Reporting from Standby

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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

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