PostgreSQL 8.4.2 Documentation
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# 44.24. `pg_listener`

The catalog `pg_listener` supports the LISTEN and NOTIFY commands. A listener creates an entry in `pg_listener` for each notification name it is listening for. A notifier scans `pg_listener` and updates each matching entry to show that a notification has occurred. The notifier also sends a signal (using the PID recorded in the table) to awaken the listener from sleep.

Table 44-24. `pg_listener` Columns

Name | Type | Description
--- | --- | ---
`relname` | `name` | Notify condition name. (The name need not match any actual relation in the database; the name `relname` is historical.)
`listenerpid` | `int4` | PID of the server process that created this entry
`notification` | `int4` | Zero if no event is pending for this listener. If an event is pending, the PID of the server process that sent the notification

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