Re: recovery from xid wraparound

On Tue, Oct 24, 2006 at 11:06:01AM +0100, Shane Wright wrote:
> If I was to abort this vacuum, given that all other tables are vacuumed
> (including system catalog tables), what's the worst case scenario? -
> given that more transactions are happening on the database

Only tables that havn't been vacuumed in the last billion transactions
are at risk. It's possible that if you've vacuumed that large table
recently by itself that all the data is actually safe, just the system
doesn't realise it.

Just make sure you've really covered *all* the system tables. If they
go you get really wierd results.

> If I understand correctly, it would be that some rows could disappear
> from this large unvacuumed table if their xid was too old - but no other
> consequence?

The VACUUM would make them reappear. To truly disappear they would
have to be 3 billion transactions old. That leaves the unique index
issue I mentioned.

> (fully aware that a db-wide vacuum is needed, but if it can [safely]
> wait for the weekend that would be preferable)

That's risk-management. For example, all the really old tuples are
possibly near the beginning of the table, thus this current vacuum will
have fixed them already. But to get a handle on that you need to
analyse your tuple turnover and usage ratio.

Have a nice day,
--
Martijn van Oosterhout <kleptog(at)svana(dot)org> http://svana.org/kleptog/
> From each according to his ability. To each according to his ability to litigate.