Re: Exposing the Xact commit order to the user

On Mon, May 24, 2010 at 11:24 AM, Kevin Grittner
<Kevin(dot)Grittner(at)wicourts(dot)gov> wrote:
> Jan Wieck wrote:
>
>> In some systems (data warehousing, replication), the order of
>> commits is important, since that is the order in which changes
>> have become visible.
>
> This issue intersects with the serializable work I've been doing.
> While in database transactions using S2PL the above is true, in
> snapshot isolation and the SSI implementation of serializable
> transactions, it's not.

I think you're confusing two subtly different things. The way to
prove that a set of transactions running under some implementation of
serializability is actually serializable is to construct a serial
order of execution consistent with the view of the database that each
transaction saw. This may or may not match the commit order, as you
say. But the commit order is still the order the effects of those
transactions have become visible - if we inserted a new read-only
transaction into the stream at some arbitrary point in time, it would
see all the transactions which committed before it and none of those
that committed afterward. So I think Jan's statement is correct.

Having said that, I think your concerns about how things will look
from a slave's point of view are possibly valid. A transaction
running on a slave is essentially a read-only transaction that the
master doesn't know about. It's not clear to me whether adding such a
transaction to the timeline could result in either (a) that
transaction being rolled back or (b) some impact on which other
transactions got rolled back. If it did, that would obviously be a
problem for serializability on slaves, though your proposed fix sounds
like it would be prohibitively expensive for many users. But can this
actually happen?

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise Postgres Company