On Fri, Jul 29, 2011 at 10:20 AM, Hannu Krosing <ha...@2ndquadrant.com> wrote:
>> An additional point to think about: if we were willing to insist on
>> streaming replication, we could send the commit sequence numbers via a
>> side channel rather than writing them to WAL, which would be a lot
>> cheaper.
> Why do you think that side channel is cheaper than main WAL ?

You don't have to flush it to disk, and you can use some other lock
that isn't as highly contended as WALInsertLock to synchronize it.

>> That might even be a reasonable thing to do, because if
>> you're doing log shipping, this is all going to be super-not-real-time
>> anyway.
> But perhaps you still may want to preserve visibility order to be able
> to do PITR to exact transaction "commit", no ?

Maybe.  In practice, I suspect most people won't be willing to pay the
price a feature like this would exact.

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

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