On Sun, 2008-12-14 at 12:57 -0500, Mark Mielke wrote: > I'm curious about your suggestion to direct queries that need the > latest > snapshot to the 'primary'. I might have misunderstood it - but it > seems > that the expectation from some is that *all* sessions see the latest > snapshot, so would this not imply that all sessions would be redirect > to > the 'primary'? I don't think it is reasonable myself, but I might be > misunderstanding something...
I said "a snapshot taken on the primary", but the query would run on the standby. Synchronising primary and standby so that they are identical from the perspective of a query requires some synchronisation delay. I'm pointing out that the synchronisation delay can occur * at the time we apply WAL - which will slow down commits (i.e. #6 on my previous list of options) * at the time we run a query that needs to see primary and standby synchronised So the same effect can be achieved in various ways. The first way would require *all* transactions to be applied on standby, i.e. option #6 for all transactions. That is a performance disaster and I would not force that onto everybody. The second way can be done by taking a snapshot on the primary, with an associated LSN, then using that snapshot on the standby. That is somewhat complex, but possible. I see the requirement for getting the same answer on multiple nodes as a further extension of "transaction isolation mode" and think that not all people will want this, so we should allow that as an option. I'm not going to worry about this at the moment. Hot standby will be useful without this and so I regard this as a secondary objective. Rome wasn't built in a single release, or something like that. -- Simon Riggs www.2ndQuadrant.com PostgreSQL Training, Services and Support -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
