Re: Why are data files getting modified?

I see, but then why is the autovacuum getting triggered after the select
command?

On Thu, Feb 10, 2022, 12:14 AM Laurenz Albe <laurenz(dot)albe(at)cybertec(dot)at>
wrote:

> On Wed, 2022-02-09 at 17:29 +0900, Abhishek Bhola wrote:
> > I have a table with monthly partitions, starting from 2011.
> > Each partition has about 2-3 billion rows and is about 40TB of data in
> total.
> > I am running a select query to \copy the data into csv files for each
> month.
> >
> > I see there is an AUTOVACUUM job started on all the partitions that have
> been
> > read (on which select has run). I am assuming the reason why Postgres is
> > autovacuuming these large partitions is this:
> >
> > db=# SELECT c.relname as table_name, c.relkind as type,
> age(c.relfrozenxid) as age,
> > c.relfrozenxid FROM pg_class AS c WHERE age(c.relfrozenxid) <>
> 2147483647 ORDER BY 3 DESC LIMIT 20;
> > table_name | type | age | relfrozenxid
> > ---------------------------+------+-----------+--------------
> > hist_omx_orderaudit201506 | r | 201463508 | 319891
> > hist_omx_orderaudit201509 | r | 201418497 | 364902
> > hist_omx_orderaudit201205 | r | 201142042 | 641357
> >
> > autovacuum_freeze_max_age
> > ---------------------------
> > 200000000
> > (1 row)
> >
> > Am I wrong in my understanding why this autovacuum is getting triggered?
> >
> > However, my main question is that why do the data files in the
> tablespace, corresponding
> > to these partition tables (that have been read or selected recently), or
> the ones
> > autovacuumed recently have a latest timestamp?
> >
> > There was no insert or update on these partitions, so why do I see an
> updated timestamp?
> > Does autovacuum change something on the datafiles?
>
> Yes, autovacuum will freeze old rows in the table, which modifies the data
> files.
>
> To keep this to a minimum, you can run VACUUM (FREEZE) on a partition as
> soon
> as you know that its data won't get modified any more.
> Still, I wouldn't be sure that the file modification timestamp won't change
> any more after that. But that should be pretty irrelevant; at any rate, no
> substantial data modifications will occur after that.
>
> Yours,
> Laurenz Albe
> --
> Cybertec | https://www.cybertec-postgresql.com
>
>

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