Re: Flush some statistics within running transactions

> > No, 0003 also changes the flush mode for the database KIND. All the fields that
> > I mentioned are inherited from relations stats and are flushed only at transaction
> > boundaries (so they don't appear in pg_stat_database until the transaction
> > finishes). Does that make sense? (if the database kind is not switched to
> > flush any time then none would appear while the transaction is in progress, even
> > the ones inherited from relations stats).
> >
> > PFA v3, also taking care of Zsolt's comment (thanks!) done up-thread.
>
> While reading through 0001, I got to question on which properties
> and/or assumptions of a stats kind one has to rely on to decide to
> what flush_mode should be set. To put is simpler, why don't we just
> do a periodic pgstat_report_stat(false) call that would flush all the
> stats for all stats kinds based on the new timeout registered,
> expanding a bit the flush we currently do when idle in
> ProcessInterrupts()?

There are some important cases in which we would want to
distinguish between a "transaction boundary" flush vs an
"anytime" flush.

For example, xact_commit/rollback. I would want those
fields to be in sync with tuples_inserted/updated/deleted
to allow for accurate calculations like number of inserts
per commit, etc.

Another one would be n_mod_since_analyze, That should
only be updated after commit (or not after rollback). Otherwise,
it may throw autovanalyze threshold calculations way off. Same
for n_dead_tup and autovacuum.

> I am also not convinced that we have to be that aggressive with these
> extra flushes. The target is long-running analytical queries, that
> could take minutes or even hours. Using the same value as
> PGSTAT_IDLE_INTERVAL (10s),

PGSTAT_IDLE_INTERVAL is flushing an idle backend every 10 seconds
IIUC. So this value only applies when outside of a transaction.

> A 1s vs 10s report interval does not really matter for long analytical queries.

Sure, Bertrand mentioned early in the thread that the anytime flushes
could be made configurable. Perhaps that is a good idea where we can
default with something large like 10s intervals for anytime flushes, but allow
the user to configure a more frequent flushes ( although I would think
that 1 sec is the minimum we should allow ).

--
Sami Imseih
Amazon Web Services (AWS)