Re: New strategies for freezing, advancing relfrozenxid early

On Mon, Jan 16, 2023 at 8:13 PM Dilip Kumar <dilipbalaut@gmail.com> wrote:
> I think that it makes sense to keep 'vacuum_freeze_strategy_threshold'
> strictly for freezing.  But the point is that the eager scanning
> strategy is driven by table freezing needs of the table (tableagefrac)
> that make sense, but if we have selected the eager freezing based on
> the table age and its freezing need then why don't we force the eager
> freezing as well if we have selected eager scanning, after all the
> eager scanning is selected for satisfying the freezing need.

Don't think of eager scanning as the new name for aggressive mode --
it's a fairly different concept, because we care about costs now.
Eager scanning can be chosen just because it's very cheap relative to
the alternative of lazy scanning, even when relfrozenxid is still very
recent. (This kind of behavior isn't really new [1], but the exact
implementation from the patch is new.)

Tables such as pgbench_branches and pgbench_tellers will reliably use
eager scanning strategy, no matter how any GUC has been set -- just
because the added cost is always zero (relative to lazy scanning). It
really doesn't matter how far along tableagefrac here, ever. These
same tables will never use eager freezing strategy, unless the
vacuum_freeze_strategy_threshold GUC is misconfigured. (This is
another example of how scanning strategy and freezing strategy may
differ for the same table.)

You do have a good point, though. I think that I know what you mean.
Note that antiwraparound autovacuums (or VACUUMs of tables very near
to that point) *will* always use both the eager freezing strategy and
the eager scanning strategy -- which is probably close to what you
meant.

The important point is that there can be more than one reason to
prefer one strategy to another -- and the reasons can be rather
different. Occasionally it'll be a combination of two factors together
that push things in favor of one strategy over the other -- even
though either factor on its own would not have resulted in the same
choice.

[1] https://wiki.postgresql.org/wiki/Freezing/skipping_strategies_patch:_motivating_examples#Constantly_updated_tables_.28usually_smaller_tables.29
-- 
Peter Geoghegan