LISTEN
Description
LISTEN registers the current session as a listener on the notification channel namedchannel. If the current session is already registered as a listener for this notification channel, nothing is done.
Whenever the commandNOTIFYchannel is invoked, either by this session or another one connected to the same database, all the sessions currently listening on that notification channel are notified, and each will in turn notify its connected client application.
A session can be unregistered for a given notification channel with theUNLISTEN command. A session's listen registrations are automatically cleared when the session ends.
The method a client application must use to detect notification events depends on whichPostgreSQL application programming interface it uses. With thelibpq library, the application issuesLISTEN as an ordinary SQL command, and then must periodically call the functionPQnotifies to find out whether any notification events have been received. Other interfaces such aslibpgtcl provide higher-level methods for handling notify events; indeed, withlibpgtcl the application programmer should not even issueLISTEN orUNLISTEN directly. See the documentation for the interface you are using for more details.
NOTIFY contains a more extensive discussion of the use ofLISTEN andNOTIFY.
Notes
LISTEN takes effect at transaction commit. IfLISTEN orUNLISTEN is executed within a transaction that later rolls back, the set of notification channels being listened to is unchanged.
A transaction that has executedLISTEN cannot be prepared for two-phase commit.