Simon Riggs wrote: > On Tue, 2010-05-04 at 13:23 -0400, Tom Lane wrote: > >> * LogStandbySnapshot is merest fantasy: no guarantee that either the >> XIDs list or the locks list will be consistent with the point in WAL >> where it will get inserted. What's worse, locking things down enough >> to guarantee consistency would be horrid for performance, or maybe >> even deadlock-inducing. Could lose both ways: list might contain an >> XID whose commit/abort went to WAL before the snapshot did, or list >> might be missing an XID started just after snap was taken, The latter >> case could possibly be dealt with via nextXid filtering, but that >> doesn't fix the former case, and anyway we have both ends of the same >> problem for locks. > > This was the only serious complaint on your list, so lets address it. > > Clearly we don't want to lock everything down, for all the reasons you > say. That creates a gap between when data is derived and when data > logged to WAL.
Right. This was discussed first in August: http://archives.postgresql.org/message-id/4a8ce561.4000...@enterprisedb.com. I concur that the idea is that we deal at replay with the fact that the snapshot lags behind. At replay, any locks/XIDs in the snapshot that have already been committed/aborted are ignored. For any locks/XIDs taken just after the snapshot was taken, the replay will see the other WAL records with that information. We need to add comments explaining all that. -- Heikki Linnakangas EnterpriseDB http://www.enterprisedb.com -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
