Re: TABLESAMPLE patch

On 04/10/15 21:57, Petr Jelinek wrote:
> On 10/04/15 21:26, Peter Eisentraut wrote:
>
> But this was not really my point, the BERNOULLI just does not work
> well with row-limit by definition, it applies probability on each
> individual row and while you can get probability from percentage very
> easily (just divide by 100), to get it for specific target number of
> rows you have to know total number of source rows and that's not
> something we can do very accurately so then you won't get 500 rows
> but approximately 500 rows.

It's actually even trickier. Even if you happen to know the exact number 
of rows in the table, you can't just convert that into a percentage like 
that and use it for BERNOULLI sampling. It may give you different number 
of result rows, because each row is sampled independently.

That is why we have Vitter's algorithm for reservoir sampling, which 
works very differently from BERNOULLI.

--
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services