On Thu, Mar 31, 2011 at 4:16 PM, Bruce Momjian <br...@momjian.us> wrote: > Robert Haas wrote: >> On Thu, Mar 31, 2011 at 2:59 PM, Bruce Momjian <br...@momjian.us> wrote: >> > Bruce Momjian wrote: >> >> > Yeah, I think this change would have the effect of moving the freeze >> >> > limit by one (or two?) counts. ?Given the moving nature of values >> >> > returned by ReadNewTransactionId this would probably have no practical >> >> > effect. ?Still, the code as is seems more natural to me (Tom wrote this >> >> > bit IIRC, not me). >> >> >> >> I am now thinking the code is correct --- it maps values from 0 to >> >> FirstNormalTransactionId into the top of the (unsigned) xid range. >> >> Unless someone objects, I will add a C comment about this behavior so >> >> future readers are not confused. >> > >> > OK, now I think it is wrong. ? :-) >> > >> > The effect is to map max xid + 1 to max xid - >> > FirstNormalTransactionId(3) + 1, which makes the xid look like it is >> > going backwards, less than max xid --- not good. >> >> The XID space is *circular*. > > Right but you would think that as the xid moves forward, the caculation > of how far back to vacuum should move only forward. In this case, > incrementing the xid by one would cause the vacuum horizon to move > backward by two.
I don't see how that would happen. The XID immediately preceding FirstNormalTransactionId is 2^32-1, and that's exactly what this calculation produces. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
