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
