NOTIFY(7)							   SQL Commands 							 NOTIFY(7)

NAME

       NOTIFY - generate a notification

SYNOPSIS

       NOTIFY name

DESCRIPTION

       The NOTIFY command sends a notification event to each client application that has previously executed LISTEN name for the specified notifi-
       cation name in the current database.

       NOTIFY provides a simple form of signal or 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 notification name)
       from notifier to listener(s).

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

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

       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 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.

       NOTIFY behaves like Unix signals in one important respect: if the same notification name is signaled multiple times  in	quick  succession,
       recipients might get only one notification event for several executions of NOTIFY. So it is a bad idea to depend on the number of notifica-
       tions 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 client that executes NOTIFY to be listening on the same notification name itself. In that case it will get back a noti-
       fication 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. (Despite
       what was said in the preceding paragraph, this is a safe technique.  PostgreSQL keeps self-notifications separate from notifications arriv-
       ing from other sessions, so you cannot miss an outside notification by ignoring your own notifications.)

PARAMETERS

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

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 NOTIFY statement in the SQL standard.

SEE ALSO

       LISTEN [listen(7)], UNLISTEN [unlisten(7)]

SQL - Language Statements					    2010-05-14								 NOTIFY(7)