Re: About varlena2

Qingqing Zhou <zhouqq@cs.toronto.edu> wrote:

> To reduce size of varlen2.vl_len to int16. This has been mentioned before,
> but is there any show-stopper reasoning preventing us from doing that or
> somebody has been working on it?

Hi, I'm rewriting the patch that I proposed before.
(http://archives.postgresql.org/pgsql-hackers/2005-09/msg00421.php)
This is another way to reduce the size of variable length types,
using variable length headers.

I'm sure that there are pros and cons of this approach.
Pros. - Optimized for short variables (length <= 127),   where the header takes only one byte. - It can represent long
data.
Cons. - More complexity and operations to extract lengths and buffers. - Needs more works to support TOAST.

To support TOAST, I think the following representations.
It might be good to use only A and B, if TOAST is not needed.
 | Representation             | Size  | Mode                |
--+----------------------------+-------+---------------------+
A | 0*******            + data | 1 + n | length <= 127       |
B | 10****** +  1 byte  + data | 2 + n | length <= 16K -1    |
C | 110----- +  4 bytes + data | 5 + n | length <= 4G  -1    |
D | 1110---- +  6 bytes + data | 7 + n | Compressed          |
E | 11110--- + 12 bytes        | 13    | External            |
F | 11111--- + 16 bytes        | 17    | External+Compressed |
('*' bits are used for length, '-' are unused.)

Comments welcome,
---
ITAGAKI Takahiro
NTT Cyber Space Laboratories