Re: Exposing the Xact commit order to the user

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