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. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
