Reporting from Standby

Good afternoon.

PG 12.12, PGDG ubuntu 18.04 LTS build.

We have a primary with a couple of physical/streaming replicas. One
intended for quick lookups and another intended for long-running reports
(aka "the reporting replica"). There's also a DR replica in another region
but we don't touch that for obvious reasons. All streaming replicas
currently make use of replication slots on the primary as well. We have WAL
archiving to cloud storage and restore/recovery handled via pgBackrest.

I'm running into the age-old dilemma on the reporting replica where queries
are being killed with the "canceling statement due to conflict with
recovery" error. We have hot_standby_feedback=on and both
max_standby_streaming_delay and max_standby_archive_delay are set to 5
minutes (300s). I'm fine increasing those delay parameters to 15 minutes or
more. Like I said this replica is dedicated to long-running reports like
month-end reports that you'd see today on the first of the month.

I searched around and I see some suggestions like disabling
hot_standby_feedback on replicas like this to minimize impact on the
primary, in addition to increasing the delays, or even disabling the delays
by setting them to -1. With replication slots in use, I want to make sure
that WAL retention doesn't fill up the WAL volume, would it make sense to
not use a slot for this replica (and/or not use streaming replication)?

I'm interested to know what people suggest in this space.

--
Don Seiler
www.seiler.us