Re: Hash Join cost estimates

On Mon, Apr 1, 2013 at 2:35 AM, Jeff Davis <pgsql(at)j-davis(dot)com> wrote:
> On Sun, 2013-03-31 at 15:45 -0400, Tom Lane wrote:
>> Really, when we're traipsing down a bucket
>> list, skipping over bucket entries with the wrong hash code is just
>> about free, or at least it's a whole lot cheaper than applying ExecQual.
>>
>> Perhaps what we should do is charge the hash_qual_cost only for some
>> small multiple of the number of tuples that we expect will *pass* the
>> hash quals, which is a number we have to compute anyway. The multiple
>> would represent the rate of hash-code collisions we expect.
>
> +1.
>
>> I'd still be inclined to charge something per bucket entry, but it
>> should be really small, perhaps on the order of 0.01 times
>> cpu_operator_cost.
>
>> Or we could just drop that term entirely.
>
> FWIW, either of those are fine with me based on my limited experience.

FWIW, I have also seen this problem and the proposed fixes sound
reasonable to me.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company