Re: Reviewing freeze map code

On 2016-05-06 18:31:03 -0300, Alvaro Herrera wrote:
> I don't know what happens when the freeze_table_age threshold is
> reached.

We scan all non-frozen pages, whereas we earlier had to scan all pages.

That's really both the significant benefit, and the danger. Because if
we screw up the all-frozen bits in the visibilitymap, we'll be screwed
soon after.

> Do we scan the whole table when that happens?

No, there's atm no way to force a whole-table vacuum, besides manually
rm'ing the _vm fork.

> Another question on this feature is what happens with the table age
> (relfrozenxid, relminmxid) when the table is not wholly scanned by
> vacuum.

Basically we increase the horizons whenever scanning all pages that are
not known to be frozen (+ potentially some frozen ones due to the
skipping logic). Without that there'd really not be a point in the
freeze map feature, as we'd continue to have the expensive anti
wraparound vacuums.

Andres