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Re: Limit + group + join

From: Mark Kirkwood <markir(at)paradise(dot)net(dot)nz>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Tobias Brox <tobias(at)nordicbet(dot)com>,pgsql-performance(at)postgresql(dot)org
Subject: Re: Limit + group + join
Date: 2005-08-26 23:29:01
Message-ID: 430FA5BD.2070407@paradise.net.nz (view raw or flat)
Thread:
Lists: pgsql-performance
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


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