Hannu Krosing wrote:
On Thu, 2008-10-16 at 18:52 +0300, Heikki Linnakangas wrote:
Also, I can't help thinking that this would be a lot simpler if we just
treated all subtransactions the same as top-level transactions. The only
problem with that is that there can be a lot of subtransactions, which
means that we'd need a large UnobservedXids array to handle the worst
case, but maybe it would still be acceptable?
I remember cases on this list where long transactions did run out of
subtransaction ids. To accommodate something approacing that we need an
array for storing (max_connections * 4G ) UnobservedXids instead of just
max_connections.
You can't have more than 4G (or 2G?) active subtransactions running in a
system, because you will simply run out of transaction ids and hit xid
wrap-around after that. So in the worst-case, you don't need space for
(max_connections * 4G) xids, just 4G. That's still a large number, of
course.
In situations like that, a bitmap, like clog, instead of an array, would
be more space efficient. But that's less efficient in the more common
case that there's few in-progress transactions, but some of them are
very old.
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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