Обсуждение: Regarding redo/undo files.

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

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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I see that most of this document is obsolete.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)

Alvaro Herrera <alvherre@dcc.uchile.cl> writes:
> 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 I see that most of this document is obsolete.

Yeah, I was planning to remove that section or at least edit it
severely...

> I'm pretty sure there's people that don't want UNDO to be implemented.

I think the general consensus is that it doesn't offer us anything we
need, and the downsides are significant.  (Oracle DBAs are well aware
of the hazards of having to rely on the transaction log for UNDO ---
it does not play well at all with long-running transactions.)
        regards, tom lane

Mamta Singh <mamta_csd@yahoo.co.in> writes:
> Also, give me some idea as to what do we
> need to change to implement undo-file. 

It's very unlikely that we'll ever implement WAL-based UNDO.
No one has given any convincing reason to think it's a good idea.
        regards, tom lane