Re: tablesample performance

On 18 October 2016 at 22:06, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Simon Riggs <simon@2ndquadrant.com> writes:
>> On 18 October 2016 at 19:34, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>>> 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?
>
> The standard tablesample methods are constrained by other requirements,
> such as repeatability.  I am not sure that loading this one on top of
> that is a good idea.  The bias I referred to above is *not* the fault
> of the sample methods, rather it's the fault of using "LIMIT 1".

Hmm, yeh, that would make it a little too much of a special case.

> It does seem like maybe it'd be nice for tsm_system_rows to start at a
> randomly chosen entry in the first block it visits, rather than always
> dumping that entire block.  Then "tablesample system_rows(1)" would
> actually give you a pretty random row, and I think we aren't giving up
> any useful properties it has now.

OK, will patch that.

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