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NOTIFY(5)
NAME
  NOTIFY - generate a notification

SYNOPSIS
  NOTIFY name

  INPUTS

  notifyname
       Notify condition to be signaled.

  OUTPUTS

  NOTIFY
       Acknowledgement that notify command has executed.

  Notify events
       Events are delivered to listening frontends; whether and how each
       frontend application reacts depends on its programming.

DESCRIPTION
  The NOTIFY command sends a notify event to each frontend application that
  has previously executed LISTEN notifyname for the specified notify
  condition in the current database.

  The information passed to the frontend for a notify event includes the
  notify condition name and the notifying backend process's PID. It is up to
  the database designer to define the condition names that will be used in a
  given database and what each one means.

  Commonly, the notify condition 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 condition names to signal different
  sorts of changes to a single table.

  NOTIFY provides a simple form of signal or IPC (interprocess communication)
  mechanism for a collection of processes accessing the same PostgreSQL
  database.  Higher-level mechanisms can be built by using tables in the
  database to pass additional data (beyond a mere condition name) from
  notifier to listener(s).

  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 rule that
  is triggered by table updates.  In this way, notification happens
  automatically when the table is changed, and the application programmer
  can't 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 we would like all the
  commands within it to have had no effect, including NOTIFY. But it can be
  disconcerting if one is expecting the notify events to be delivered
  immediately. Secondly, if a listening backend receives a notify signal
  while it is within a transaction, the notify event will not be delivered to
  its connected frontend until just after the transaction is completed
  (either committed or aborted). Again, the reasoning is that if a notify
  were delivered within a transaction that was later aborted, one would want
  the notification to be undone somehow---but the backend cannot ``take
  back'' a notify once it has sent it to the frontend.	So notify 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.

  NOTIFY behaves like Unix signals in one important respect: if the same
  condition name is signaled multiple times in quick succession, recipients
  may get only one notify event for several executions of NOTIFY. So it is a
  bad idea to depend on the number of notifies received. Instead, use NOTIFY
  to wake up applications that need to pay attention to something, and use a
  database object (such as a sequence) to keep track of what happened or how
  many times it happened.

  It is common for a frontend that sends NOTIFY to be listening on the same
  notify name itself. In that case it will get back a notify event, just like
  all the other listening frontends. Depending on the application logic, this
  could result in useless work---for example, re-reading a database table to
  find the same updates that that frontend just wrote out. In PostgreSQL 6.4
  and later, it is possible to avoid such extra work by noticing whether the
  notifying backend process's PID (supplied in the notify event message) is
  the same as one's own backend's PID (available from libpq). When they are
  the same, the notify event is one's own work bouncing back, and can be
  ignored. (Despite what was said in the preceding paragraph, this is a safe
  technique.  PostgreSQL keeps self-notifies separate from notifies arriving
  from other backends, so you cannot miss an outside notify by ignoring your
  own notifies.)

  NOTES

  name can be any string valid as a name; it need not correspond to the name
  of any actual table. If name 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 PostgreSQL, 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.

  In PostgreSQL releases prior to 6.4, the backend PID delivered in a notify
  message was always the PID of the frontend's own backend. So it was not
  possible to distinguish one's own notifies from other clients' notifies in
  those earlier releases.

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 NOTIFY statement in SQL92.

Index for Section 5

Alphabetical listing for N

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