LISTEN

LISTEN

7

SQL - Language Statements

LISTEN

listen for a notification

LISTEN

channel

Description
===========

`LISTEN` registers the current session as a listener on the notification
channel named channel. If the current session is already registered as a
listener for this notification channel, nothing is done.

Whenever the command `NOTIFY channel` 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 the
`UNLISTEN` 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 which PostgreSQL application programming interface it uses.
With the libpq library, the application issues `LISTEN` as an ordinary
SQL command, and then must periodically call the function `PQnotifies`
to find out whether any notification events have been received. Other
interfaces such as libpgtcl provide higher-level methods for handling
notify events; indeed, with libpgtcl the application programmer should
not even issue `LISTEN` or `UNLISTEN` directly. See the documentation
for the interface you are using for more details.

Parameters
==========

channel

:   Name of a notification channel (any identifier).

Notes
=====

`LISTEN` takes effect at transaction commit. If `LISTEN` or `UNLISTEN`
is executed within a transaction that later rolls back, the set of
notification channels being listened to is unchanged.

A transaction that has executed `LISTEN` cannot be prepared for
two-phase commit.

There is a race condition when first setting up a listening session: if
concurrently-committing transactions are sending notify events, exactly
which of those will the newly listening session receive? The answer is
that the session will receive all events committed after an instant
during the transaction\'s commit step. But that is slightly later than
any database state that the transaction could have observed in queries.
This leads to the following rule for using `LISTEN`: first execute (and
commit!) that command, then in a new transaction inspect the database
state as needed by the application logic, then rely on notifications to
find out about subsequent changes to the database state. The first few
received notifications might refer to updates already observed in the
initial database inspection, but this is usually harmless.

[???](#sql-notify) contains a more extensive discussion of the use of
`LISTEN` and `NOTIFY`.

Examples
========

Configure and execute a listen/notify sequence from psql:

    LISTEN virtual;
    NOTIFY virtual;
    Asynchronous notification "virtual" received from server process with PID 8448.

Compatibility
=============

There is no `LISTEN` statement in the SQL standard.

See Also
========
