Re: Proposal for Null Bitmap Optimization(for Trailing NULLs)

On Mon, 2007-12-17 at 08:47 -0500, Andrew Dunstan wrote:

> This strikes me as such a corner case that it's likely not to be worth it.
> 
> If you really want to save space along these lines, one better place to 
> start might  be mutable with column ordering - see 
> http://archives.postgresql.org/pgsql-hackers/2006-12/msg00983.php . That 
> would mean that we would be able to move nullable columns physically to 
> the tail which in turn might help this suggestion have more effect.

Could be a good idea.

Currently on a 64-bit system we occupy 23 bytes for row header, so any
table with more than 8 columns will cause the null bitmap to overflow
and for us to use another 8 bytes.

OP's idea could avoid that in many cases, so the saving isn't 1 byte it
is fairly frequently going to be an 8 byte saving.

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