Michael Lewis <mlewis@xxxxxxxxxxx> wrote on 06/23/2020 04:44:51 PM: > Long running transactions are the common one that I see. You might > be dealing with replication slots or prepared transactions. > Basically, if some process might see that "old truth", then it can't > be vacuumed away yet. Thanks, those links you provided were informative. Our application doesn't use prepared transactions, so that one is easy to eliminate. Our database does use replication, but there's only one replication slot and the xmin and catalog_xmin columns are blank. I presume the only replication slot that exists is the one that the other replica is in fact using. I *think* this means replication isn't the issue, but it's hard to say since the xmin column is blank (and all the reading I've done doesn't even mention that the xmin on replication slots can be null). That brings us to transactions. I looked for any long-running idle transaction (transactions that have been idle for more than 15 or 5 minutes), but found none. I tried: SELECT pid, datname, usename, state, backend_xmin FROM pg_stat_activity WHERE backend_xmin IS NOT NULL ORDER BY age(backend_xmin) DESC; But the backend_xmin for all of the rows returned is exactly the same, and that xmin is greater than the oldest xmin reported in the autovacuum logs. It does seem odd that the backend_xmin value isn't changing though. Is that normal? So, for replication slots, I'm seeing a null xmin value, which the articles do not comment on how that should be interpreted. And for transactions, all of the transaction xmins are the same, which also seems odd and not what the articles suggested. I know the transactions themselves are coming and going because I can see the pids changing, but the xmins are always the same. Strange. Having to check the logs to see what the oldest xmin is painful, and requires that a vacuum or autovacuum has been executed. Is there another way to check what the oldest xmin is on the dead tuples of a table? Regards, Jim Hurne
