Re: tablesample performance

On 18 October 2016 at 19:34, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Andy Colson <andy(at)squeakycode(dot)net> writes:
>> On 10/18/2016 11:44 AM, Francisco Olarte wrote:
>>> This should be faster, but to me it seems it does a different thing.
>
>> Ah, yes, you're right, there is a bit of a difference there.
>
> If you don't want to have an implicit bias towards earlier blocks,
> I don't think that either standard tablesample method is really what
> you want.
>
> The contrib/tsm_system_rows tablesample method is a lot closer, in
> that it will start at a randomly chosen block, but if you just do
> "tablesample system_rows(1)" then you will always get the first row
> in whichever block it lands on, so it's still not exactly unbiased.

Is there a reason why we can't fix the behaviours of the three methods
mentioned above by making them all start at a random block and a
random item between min and max?

It wasn't ever intended to be biased and bernoulli in particular ought
to have a strict no bias.

Happy to patch if we agree.

--
Simon Riggs http://www.2ndQuadrant.com/
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