On 05/01/15 00:18, Robert Haas wrote:
> On Thu, Apr 30, 2015 at 5:31 PM, Heikki Linnakangas <hlinnaka@iki.fi> wrote:
>>
>> You can override the ndistinct estimate with ALTER TABLE. I think
>> that's enough for an escape hatch.
>
> I'm not saying that isn't nice to have, but I don't think it really
> helps much here. Setting the value manually requires that you know
> what value to set, and you might not. If, on some workloads, the old
> algorithm beats the new one reliably, you want to be able to
> actually go back to the old algorithm, not manually override every
> wrong decision it makes. A GUC for this is pretty cheap insurance.
IMHO this is exactly the same situation as with the current ndistinct
estimator. If we find out we'd have to use this workaround more
frequently than before, then clearly the new estimator is rubbish and
should not be committed.
In other words, I agree with Heikki.
--
Tomas Vondra http://www.2ndQuadrant.com
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