Re: reloption to prevent VACUUM from truncating empty pages at theend of relation

On Mon, Feb 25, 2019 at 08:47:28AM +0100, Julien Rouhaud wrote:
> Ah good point.  We could also use something like
> pg_relation_size('reloptions_test') /
> current_setting('block_size')::bigint but >0 should be enough for this
> test.

Also, shouldn't the relopt check happen in
should_attempt_truncation()?  It seems to me that if we use this
routine somewhere else then it should be controlled by the option.

At the same time, we also have REL_TRUNCATE_FRACTION and
REL_TRUNCATE_MINIMUM which could be made equally user-tunnable.
That's more difficult to control, still why don't we also consider
this part?

Another thing that seems worth thinking about is a system-level GUC,
and an option in the VACUUM command to control if truncation should
happen or not.  We have a lot of infrastructure to control such
options between vacuum and autovacuum, so it could be a waste to not
consider potential synergies.
--
Michael