On 6/22/06, Gavin Sherry <[EMAIL PROTECTED]> wrote:
There seems to be a bit of confusion about what MVCC is.
Yes, PostgreSQL implements MVTO-style concurrency, Oracle implements MVRC, ... Let's not go into theory here, because there's plenty of papers and books on the subject.
The other MVCC databases maintain UNDO logs outside of the table's data files.
*most* other MVCC systems act this way, but not all.
When an update occurs, the existing row version is copied to te UNDO file
Not in all systems. A few now perform in-memory UNDO and only write it to disk if and when it is required.
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. As you're probably haven't done consulting for Oracle recently, I can tell you that the once-dreaded ORA-01555 is nearly a thing of the past. Continuing to dispense misinformation doesn't help anyone so let's stop trying to use Oracle's past problems (which are largely due to improper tuning) as preferring PostgreSQL's methods over others.
Also, it leads to inefficiency with variable size data types. The new version of a row might be longer or shorter than the previous version and this has to be causing them a headaches and performance penalties.
I've seen many MVCC implementations using update-in-place (overwriting storage manager) with plenty of variable-length attributes; not all have these problems.
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.
US Patent Number 5,870,758 -- (one of?) Oracle's snapshot isolation patent
Again, not the most optimal implementation of MVCC, but does certainly have advantages.
Tom Lane's MVCC talk: http://www.postgresql.org/files/developer/transactions.pdf
Always a good read. I suggest reading Transactional Information Systems: Theory, Algorithms, and the Practice of Concurrency Control by Weikum and Vossen... it's basically the new Gray and Reuter. It covers multiple MVCC techniques as well as differences between the different UNDO/REDO database models. -- Jonah H. Harris, Software Architect | phone: 732.331.1300 EnterpriseDB Corporation | fax: 732.331.1301 33 Wood Ave S, 2nd Floor | [EMAIL PROTECTED] Iselin, New Jersey 08830 | http://www.enterprisedb.com/ ---------------------------(end of broadcast)--------------------------- TIP 4: Have you searched our list archives? http://archives.postgresql.org
