Re: Consider low startup cost in add_partial_path

On Fri, Sep 27, 2019 at 2:24 PM James Coleman <jtc331(at)gmail(dot)com> wrote:
> Over in the incremental sort patch discussion we found [1] a case
> where a higher cost plan ends up being chosen because a low startup
> cost partial path is ignored in favor of a lower total cost partial
> path and a limit is a applied on top of that which would normal favor
> the lower startup cost plan.
>
> 45be99f8cd5d606086e0a458c9c72910ba8a613d originally added
> `add_partial_path` with the comment:
>
> > Neither do we need to consider startup costs:
> > parallelism is only used for plans that will be run to completion.
> > Therefore, this routine is much simpler than add_path: it needs to
> > consider only pathkeys and total cost.
>
> I'm not entirely sure if that is still true or not--I can't easily
> come up with a scenario in which it's not, but I also can't come up
> with an inherent reason why such a scenario cannot exist.

I think I just didn't think carefully about the Limit case.

--
Robert Haas
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