"Christian Plattner" <postgresql@sioux.ch> writes:
> From: "Tom Lane" <tgl@sss.pgh.pa.us>
>> How much later? clog is not kept forever.
> As far as I understand your code there is only one chance that information
> in clog gets lost:
> If XIDs are reused then ExtendCLOG will overwrite existing entries.
> Also, it seems to me that VACCUM has not effect on the clog.
You're quite mistaken. clog is truncated during vacuum, once we are
confident that there are no unvacuumed rows in the database with XIDs
older than a certain point. This is to keep clog space requirements
within reason.
regards, tom lane