On Thu, Jan 19, 2006 at 09:18:55AM +0800, Christopher Kings-Lynne wrote:
> >Oracle does, but you pay in other ways. Instead of keeping dead tuples
> >in the main heap, they shuffle them off to an 'undo log'. This has some
> >downsides:
> >
> >Rollbacks take *forever*, though this usually isn't much of an issue
> >unless you need to abort a really big transaction.
>
> It's a good point though. Surely a database should be optimised for the
> most common operation - commits, rather than rollbacks?
Generally true, but keep in mind this counter-argument... our MVCC
performs fewer disk writes (since generally you can find some free space
on the page you're modifying) and you can control when you take the hit
of cleaning up dead space. In fact, you can take that hit at a reduced
priority (vacuum_cost_*).
--
Jim C. Nasby, Sr. Engineering Consultant jnasby@pervasive.com
Pervasive Software http://pervasive.com work: 512-231-6117
vcard: http://jim.nasby.net/pervasive.vcf cell: 512-569-9461