On Wed, Apr 5, 2017 at 3:37 AM, Александр Киселев <flykens@xxxxxxxxx> wrote: > Hello! > > My name is Alexander. I am an administarator PostgreSQL. > I am studying PostgreSQL's 9.6 documentation. > I am interested in parameter hot_standby_feedback in postgresql.conf > Can you explain more detail than in documentation for what this parameter is > used? > In what situations the parameter can show itself? > How to test influence of the parameter on database's behavour. It's possible for a long running query on a slave to live past the time the tuples it needs exist on the master. This is because the transaction the read only query is running under cannot "lock" those tuples against vacuum on the master. The master gets vacuumed, those tuples go away, the streaming / wal log replication gets relayed to the read only slave, it loses the tuples and your query fails because it can no longer access the snapshot of those tuples it started under. With feedback, the slave can tell the master "I'm using these tuples, please don't delete them just yet" and the master will skip vacuuming them up. It's darned useful for long running queries on a slave. -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
