On 5/24/2010 9:30 AM, Heikki Linnakangas wrote:
On 22/05/10 16:35, Tom Lane wrote:
Josh Berkus<j...@agliodbs.com>  writes:
   From a discussion at dinner at pgcon, I wanted to send this to the list
for people to poke holes in it:

Somebody (I think Joe or Heikki) poked a big hole in this last night at
the Royal Oak.


 Although the scheme would get rid of the need to replace
old XIDs with FrozenXid, it does not get rid of the need to set hint
bits before you can truncate CLOG.

Hmm, we don't rely on setting hint bits to truncate CLOG anymore (http://archives.postgresql.org/pgsql-committers/2006-11/msg00026.php). It's the replacement of xids with FrozenXid that matters, the hint bits are really just hints.

Doesn't change the conclusion, though: you still need to replace XIDs with FrozenXids to truncate the clog. Conceivably we could keep around more than 2^32 transactions in clog with this scheme, but then you need a lot more space for the clog. But perhaps it would be better to do that than to launch anti-wraparound vacuums, or to refuse more updates in the extreme cases.

Correct. The problem actually are aborted transactions. Just because an XID is really old doesn't mean it was committed.


Anyone who trades liberty for security deserves neither
liberty nor security. -- Benjamin Franklin

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