On 2013-06-05 15:28:09 +0100, Greg Stark wrote: > On Wed, May 22, 2013 at 3:18 AM, Robert Haas <robertmh...@gmail.com> wrote: > > We've had a number of discussions about the evils of SnapshotNow. As > > far as I can tell, nobody likes it and everybody wants it gone, but > > there is concern about the performance impact. > I thought there were many call sites that were specifically depending > on seeing dirty reads to avoid race conditions with other backends -- > which probably just narrowed the race condition or created different > ones.
But SnapshotNow doesn't allow you to do actual dirty reads? It only gives you rows back that were actually visible when we checked. The difference to SnapshotMVCC is that during a scan the picture of which transactions are committed can change. > I'm not even sure what "clean them up" means. You can replace checks > with things like constraints and locks but the implementation of > constraints and locks will still need to use SnapshotNow surely? The places that require this should already use HeapTupleSatisfiesDirty which is something different. Greetings, Andres Freund -- Andres Freund http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
