On 12/12/13 08:39, Gavin Flower wrote:
On 12/12/13 08:31, Kevin Grittner wrote:
Gavin Flower <gavinflo...@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
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