Re: why not parallel seq scan for slow functions

On Wed, Sep 6, 2017 at 1:47 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>> On Tue, Sep 5, 2017 at 4:34 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>>> Okay, now I understand your point, but I think we already change the
>>> cost of paths in apply_projection_to_path which is done after add_path
>>> for top level scan/join paths.
>
>> Yeah. I think that's a nasty hack, and I think it's Tom's fault. :-)
>
> Yeah, and it's also documented:
>
> * This has the same net effect as create_projection_path(), except that if
> * a separate Result plan node isn't needed, we just replace the given path's
> * pathtarget with the desired one. This must be used only when the caller
> * knows that the given path isn't referenced elsewhere and so can be modified
> * in-place.
>
> If somebody's applying apply_projection_to_path to a path that's already
> been add_path'd, that's a violation of the documented restriction.

/me is confused. Isn't that exactly what grouping_planner() is doing,
and has done ever since your original pathification commit
(3fc6e2d7f5b652b417fa6937c34de2438d60fa9f)? It's iterating over
current_rel->pathlist, so surely everything in there has been
add_path()'d.

--
Robert Haas
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