Re: Bug in autovacuum.c?

Alvaro Herrera wrote:
> > That would probably work, but the existing coding actually makes more
> > sense.  It's essentially trying to scan backwards by
> > autovacuum_freeze_max_age XIDs through the circular XID space.  But
> > the XID space isn't actually circular, because there are 3 special
> > values.  So if we land on one of those values we want to skip backward
> > by 3.  Here FirstNormalTransactionId doesn't represent itself, but
> > rather the number of special XIDs that exist.
> 
> 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.

--  Bruce Momjian  <bruce@momjian.us>        http://momjian.us EnterpriseDB
http://enterprisedb.com
 + It's impossible for everything to be true. +