Re: Exposing the Xact commit order to the user

On Wed, May 26, 2010 at 4:11 PM, Dimitri Fontaine
<dfontaine(at)hi-media(dot)com> wrote:
> Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com> writes:
>> Perhaps I'm missing something, but I thought that Slony currently uses a
>> heartbeat, and all transactions committed between two beats are banged
>> together and committed as one in the slave so that their relative commit
>> order doesn't matter.
>
> I guess Slony does the same as pgq here: all events of all those
> transactions between two given ticks are batched together in the order
> of the event commits. (In fact the batches are made at the consumer
> request, so possibly spreading more than 2 ticks at a time).
>
> If you skip that event ordering (within transactions), you can't
> maintain foreign keys on the slaves, among other things.
>
> The idea of this proposal is to be able to get this commit order
> directly from where the information is maintained, rather than use some
> sort of user sequence for that.

Exactly.

> So even ordering the txid and txid_snapshots with respect to WAL commit
> time (LSN) won't be the whole story, for any given transaction
> containing more than one event we also need to have them in order. I
> know Jan didn't forget about it so it must either be in the proposal or
> easily derived, too tired to recheck.

Right, so the point is - with this proposal, he can switch to using a
LOCAL sequence number to order events within the session and then
order the sessions using the commit ordering. Right now, he has to
use a GLOBAL sequence number because there's no way to know the commit
order.

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