man > NOTIFY(7)

NOTIFY(7)                          PostgreSQL 16.14 Documentation                          NOTIFY(7)

NAME
       NOTIFY - generate a notification

SYNOPSIS
       NOTIFY channel [ , payload ]

DESCRIPTION
       The NOTIFY command sends a notification event together with an optional “payload” string to
       each client application that has previously executed LISTEN channel for the specified channel
       name in the current database. Notifications are visible to all users.

       NOTIFY provides a simple interprocess communication mechanism for a collection of processes
       accessing the same PostgreSQL database. A payload string can be sent along with the
       notification, and higher-level mechanisms for passing structured data can be built by using
       tables in the database to pass additional data from notifier to listener(s).

       The information passed to the client for a notification event includes the notification
       channel name, the notifying session's server process PID, and the payload string, which is an
       empty string if it has not been specified.

       It is up to the database designer to define the channel names that will be used in a given
       database and what each one means. Commonly, the channel name is the same as the name of some
       table in the database, and the notify event essentially means, “I changed this table, take a
       look at it to see what's new”. But no such association is enforced by the NOTIFY and LISTEN
       commands. For example, a database designer could use several different channel names to
       signal different sorts of changes to a single table. Alternatively, the payload string could
       be used to differentiate various cases.

       When NOTIFY is used to signal the occurrence of changes to a particular table, a useful
       programming technique is to put the NOTIFY in a statement trigger that is triggered by table
       updates. In this way, notification happens automatically when the table is changed, and the
       application programmer cannot accidentally forget to do it.

       NOTIFY interacts with SQL transactions in some important ways. Firstly, if a NOTIFY is
       executed inside a transaction, the notify events are not delivered until and unless the
       transaction is committed. This is appropriate, since if the transaction is aborted, all the
       commands within it have had no effect, including NOTIFY. But it can be disconcerting if one
       is expecting the notification events to be delivered immediately. Secondly, if a listening
       session receives a notification signal while it is within a transaction, the notification
       event will not be delivered to its connected client until just after the transaction is
       completed (either committed or aborted). Again, the reasoning is that if a notification were
       delivered within a transaction that was later aborted, one would want the notification to be
       undone somehow — but the server cannot “take back” a notification once it has sent it to the
       client. So notification events are only delivered between transactions. The upshot of this is
       that applications using NOTIFY for real-time signaling should try to keep their transactions
       short.

       If the same channel name is signaled multiple times with identical payload strings within the
       same transaction, only one instance of the notification event is delivered to listeners. On
       the other hand, notifications with distinct payload strings will always be delivered as
       distinct notifications. Similarly, notifications from different transactions will never get
       folded into one notification. Except for dropping later instances of duplicate notifications,
       NOTIFY guarantees that notifications from the same transaction get delivered in the order
       they were sent. It is also guaranteed that messages from different transactions are delivered
       in the order in which the transactions committed.

       It is common for a client that executes NOTIFY to be listening on the same notification
       channel itself. In that case it will get back a notification event, just like all the other
       listening sessions. Depending on the application logic, this could result in useless work,
       for example, reading a database table to find the same updates that that session just wrote
       out. It is possible to avoid such extra work by noticing whether the notifying session's
       server process PID (supplied in the notification event message) is the same as one's own
       session's PID (available from libpq). When they are the same, the notification event is one's
       own work bouncing back, and can be ignored.

PARAMETERS
       channel
           Name of the notification channel to be signaled (any identifier).

       payload
           The “payload” string to be communicated along with the notification. This must be
           specified as a simple string literal. In the default configuration it must be shorter
           than 8000 bytes. (If binary data or large amounts of information need to be communicated,
           it's best to put it in a database table and send the key of the record.)

NOTES
       There is a queue that holds notifications that have been sent but not yet processed by all
       listening sessions. If this queue becomes full, transactions calling NOTIFY will fail at
       commit. The queue is quite large (8GB in a standard installation) and should be sufficiently
       sized for almost every use case. However, no cleanup can take place if a session executes
       LISTEN and then enters a transaction for a very long time. Once the queue is half full you
       will see warnings in the log file pointing you to the session that is preventing cleanup. In
       this case you should make sure that this session ends its current transaction so that cleanup
       can proceed.

       The function pg_notification_queue_usage returns the fraction of the queue that is currently
       occupied by pending notifications. See Section 9.26 for more information.

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

   pg_notify
       To send a notification you can also use the function pg_notify(text, text). The function
       takes the channel name as the first argument and the payload as the second. The function is
       much easier to use than the NOTIFY command if you need to work with non-constant channel
       names and payloads.

EXAMPLES
       Configure and execute a listen/notify sequence from psql:

           LISTEN virtual;
           NOTIFY virtual;
           Asynchronous notification "virtual" received from server process with PID 8448.
           NOTIFY virtual, 'This is the payload';
           Asynchronous notification "virtual" with payload "This is the payload" received from server process with PID 8448.

           LISTEN foo;
           SELECT pg_notify('fo' || 'o', 'pay' || 'load');
           Asynchronous notification "foo" with payload "payload" received from server process with PID 14728.

COMPATIBILITY
       There is no NOTIFY statement in the SQL standard.

SEE ALSO
       LISTEN(7), UNLISTEN(7)

PostgreSQL 16.14                                2026                                       NOTIFY(7)