On Tue, Dec 8, 2020 at 6:24 PM Hou, Zhijie <houzj.f...@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
