From: | Bruce Momjian <bruce(at)momjian(dot)us> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: Getting rid of cheap-startup-cost paths earlier |
Date: | 2012-09-01 22:26:55 |
Message-ID: | 20120901222655.GA21043@momjian.us |
Views: | Raw Message | Whole Thread | Download mbox | Resend email |
Thread: | |
Lists: | pgsql-hackers |
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. +
From | Date | Subject | |
---|---|---|---|
Next Message | Tom Lane | 2012-09-02 00:47:41 | Re: Estimated rows question |
Previous Message | Tom Lane | 2012-09-01 22:23:59 | Re: Getting rid of cheap-startup-cost paths earlier |