From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
Cc: | Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: why not parallel seq scan for slow functions |
Date: | 2017-09-06 19:41:42 |
Message-ID: | 1389.1504726902@sss.pgh.pa.us |
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Lists: | pgsql-hackers |
Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> In particular, as Jeff and Amit point out, it
> may well be that (a) before apply_projection_to_path(), the cheapest
> plan is non-parallel and (b) after apply_projection_to_path(), the
> cheapest plan would be a Gather plan, except that it's too late
> because we've already thrown that path out.
I'm not entirely following. I thought that add_path was set up to treat
"can be parallelized" as an independent dimension of merit, so that
parallel paths would always survive.
> What we ought to do, I think, is avoid generating gather paths until
> after we've applied the target list (and the associated costing
> changes) to both the regular path list and the partial path list.
Might be a tad messy to rearrange things that way.
regards, tom lane
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