Re: xmin value from pg_stat_replication

Thanks for the feedva k,

It turns out we were not looking at master & skave. The slave has no replication slot. We are seeing lots of row versions not getting cleaned up, trying toinvestigate all the possible causes. Is there a way to determine which connections (on the master or the slave) are the cause of the row versions not getting cleaned up?

Thanks

On Wed, Jul 17, 2019, at 4:32 PM, Andres Freund wrote:
> Hi,
>
> On 2019-07-17 12:31:29 -0600, S. Bob wrote:
> > We have a master and a standby (Streaming replication).
> >
> > This query produces a value of approx 1.5million on the master and approx
> > 80million on the standby:
> >
> >
> > select slot_name, slot_type, database, xmin from pg_replication_slots order
> > by age(xmin);
>
> I'd suggest using age(xmin) in the output as well, not just in the ORDER
> BY, otherwise it's hard to make sense of the value.
>
> When you say a value of '1.5 million' on the master, how did you
> determine that? Because the above doesn't return anything for the
> master. The simplest way to determine that is probably:
>
> SELECT max(age(backend_xmin)) FROM pg_stat_activity;
>
>
> When you say '80 million', do you mean that pg_replication_slots.xmin
> was 80 million, or age(pg_replication_slots.xmin)? If it's the former,
> then that would indicate that the standby is *not* a problem, but that
> you have a long-running transaction or such on the primary.
>
> Basically, age(xmin) has to be large, not xmin itself.
>
>
> > We are running replication in async mode.
> >
> >
> > A couple of questions:
> >
> >
> > - Is it unusual for the master & standby to differ dramatically like this?
>
> Do you have very longrunning queries on the standby or primary? That's
> typically the reason why xmin is old.
>
>
> > - could the replication slot (i.e. the xmin on the standby) be preventing
> > vacuum from cleaning up row versions on the master?
>
> Yes.
>
> Greetings,
>
> Andres Freund
>