Re: Why is a Read-only Table Gets Autovacuumed "to prevent wraparound"

On Mon, 2023-01-16 at 07:48 -0500, Fred Habash wrote:
> This is a puzzle I have not been able to crack yet. 
> 
> We have a single-page table with 28 rows that is purely read-only. There isn't a way in postgres to make a table RO, but I say this with confidence because pg_stat_user_tables has always showed 0
> updates/deletes/inserts. 
> 
> Furthermore, the schema app developers know, for certain, this table does not get changed at all. 
> 
> We installed scripts that run every few minutes that do a 'select *' and over a period of days, we have not seen a change.
> 
> We disabled autovacuum on this table '{autovacuum_enabled=false}'. But, despite the fact that this table is read-only (by design) and autovac id is disabled, it got autovac'd twice in less than 10
> days and on both occasions, pg_stat_activity showed the worker with 'to prevent wraparound'. This explains why autovac did not honor the disabled status. 
> 
> But why is this table autovac'd at all? 

For every table PostgreSQL stores the oldest transaction ID in an unfrozen tuple
in "pg_class.relfrozenxid".  Once that is more than "autovacuum_freeze_max_age",
the table gets autovacuumed.  If the table is already all-frozen, that is a short
operation and will just advance "pg_class.relfrozenxid".

Yours,
Laurenz Albe