On Thu, Mar 31, 2011 at 4:35 PM, Bruce Momjian <br...@momjian.us> wrote: > Robert Haas wrote: >> >> > 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. > > OK, let me see if I understand --- the caculation is below: > > xidForceLimit = recentXid - autovacuum_freeze_max_age; > if (xidForceLimit < FirstNormalTransactionId) > xidForceLimit -= FirstNormalTransactionId; > > The values: > > xidForceLimit Result > --------------------------- > max_xid-2 max_xid-2 > max_xid-1 max_xid-1 > max_xid max_xid > 0 max_xid-3 <- backward here > 1 max_xid-2 > 2 max_xid-1 > 3 3
You have to consider those three lines all of a piece. Suppose autovacuum_freeze_age is 100. Then: 105 -> 5 104 -> 4 103 -> 3 102 -> max_xid 101 -> max_xid - 1 100 -> max_xid - 2 -- 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
