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 43-20. pg_listener Columns
Name
Type
References
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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