Re: AW: [HACKERS] RE: [GENERAL] Transaction logging

Zeugswetter Andreas IZ5 wrote:
> 
> > 2. all datafiles have to be backed up _before_ log backup
> >    due to WAL logic: changes must be written to log before
> >    they'll be written to on-disk data pages.
> >
> When I was talking about pg_log, I meant pg_log as it is now.
> As I understand it, it only stores commit/rollback info for each used xtid
> and no other info.

Actually, I would like to implement WAL as it's done in other systems.
There would be no more pg_log with xact statuses as now. But for
the first implementation it's easy to leave pg_log as is (UNDO
is hard to implement). In any case WAL will be _single_ source
about everything - what's changes and what transactions were
commited/aborted. From this point of view pg_log will be just
one of datafiles: on recovery changes (commit/abort statuses)
will be applied to pg_log just like to other datafiles.

> PS: for rollback you need the before image of rows, I would keep 
> this in a separate place like Oracle (rollback segment) and Informix
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Oracle places rollback segments in memory to speedup abort/MVCC.
Before images are in WAL and used to restore rollback segments
on recovery.

> (physical log) since this info does not need to go to the 
> rollforward tape.

Vadim