Re: Problem with estimating pages for a table

Cristina M wrote:

> I posted to the general list, and didn't receive any replies.
> Therefore, I am trying this list now, hopefully this is the right
> mailing list for this type of questions.
> 
> I am trying to compute the no of pages of a table. I am using the formula :
> 
> pages = ( columns width + 28) * no. of rows / block size

Keep in mind that if you have varchar(1000) and store 30 bytes of text,
it will use 30+4, not 1000+4.

Very long attributes may be compressed and/or stored in a side table
called the TOAST table; only a pointer to it remains on the base table
(which is some 20 bytes long I think).

Also keep in mind that there's a lot of space lost to alignment
considerations, so don't expect things to match down to the last byte.

-- 
Alvaro Herrera                                http://www.CommandPrompt.com/
PostgreSQL Replication, Consulting, Custom Development, 24x7 support