPER_REL (doesn't have grouping, order, limit, etc clause) then we can
> > probably assume that the Gather will be top_node. I am not sure about this
> > but I think it is worth exploring.
> >
>
> I took a look at the parallel insert patch and have the same idea.
> https://commitfest.postgresql.org/31/2844/
>
>          * Consider generating Gather or Gather Merge paths.  We must only do this
>          * if the relation is parallel safe, and we don't do it for child rels to
>          * avoid creating multiple Gather nodes within the same plan. We must do
>          * this after all paths have been generated and before set_cheapest, since
>          * one of the generated paths may turn out to be the cheapest one.
>          */
>         if (rel->consider_parallel && !IS_OTHER_REL(rel))
>                 generate_useful_gather_paths(root, rel, false);
>
> IMO Gatherpath created here seems the right one which can possible ignore parallel cost if in CTAS.
> But We need check the following parse option which will create path to be the parent of Gatherpath here.
>
> if (root->parse->rowMarks)
> if (limit_needed(root->parse))
> if (root->parse->sortClause)
> if (root->parse->distinctClause)
> if (root->parse->hasWindowFuncs)
> if (root->parse->groupClause || root->parse->groupingSets || root->parse->hasAggs || root->root->hasHavingQual)
>

Thanks Amit and Hou. I will look into these areas and get back soon.

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