Re: Sync Rep: First Thoughts on Code

Mark Mielke wrote:
> Mark Mielke wrote:
>> Forget replication - even for the exact same server - I don't expect
>> that if I commit from one session, I will be able to see the change
>> immediately from my other session or a new session that I just opened.
>> Perhaps this is often stable to rely on this, and it is useful for the
>> database server to minimize the window during which the commit becomes
>> visible to others, but I think it's a false expectation from the start
>> that it absolutely will be immediately visible to another session. I'm
>> thinking of situations where some part of the table is in cache. The
>> only way the commit can communicate that the new transaction is
>> available is by during communication between the processes or threads,
>> or between the multiple CPUs on the machine. Do I want every commit to
>> force each session to become fully in alignment before my commit
>> completes? Does PostgreSQL make this guarantee today? I bet it doesn't
>> if you look far enough into the guts. It might be very fast - I don't
>> think it is infinitely fast.
>
> FYI: I haven't been able to prove this. Multiple sessions running on my
> dual-core CPU seem to be able to see the latest commits before they
> begin executing. Am I wrong about this? Does PostgreSQL provide a
> intentional guarantee that a commit from one session that completes
> immediately followed by a query from another session will always find
> the commit effect visible (provide the transaction isolation level
> doesn't get in the way)?

Yes. PostgreSQL does guarantee that, and I would expect any other DBMS
to do the same.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com