PostgreSQL
Prev |  | Next

# LISTEN

## Name

LISTEN — Listen for a response on a notify condition

    LISTEN name

### Inputs

`name`

  Name of notify condition.

### Outputs

`LISTEN`

  Message returned upon successful completion of registration.

`NOTICE Async_Listen: We are already listening on `name``

  If this backend is already registered for that notify condition.

## Description

LISTEN registers the current Postgres backend as a listener on the notify condition `name`.

Whenever the command NOTIFY `name` is invoked, either by this backend or another one connected to the same database, all the backends currently listening on that notify condition are notified, and each will in turn notify its connected frontend application. See the discussion of NOTIFY for more information.

A backend can be unregistered for a given notify condition with the UNLISTEN command. Also, a backend's listen registrations are automatically cleared when the backend process exits.

The method a frontend application must use to detect notify events depends on which Postgres application programming interface it uses. With the basic libpq library, the application issues LISTEN as an ordinary SQL command, and then must periodically call the routine `PQnotifies` to find out whether any notify 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 library you are using for more details.

NOTIFY contains a more extensive discussion of the use of LISTEN and NOTIFY.

### Notes

`name` can be any string valid as a name; it need not correspond to the name of any actual table. If `notifyname` is enclosed in double-quotes, it need not even be a syntactically valid name, but can be any string up to 31 characters long.

In some previous releases of Postgres, `name` had to be enclosed in double-quotes when it did not correspond to any existing table name, even if syntactically valid as a name. That is no longer required.

## Usage

Configure and execute a listen/notify sequence from psql:

    LISTEN virtual;
    NOTIFY virtual;

    Asynchronous NOTIFY 'virtual' from backend with pid '8448' received.

## Compatibility

### SQL92

There is no LISTEN in SQL92.

Prev | Home | Next
INSERT | Up | LOAD