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

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.

--
Robert Haas
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