Re: why not parallel seq scan for slow functions

On Wed, Sep 6, 2017 at 1:47 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
>> On Tue, Sep 5, 2017 at 4:34 AM, Amit Kapila <amit.kapila16@gmail.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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