Re: Bug in autovacuum.c?

Alvaro Herrera wrote:
> Excerpts from Bruce Momjian's message of vie abr 01 16:50:29 -0300 2011:
> 
> > To do the right thing every computation that passes over the xid
> > wraparound bounary should subtract FirstNormalTransactionId, not just
> > those that fall in the boundry.  That would prevent the value from going
> > backward and still allow the mapping you liked;  it isn't worth it, but
> > that is the right answer.
> 
> This code is only concerned calculating an immediate the wrap horizon
> for the autovacuuming run that's about to take place.  If it's wrong in
> one or three counts doesn't mean much.  Consider what would happen if
> load was high and it would have taken 100 extra milliseconds to get to
> that bit: ReadNewTransactionId would have returned a value 3
> transactions later.  Furthermore, before this value is even used at all
> for vacuuming, there has to be a whole lot of inter-process signalling,
> a fork, and a new backend startup.
> 
> I think this should be left alone.  As you said, it isn't worth it.

Agreed it is not worth it but I think we should at least C comment
something.   I think at a minimum we should set it to
FirstNormalTransactionId.

I am not so concerned about this case but about other cases where we are
computing xid distances across the invalid range.

-- 
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + It's impossible for everything to be true. +