Re: Regarding redo/undo files.

On Fri, Jul 30, 2004 at 05:53:02AM +0100, Mamta Singh wrote:

> I went thru the site
> http://www.postgresql.org/docs/7.4/static/wal-benefits-later.html

I see that most of this document is obsolete. It mentions:

1. with UNDO it will be possible to remove pg_clog.
(already possible, another mechanism)
2. with UNDO it will be possible to implement savepoints.
(already possible, another mechanism)
3. with WAL, PITR is possible.
(already done)
4. we need a compressed WAL format.

Why not rip that section completely?

> Undo operation is not implemented. and information of
> the status regarding the transaction is stored in the
> permanent file pg_clog. But I have not been able to
> see the format of the file, as this file is in binary
> format. Same for the redo file.

pg_clog is documented in slru.c, clog.c and the README file in current
CVS tip src/backend/access/transam. It's an array of "doublebits."

REDO files are the WAL archives. The format is also binary. See xlog.c
for a start.

> Could you please send me the format of the redo and
> pg_clog file. Also, give me some idea as to what do we
> need to change to implement undo-file.

I'm pretty sure there's people that don't want UNDO to be implemented.
Don't waste your time ...

--
Alvaro Herrera (<alvherre[a]dcc.uchile.cl>)
"A wizard is never late, Frodo Baggins, nor is he early.
He arrives precisely when he means to." (Gandalf, en LoTR FoTR)