PostgreSQL NOTIFY queue empty after LISTEN

Question

I am new at PostgreSQL LISTEN/NOTIFY features. After the LISTEN command,

LISTEN mychannel;

We can NOTIFY messages to the channel.

NOTIFY mychannel 'Hello';

This NOTIFY messages is saved somewhere in database. Where? Can I see messages using select query from a table?
If I NOTIFY 3 or 5 messages, these are saved on queue. I run the LISTEN command using a programming language, all messages handled and messages removed from queue. If my client crashed, all messages will be lost. How can I solve this problem?

Laurenz Albe · Accepted Answer · 2019-02-19 06:25:21Z

2

I am afraid that you misunderstand LISTEN. It is for asynchronous communication between active sessions. The messages are not stored anywhere.

If you want a persistent dara structure for communication between two parts of your application, you are probably looking for a queue.

Since queues are notoriously hard to implement in relational databases, I suggest that you use one of the existing third party implementations for PostgreSQL.

LISTEN / NOTIFY can be part of such a queue implementation to make it unnecessary for the recipients to poll the queue regularly.

edited Feb 19, 2019 at 6:25

answered Feb 19, 2019 at 6:20

Laurenz Albe

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