Re: ANALYZE sampling is too good

On 12/12/13 08:39, Gavin Flower wrote:
> On 12/12/13 08:31, Kevin Grittner wrote:
>> Gavin Flower <GavinFlower@archidevsys.co.nz> wrote:
>>
>>> For example, assume 1000 rows of 200 bytes and 1000 rows of 20 bytes,
>>> using 400 byte pages.  In the pathologically worst case, assuming
>>> maximum packing density and no page has both types: the large rows 
>>> would
>>> occupy  500 pages and the smaller rows 50 pages. So if one selected 11
>>> pages at random, you get about 10 pages of large rows and about one for
>>> small rows!
>> With 10 * 2 = 20 large rows, and 1 * 20 = 20 small rows.
>>
>> -- 
>> Kevin Grittner
>> EDB: http://www.enterprisedb.com
>> The Enterprise PostgreSQL Company
> Sorry, I've simply come up with well argued nonsense!
>
> Kevin, you're dead right.
>
>
> Cheers,
> Gavin
>
>
I looked at:
http://www.postgresql.org/docs/current/interactive/storage-page-layout.html
this says that each row has an overhead, which suggests there should be 
a bias towards small rows.

There must be a lot of things going on, that I'm simply not aware of, 
that affect sampling bias...


Cheers,
Gavin