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

On Tue, May 22, 2012 at 08:29:48AM -0400, Robert Haas wrote:
> On Tue, May 22, 2012 at 1:50 AM, Tom Lane <tgl@sss.pgh.pa.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.

Is this a TODO?

-- 
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

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