Thread: Mitigating impact of long running read-only queries

Mitigating impact of long running read-only queries

From

Michael Lewis

Date:

14 December 2020, 17:41:11

https://www.postgresql.org/docs/current/sql-set-transaction.html

"The DEFERRABLE transaction property has no effect unless the transaction is also SERIALIZABLE and READ ONLY. When all three of these properties are selected for a transaction, the transaction may block when first acquiring its snapshot, after which it is able to run without the normal overhead of a SERIALIZABLE transaction and without any risk of contributing to or being canceled by a serialization failure. This mode is well suited for long-running reports or backups."

If I use these three properties together as below, would WAL buildup be prevented, or will the system still be causing a backup of the WAL for logical replication during the entire duration of my long reporting query?

BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE READ ONLY DEFERRABLE;

--long report query

commit;