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Re: 7.4 vs 7.3 ( hash join issue )

From: Gaetano Mendola <mendola(at)bigfoot(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Subject: Re: 7.4 vs 7.3 ( hash join issue )
Date: 2004-09-23 14:04:20
Message-ID: 4152D7E4.3010409@bigfoot.com (view raw or flat)
Thread:
Lists: pgsql-patchespgsql-performance
Tom Lane wrote:
> Greg Stark <gsstark(at)mit(dot)edu> writes:
> 
>>No, postgres didn't do things in reverse order. It hashed the empty table and
>>then went ahead and checked every record of the non-empty table against the
>>empty hash table.
> 
> 
>>Reading the code there's no check for this, and it seems like it would be a
>>useful low-cost little optimization.
> 
> 
> Yeah, I was just looking at doing that.
> 
> It would also be interesting to prefetch one row from the outer table and fall
> out immediately (without building the hash table) if the outer table is
> empty.  This seems to require some contortion of the code though :-(
> 
> 
>>I think postgres normally hashes the table it thinks is smaller,
> 
> 
> Right, it will prefer to put the physically smaller table (estimated
> width*rows) on the inside.

Do you plan to do a patch for the 7.4, so I'll wait for a 7.4.6 ( that IIRC have already
two important patches pending ) or is 8.0 stuff ?



Regards
Gaetano Mendola




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