Re: Visibility map and freezing

What's about add second bit which mark frozen page (all tuples have freeze XID)? 
It should avoid full scan, but extend size of map.    Zdenek



Heikki Linnakangas napsal(a):
> The way VACUUM works with the visibility map is that if any pages are 
> skipped, relfrozenxid can't be updated. That means that plain VACUUM 
> won't advance relfrozenxid, and doesn't protect from XID wraparound.
> 
> We discussed this in the context of autovacuum before, and we have that 
> covered now. Autovacuum will launch a full-scanning vacuum that advances 
> relfrozenxid, when autovacuum_freeze_max_age is reached, and partial 
> vacuums otherwise.
> 
> Autovacuum will launch anti-wraparound vacuum even if it's otherwise 
> disabled. Which is good, but it'll be an unpleasant surprise for someone 
> who performs a simple manual database-wide "VACUUM", for example, every 
> night from a cron job. You could run VACUUM FREEZE, say monthly, to 
> force a full-scanning vacuum, but that's unnecessarily aggressive, and 
> you need to know about the issue to set that up in the first place.
> 
> I think we need a threshold similar to autovacuum_freeze_max_age for 
> manual vacuums as well: vacuum_freeze_max_age. If you run VACUUM, and 
> relfrozenxid is older than vacuum_freeze_max_age, the visibility map is 
> ignored and all pages are scanned.
> 
> This ensures that you don't run into forced anti-wraparound autovacuums 
> if you do your VACUUMs manually.
>