On Tue, May 15, 2018 at 08:45:07AM +0530, Amit Kapila wrote:
> No, it is not like that. We divide the scan among workers and each
> worker should perform projection of the rows it scanned (after
> applying filter). Now, if the expensive functions are part of target
> lists, then we can push the computation of expensive functions (as
> part of target list) in workers which will divide the work.
>
> > Really? Do
> > we run each column in its own worker or do we split the result set into
> > parts and run those in parallel? How do we know, just the function call
> > costs?
> >
>
> The function's cost can be determined via pg_proc->procost. For this
> particular case, you can refer the call graph -
> create_pathtarget->set_pathtarget_cost_width->cost_qual_eval_node->cost_qual_eval_walker->get_func_cost
>
> > I can admit I never saw that coming.
> >
>
> I think the use case becomes interesting with parallel query because
> now you can divide such cost among workers.
>
> Feel free to ask more questions if above doesn't clarify the usage of
> these features.
OK, I have added the following release note item for both of these:
2017-11-16 [e89a71fb4] Pass InitPlan values to workers via Gather (Merge).
2018-03-29 [3f90ec859] Postpone generate_gather_paths for topmost scan/join rel
2018-03-29 [11cf92f6e] Rewrite the code that applies scan/join targets to paths
Allow single-evaluation queries, e.g. <literal>FROM</literal>
clause queries, and functions in the target list to be
parallelized (Amit Kapila, Robert Haas)
--
Bruce Momjian <bruce@momjian.us> http://momjian.us
EnterpriseDB http://enterprisedb.com
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