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.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
