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postgresqlnotifylisten

What happens with a NOTIFY when no session has issued LISTEN in PostgreSQL?

PostgreSQL has a good listen/notify system. Documentation says:

There is a queue that holds notifications that have been sent but not yet processed by all listening sessions. If this queue becomes full, transactions calling NOTIFY will fail at commit.

But I can't find out what happening with events in a specified channel that doesn't have listeners. Will notification queue overflow or will PG drop these events from queue?

Solution

  • It could be more clear in the manual, but there is definitive indication that the queue is cleaned as soon as no session is actively waiting for the notification. Per documentation:

    However, no cleanup can take place if a session executes LISTEN and then enters a transaction for a very long time. Once the queue is half full you will see warnings in the log file pointing you to the session that is preventing cleanup. In this case you should make sure that this session ends its current transaction so that cleanup can proceed.

    That means, if nobody is listening (no active session has issued a LISTEN command on the same channel), nothing keeps Postgres from cleaning the queue instantly.