PostgreSQL ERROR: canceling statement due to conflict with recovery

No need to touch hot_standby_feedback. As others have mentioned, setting it to on can bloat master. Imagine opening transaction on a slave and not closing it.

Instead, set max_standby_archive_delay and max_standby_streaming_delay to some sane value:

# /etc/postgresql/10/main/postgresql.conf on a slave
max_standby_archive_delay = 900s
max_standby_streaming_delay = 900s

This way queries on slaves with a duration less than 900 seconds won't be cancelled. If your workload requires longer queries, just set these options to a higher value.

There's no need to start idle transactions on the master. In postgresql-9.1 the most direct way to solve this problem is by setting

hot_standby_feedback = on

This will make the master aware of long-running queries. From the docs:

The first option is to set the parameter hot_standby_feedback, which prevents VACUUM from removing recently-dead rows and so cleanup conflicts do not occur.

Why isn't this the default? This parameter was added after the initial implementation and it's the only way that a standby can affect a master.

As stated here about hot_standby_feedback = on :

Well, the disadvantage of it is that the standby can bloat the master, which might be surprising to some people, too

And here:

With what setting of max_standby_streaming_delay? I would rather default that to -1 than default hot_standby_feedback on. That way what you do on the standby only affects the standby

So I added

max_standby_streaming_delay = -1

And no more pg_dump error for us, nor master bloat :)

For AWS RDS instance, check http://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/Appendix.PostgreSQL.CommonDBATasks.html

Running queries on hot-standby server is somewhat tricky — it can fail, because during querying some needed rows might be updated or deleted on primary. As a primary does not know that a query is started on secondary it thinks it can clean up (vacuum) old versions of its rows. Then secondary has to replay this cleanup, and has to forcibly cancel all queries which can use these rows.

Longer queries will be canceled more often.

You can work around this by starting a repeatable read transaction on primary which does a dummy query and then sits idle while a real query is run on secondary. Its presence will prevent vacuuming of old row versions on primary.

More on this subject and other workarounds are explained in Hot Standby — Handling Query Conflicts section in documentation.