Bruce Momjian wrote:
> > > I am not so concerned about this case but about other cases where we are
> > > computing xid distances across the invalid range.
> >
> > Such as?
>
> Not sure. I have not had time to research this, but there might be
> cases where this backward movement matters --- remember our XIDs are
> valid only within about a 2 billion range, and we do less/greater
> comparisons in that range (using a macro). That macro is not going to
> cover over backward xid movement.
OK, I am done training for the day, and found this macro:/* advance a transaction ID variable, handling wraparound
correctly*/#define TransactionIdAdvance(dest) \ do { \ (dest)++; \ if ((dest) <
FirstNormalTransactionId)\ (dest) = FirstNormalTransactionId; \ } while(0)
which seems OK, but we the -= all over varsup.c
/* * We'll refuse to continue assigning XIDs in interactive mode once we get * within 1M transactions of data
loss. This leaves lots of room for the * DBA to fool around fixing things in a standalone backend, while not *
beingsignificant compared to total XID space. (Note that since * vacuuming requires one transaction per table
cleaned,we had better be * sure there's lots of XIDs left...) */ xidStopLimit = xidWrapLimit - 1000000; if
(xidStopLimit< FirstNormalTransactionId) xidStopLimit -= FirstNormalTransactionId;
Now I am not sure where to add a C comment. :-(
-- Bruce Momjian <bruce@momjian.us> http://momjian.us EnterpriseDB
http://enterprisedb.com
+ It's impossible for everything to be true. +