Re: Visibility map and freezing

On Mon, 2008-12-22 at 21:24 +0200, Heikki Linnakangas wrote:
> Introduce new vacuum_freeze_max_age setting. Manual VACUUM will scan the
> whole table and advance relfrozenxid, if relfrozenxid is older than
> vacuum_freeze_max_age.
>

It's confusing to have two GUCs named vacuum_freeze_min_age and
vacuum_freeze_max_age with incompatible definitions. The former is the
minimum transaction age of a tuple found during the scan of a table,
while the latter is the maximum transaction age of the relfrozenxid of
the table.

> If you set vacuum_freeze_max_age to 0, the visibility map is not used to
> skip pages, so you'll get the pre-8.4 old behavior.

Seems like a strange way to turn off visibility maps, and the meaning
doesn't seem to fit with either vacuum_freeze_min_age or
autovacuum_freeze_max_age.

The proposal itself makes sense, but I think we need to decide on some
better names. Right now the meanings of autovacuum_freeze_max_age and
vacuum_freeze_min_age are incompatible, so we're not in a good position,
but there has to be something less confusing.

For one thing, there isn't even a common definition of "min" or "max"
between them. They both trigger an event (freezing or vacuuming) when
something (tuple xmin or relfrozenxid) exceeds some number. Why is one
called a min and the other a max?

Regards,
Jeff Davis