On 12/12/13 06:22, Tom Lane wrote:
> I wrote:
>> Hm. You can only take N rows from a block if there actually are at least
>> N rows in the block. So the sampling rule I suppose you are using is
>> "select up to N rows from each sampled block" --- and that is going to
>> favor the contents of blocks containing narrower-than-average rows.
> Oh, no, wait: that's backwards. (I plead insufficient caffeine.)
> Actually, this sampling rule discriminates *against* blocks with
> narrower rows. You previously argued, correctly I think, that
> sampling all rows on each page introduces no new bias because row
> width cancels out across all sampled pages. However, if you just
> include up to N rows from each page, then rows on pages with more
> than N rows have a lower probability of being selected, but there's
> no such bias against wider rows. This explains why you saw smaller
> values of "i" being undersampled.
>
> Had you run the test series all the way up to the max number of
> tuples per block, which is probably a couple hundred in this test,
> I think you'd have seen the bias go away again. But the takeaway
> point is that we have to sample all tuples per page, not just a
> limited number of them, if we want to change it like this.
>
> regards, tom lane
>
>
Hmm...
In my previous reply, which hasn't shown up yet!
I realized I made a mistake!
The fraction/probability could any of 0.25. 0.50, and 0.75.
Cheers,
Gavin