Re: Postgres 11 release notes - Mailing list pgsql-hackers

On Wed, May 16, 2018 at 09:55:05AM +0530, Amit Kapila wrote:
> On Wed, May 16, 2018 at 12:17 AM, Bruce Momjian <bruce@momjian.us> wrote:
> > 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)
> >
> 
> Sorry, but it is not clear to me what you intend to say by "e.g.
> <literal>FROM</literal> clause queries"?   What I could imagine is
> something like "e.g. queries in <literal>WHERE</literal> clause that
> return aggregate value"

Oh, sorry, changed to:

        Allow single-evaluation queries, e.g. <literal>WHERE</literal>
        clause aggregate 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

+ As you are, so once was I.  As I am, so you will be. +
+                      Ancient Roman grave inscription +