Re: storage size of "bit" data type..

On Dec 5, 2007, at 7:23 PM, Michael Glaesemann wrote:
> On Dec 5, 2007, at 14:19 , Alex Mayrhofer wrote:
>> i'm trying to find out the storage size for bit(n) data. My
>> initial assumption would be that for any 8 bits, one byte of
>> storage is required.
>
> select pg_column_size(B'1') as "1bit",
>        pg_column_size(B'1111') as "4bits",
>        pg_column_size(B'11111111') as "1byte",
>        pg_column_size(B'111111111111') as "12bits",
>        pg_column_size(B'1111111111111111') as "2bytes",
>        pg_column_size(B'11111111111111111') as "17bits",
>        pg_column_size(B'111111111111111111111111') as "3bytes";
> 1bit | 4bits | 1byte | 12bits | 2bytes | 17bits | 3bytes
> ------+-------+-------+--------+--------+--------+--------
>     9 |     9 |     9 |     10 |     10 |     11 |     11
> (1 row)
>
> Looks like there's 8 bytes of overhead as well, probably because a
> bit string is a varlena type.

Wow, that's screwed up... that's a lot more than varlena overhead:

select pg_column_size('a'::text), pg_column_size(1::numeric),
pg_column_size(3111234::numeric);
  pg_column_size | pg_column_size | pg_column_size
----------------+----------------+----------------
               5 |             10 |             12

Apparently it's something related to numeric.
--
Decibel!, aka Jim C. Nasby, Database Architect  decibel@decibel.org
Give your computer some brain candy! www.distributed.net Team #1828