Re: [HACKERS] parallelize queries containing subplans

On Tue, Jan 10, 2017 at 11:55 AM, Dilip Kumar <dilipbalaut@gmail.com> wrote:
> On Wed, Dec 28, 2016 at 11:47 AM, Amit Kapila <amit.kapila16@gmail.com> wrote:
>> Attached patch implements the above idea.  This will enable
>> parallelism for queries containing un-correlated subplans, an example
>> of which is as follows:
>
> I have reviewed the patch (pq_pushdown_subplan_v1.patch), Mostly patch
> looks clean to me.
> I have also done some basic functional testing and it's working fine.
>
> I have one comment.
>
> + else if (IsA(node, AlternativeSubPlan))
> + {
> + AlternativeSubPlan *asplan = (AlternativeSubPlan *) node;
> + ListCell   *lc;
> +
> + Assert(context->root);
> +
> + foreach(lc, asplan->subplans)
> + {
> + SubPlan    *splan = (SubPlan *) lfirst(lc);
> + Plan   *plan;
> +
> + Assert(IsA(splan, SubPlan));
> +
> + plan = planner_subplan_get_plan(context->root, splan);
> + if (!plan->parallel_safe)
> + return true;
> + }
>   }
>
> I see no reason why we need to process the subplan list of
> AlternativeSubPlan here, Anyway expression_tree_walker will take care
> of processing each subplan of AlternativeSubPlan. Now in the case
> where all the subplans in AlternativeSubPlan are parallel_safe we will
> process this list twice.
>

Valid point, but I think we can avoid that by returning false after
foreach(..) loop.  I think one improvement could be that instead of
manually checking the parallel safety of each subplan, we can
recursively call max_parallel_hazard_walker for each subplan.


> But, more than that it will be cleaner to not
> handle AlternativeSubPlan here unless there is some strong reason?
>

Yeah, the reason is to avoid unnecessary recursion.  Also, in all
other places in the code, we do handle both of them separately, refer
cost_qual_eval_walker for somewhat similar usage.


-- 
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com