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 notification 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 data-
       base 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 transac-
       tion  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 notifications 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 hap-
       pened.

       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 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. (Despite what was said in the preceding para-
       graph, this is a safe technique.  PostgreSQL keeps self-notifications separate from  notifications  arriving  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                                  2011-09-22                                                  NOTIFY(7)