Re: Hot standby and b-tree killed items

On Wed, Dec 24, 2008 at 5:26 PM, Simon Riggs <simon@2ndquadrant.com> wrote:
>

>
> The patch does go to some trouble to handle that case, as I'm sure
> you've seen. Are you saying that part of the patch is ineffective and
> should be removed, or?
>

Umm.. are you talking about the "wait" mechanism ? That's the only
thing I remember. Otherwise, prune record is pretty much same as any
vacuum cleanup record.

> Should/could there be a way to control frequency of prune operations? We
> could maintain cleanupxmin as a constant minimum distance from xmax, for
> example.
>

Well, there can be. But tuning any such thing might be difficult and
would have implications on the primary. I am not saying we can do
that, but we will need additional tests to see its impact.

> Are we saying we should take further measures, as I asked upthread? If
> it is a consensus that I take some action, then I will.
>

Again, I haven't seen how frequently queries may get canceled. Or if
the delay is set to a large value, how far behind standby may get
during replication, so I can't really comment. Have you done any tests
on a reasonable hardware and checked if moderately long read queries
can be run on standby without standby lagging behind a lot.

I would prefer to have a solution which can be smarter than canceling
all queries as soon as a cleanup record comes and timeout occurs. For
example, if the queries are being run on a completely different set of
tables where as the updates/deletes are happening on another set of
tables, there is no reason why those queries should be canceled. I
think it would be very common to have large history tables which may
receive long read-only queries, but no updates/deletes. Whereas other
frequently updated tables which receive very few queries on the
standby.

Thanks,
Pavan

-- 
Pavan Deolasee
EnterpriseDB     http://www.enterprisedb.com