Re: Report oldest xmin source when autovacuum cannot remove tuples

Hello.

This may be just my personal preference, but I'd rather see
information about xid horizon retention exposed through a SQL-visible
interface than added to the VACUUM log.

VACUUM VERBOSE already reports the removable cutoff and how old it was
when the operation ended. That seems sufficient as a signal that the
xid horizon is being held back.

What I would want to investigate after seeing such a signal is not
necessarily what happened to be blocking that particular VACUUM
operation at some earlier point, but what is holding the xid horizon
back now. For that purpose, a view exposing the current xid/xmin
holders seems more useful and less misleading than adding a
best-effort blocker guess to the VACUUM log.

Also, VACUUM output has traditionally reported facts observed during
the operation. Since the blocker information is reconstructed
afterwards and is explicitly best-effort, it feels somewhat different
in nature from the rest of the information reported there.

Regards,

--
Kyotaro Horiguchi
NTT Open Source Software Center