Re: Do log files demand serialization?
Date: Fri, 15 Feb 2002 07:32:00 GMT
> If it's any help, logs don't have to record the updates for a transaction
> contiguously. Each action in the AIJ log is keyed by transaction ID, and
IIRC, (and it's been a while since I studied the internals of this) Transaction ID's are assigned serially, at the inception of a transaction, but that isn't the serial order that the term "serializability" refers to, when we are talking about the ACID properties of transactions.
Consider the following scenario:
Transaction A starts, and gets assigned ID = 126. Transaction B starts, and gets assigned ID = 127.
Transaction B updates column X of Row Y of table Z.
Transaction B commits.
Transaction A updates column W of row Y of table Z. The value of column X delivered by transaction B is copied by this action.
Transaction A commits.
If you want to serialize these transactions, it's clear that B precedes A,
and not vice versa.
The fact that A has the "earlier" ID than B is of no consequence.
How this applies to AIJ records is complicated, but it works.
-- Regards, David Cressey www.dcressey.com "Kendall" <kendallwillets_at_yahooooo.com> wrote in message news:u6p4n3ftfblo20_at_news.supernews.com...Received on Fri Feb 15 2002 - 08:32:00 CET