Re: Hash Join cost estimates

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Jeff Davis <pgsql(at)j-davis(dot)com>
Cc: Stephen Frost <sfrost(at)snowman(dot)net>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Hash Join cost estimates
Date: 2013-03-29 20:37:25
Message-ID: 2942.1364589445@sss.pgh.pa.us
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Jeff Davis <pgsql(at)j-davis(dot)com> writes:
> Yes, I have run into this issue (or something very similar). I don't
> understand why the bucketsize even matters much -- assuming few hash
> collisions, we are not actually evaluating the quals any more times than
> necessary. So why all of the hashjoin-specific logic in determining the
> number of qual evaluations? The only reason I can think of is to model
> the cost of comparing the hashes themselves.

I think the point is that there may *not* be few hash collisions ...

regards, tom lane

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