Re: Limit + group + join

Tom Lane wrote:
> Mark Kirkwood <markir(at)paradise(dot)net(dot)nz> writes:
>
>>What is interesting is why this plan is being rejected...
>
>
> Which PG version are you using exactly? That mistake looks like an
> artifact of the 8.0 "fuzzy plan cost" patch, which we fixed recently:
> http://archives.postgresql.org/pgsql-committers/2005-07/msg00474.php
>

Right on - 8.0.3 (I might look at how CVS tip handles this, could be
interesting).

> But Tobias wasn't happy with 7.4 either, so I'm not sure that the fuzzy
> cost issue explains his results.
>
> As far as the "desc" point goes, the problem is that mergejoins aren't
> capable of dealing with backward sort order, so a merge plan isn't
> considered for that case (or at least, it would have to have a sort
> after it, which pretty much defeats the point for a fast-start plan).
> I have some ideas about fixing this but it won't happen before 8.2.

That doesn't explain why the nested loop is being kicked tho', or have I
missed something obvious? - it's been known to happen :-)...

Cheers

Mark