On Sat, Sep 28, 2019 at 12:16:05AM -0400, Robert Haas wrote:
On Fri, Sep 27, 2019 at 2:24 PM James Coleman <jtc...@gmail.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.
Thanks! In that case I suggest we treat it as a separate patch/fix,
independent of the incremental sort patch. I don't want to bury it in
that patch series, it's already pretty large.
regards
--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
