On 24/05/10 19:51, Kevin Grittner wrote:
> The only thing I'm confused about is what benefit anyone expects to
> get from looking at data between commits in some way other than our
> current snapshot mechanism. Can someone explain a use case where
> what Jan is proposing is better than snapshot isolation? It doesn't
> provide any additional integrity guarantees that I can see.
Right, it doesn't. What it provides is a way to reconstruct a snapshot
at any point in time, after the fact. For example, after transactions A,
C, D and B have committed in that order, it allows you to reconstruct a
snapshot just like you would've gotten immediately after the commit of
A, C, D and B respectively. That's useful replication tools like Slony
that needs to commit the changes of those transactions in the slave in
the same order as they were committed in the master.
I don't know enough of Slony et al. to understand why that'd be better
than the current heartbeat mechanism they use, taking a snapshot every
few seconds, batching commits.
-- Heikki Linnakangas EnterpriseDB http://www.enterprisedb.com