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

On 1/16/23 07:11, Laurenz Albe wrote:
> 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".

So OP should VACUUM FREEZE the table.

--
Born in Arizona, moved to Babylonia.