Re: vacuum, performance, and MVCC

On Thu, 22 Jun 2006, Jonah H. Harris wrote:

> Not in all systems.  A few now perform in-memory UNDO and only write
> it to disk if and when it is required.

Interesting...

>
> > Overwriting MVCC comes with its own baggage. Ask any Oracle user about
> > error ORA-01555[1]. There's also the added cost of managing the UNDO logs,
> > the cost of jumping around between files to get row versions and so on.
>
> This seems to be going in the direction of our common MySQL
> discussions; relying on old failures and mistakes to base our
> assumptions on the current version.  Please stay apprised of current
> developments in other systems.

Erm. Perhaps a poor example as I was not trying to put Oracle in a poor
light. Rather, I was trying to point out that each method has its
disadvantages. If the update in place method has no detractions we
shouldn't be hanging on to our existing mechanism.

> > J. Gray & A Reuter, Transaction Processing: Concepts and Techniques
>
> Pretty much older than dirt, discusses locking, and barely touches on
> MVCC.  Still has some good concepts though.

The really useful section of this book is the discussion of snapshot
isolation. That's the important thing here. Conceivably we could have a
higher performance storage system but, IMHO, it must implement snapshot
isolation.

Thanks,

Gavin