Re: Aggressive freezing in lazy-vacuum

Tom Lane wrote:
> ITAGAKI Takahiro <itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp> writes:
>> This is a stand-alone patch for aggressive freezing. I'll propose
>> to use OldestXmin instead of FreezeLimit as the freeze threshold
>> in the circumstances below:
>
> I think it's a really bad idea to freeze that aggressively under any
> circumstances except being told to (ie, VACUUM FREEZE). When you
> freeze, you lose history information that might be needed later --- for
> forensic purposes if nothing else. You need to show a fairly amazing
> performance gain to justify that, and I don't think you can.

There could be a GUC vacuum_freeze_limit, and the actual FreezeLimit
would be calculated as
GetOldestXmin() - vacuum_freeze_limit

The default for vacuum_freeze_limit would be MaxTransactionId/2, just
as it is now.

greetings, Florian Pflug