On Sat, Sep 1, 2012 at 06:23:59PM -0400, Tom Lane wrote: > Robert Haas <robertmh...@gmail.com> writes: > > On Tue, May 22, 2012 at 1:50 AM, Tom Lane <t...@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. > > 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 <br...@momjian.us> http://momjian.us EnterpriseDB http://enterprisedb.com + It's impossible for everything to be true. + -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
