On Fri, Apr 08, 2011 at 10:05:01AM -0700, Jeff Davis wrote: > On Fri, 2011-04-08 at 07:08 -0400, Noah Misch wrote: > > > Right, VACUUM FREEZE. I now see I don't need to set > > > vacuum_freeze_table_age if I use the FREEZE keyword, e.g. gram.y has: > > > > > > if (n->options & VACOPT_FREEZE) > > > n->freeze_min_age = n->freeze_table_age = 0; > > > > True; it just performs more work than strictly necessary. We don't actually > > need earlier-than-usual freezing. We need only ensure that the relfrozenxid > > will guide future VACUUMs to do that freezing early enough. However, I'm > > not > > sure how to do that without directly updating relfrozenxid, so it's probably > > just as well to cause some extra work and stick to the standard interface. > > If there are tuples in a toast table containing xids that are older than > the toast table's relfrozenxid, then there are only two options: > > 1. Make relfrozenxid go backward to the right value. There is currently > no mechanism to do this without compiling C code into the server, > because (a) VACUUM FREEZE will never move the relfrozenxid backward; and > (b) there is no way to find the oldest xid in a table with a normal > snapshot.
Couldn't you set relfrozenxid and datfrozenxid to txid_current() - 1100000000 (the highest possible vacuum_freeze_min_age, plus some slop), then run "SET vacuum_freeze_table_age = 0; VACUUM tbl" on all tables for which you did this? There's no need to set relfrozenxid back to a particular "right" value. Not suggesting we recommend this, but I can't think offhand why it wouldn't suffice. > 2. Get rid of those xids older than relfrozenxid (i.e. VACUUM FREEZE). > > I don't know what you mean about VACUUM FREEZE doing extra work. I > suppose you could set the vacuum_freeze_min_age to be exactly the right > value such that it freezes everything before the existing (and wrong) > relfrozenxid, but in practice I think it would be the same amount of > work. Suppose that your next xid at pg_upgrade time was 500M, and it's now 505M. If you're using the default vacuum_freeze_min_age = 50M, "SET vacuum_freeze_table_age = 0; VACUUM tbl" will only freeze tuples covering 5M transaction ids. "VACUUM FREEZE tbl" (a.k.a "SET vacuum_freeze_table_age = 0; SET vacuum_freeze_min_age = 0; VACUUM tbl") will freeze tuples covering 55M transaction ids. nm -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
