On Tue, Jan 17, 2023 at 10:05 AM Peter Geoghegan <p...@bowt.ie> wrote: > > On Mon, Jan 16, 2023 at 8:13 PM Dilip Kumar <dilipbal...@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.)
Yes, I agree with that. Thanks for explaining in detail. > 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. Right > 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. Yes, that makes sense to me. -- Regards, Dilip Kumar EnterpriseDB: http://www.enterprisedb.com