Re: Getting rid of cheap-startup-cost paths earlier

On Sat, Sep 1, 2012 at 06:23:59PM -0400, Tom Lane wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> > On Tue, May 22, 2012 at 1:50 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> >> Currently, the planner keeps paths that appear to win on the grounds of
> >> either cheapest startup cost or cheapest total cost. It suddenly struck
> >> me that in many simple cases (viz, those with no LIMIT, EXISTS, cursor
> >> fast-start preference, etc) we could know a-priori that cheapest startup
> >> cost is not going to be interesting, and hence immediately discard any
> >> path that doesn't win on total cost.
> >>
> >> This would require some additional logic to detect whether the case
> >> applies, as well as extra complexity in add_path. So it's possible
> >> that it wouldn't be worthwhile overall. Still, it seems like it might
> >> be a useful idea to investigate.
> >>
> >> Thoughts?
>
> > Yeah, I think we should investigate that. Presumably you could easily
> > have a situation where one part of the tree is under a LIMIT or EXISTS
> > and therefore needs to preserve fast-start plans but the rest of the
> > (potentially large) tree isn't, so we need something fairly
> > fine-grained, I think. Maybe we could add a flag to each RelOptInfo
> > indicating whether fast-start plans should be kept, or something like
> > that.
>
> I got around to looking at this finally. It turns out to be a big win,
> at least for queries without any LIMIT or other reason to worry about
> fast-start plans.

Yes, I remember from the early days how quickly the number of considred
paths can grow.

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ It's impossible for everything to be true. +