Re: shared-memory based stats collector - v67

At Mon, 21 Mar 2022 14:30:17 -0700, Andres Freund <andres(at)anarazel(dot)de> wrote in
> > Right now we reset stats for replicas, even if we start from a shutdown
> > checkpoint. That seems pretty unnecessary with this patch:
> > - 0021-pgstat-wip-only-reset-pgstat-data-after-crash-re.patch
>
> Might raise this in another thread for higher visibility.

+ /*
+ * When starting with crash recovery, reset pgstat data - it might not be
+ * valid. Otherwise restore pgstat data. It's safe to do this here,
+ * because postmaster will not yet have started any other processes
+ *
+ * TODO: With a bit of extra work we could just start with a pgstat file
+ * associated with the checkpoint redo location we're starting from.
+ */
+ if (ControlFile->state == DB_SHUTDOWNED ||
+ ControlFile->state == DB_SHUTDOWNED_IN_RECOVERY)
+ pgstat_restore_stats();
+ else
+ pgstat_discard_stats();
+

Before there, InitWalRecovery changes the state to
DB_IN_ARCHIVE_RECOVERY if it was either DB_SHUTDOWNED or
DB_IN_PRODUCTION. So the stat seems like always discarded on standby.

In the first place, I'm not sure it is valid that a standby from a
cold backup takes over the stats from the primary.

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center