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<REFENTRY ID="SQL-NOTIFY">
<REFMETA>
<REFENTRYTITLE>
NOTIFY
</REFENTRYTITLE>
<REFMISCINFO>SQL - Language Statements</REFMISCINFO>
</REFMETA>
<REFNAMEDIV>
<REFNAME>
NOTIFY
</REFNAME>
<REFPURPOSE>
Signals all frontends and backends listening on a class
</REFPURPOSE>
<REFSYNOPSISDIV>
<REFSYNOPSISDIVINFO>
<DATE>1998-09-24</DATE>
</REFSYNOPSISDIVINFO>
<SYNOPSIS>
<REPLACEABLE CLASS="PARAMETER">
</REPLACEABLE>
NOTIFY <REPLACEABLE CLASS="PARAMETER">classname</REPLACEABLE>
</SYNOPSIS>
<REFSECT2 ID="R2-SQL-NOTIFY-1">
<REFSECT2INFO>
<DATE>1998-09-24</DATE>
</REFSECT2INFO>
<TITLE>
Inputs
</TITLE>
<PARA>
<VARIABLELIST>
<VARLISTENTRY>
<TERM>
<REPLACEABLE CLASS="PARAMETER">classname</REPLACEABLE>
</TERM>
<LISTITEM>
<PARA>
Table or arbitrary relation class used for notification.
</VARIABLELIST>
</REFSECT2>
<REFSECT2 ID="R2-SQL-NOTIFY-2">
<REFSECT2INFO>
<DATE>1998-09-24</DATE>
</REFSECT2INFO>
<TITLE>
Outputs
</TITLE>
<PARA>
<VARIABLELIST>
<VARLISTENTRY>
<TERM>
<replaceable>status</replaceable>
</TERM>
<LISTITEM>
<PARA>
<VARIABLELIST>
<VARLISTENTRY>
<TERM>
NOTIFY
</TERM>
<LISTITEM>
<PARA>
Notification message from backend.
</VARIABLELIST>
</VARIABLELIST>
</REFSECT2>
</REFSYNOPSISDIV>
<REFSECT1 ID="R1-SQL-NOTIFY-1">
<REFSECT1INFO>
<DATE>1998-09-24</DATE>
</REFSECT1INFO>
<TITLE>
Description
</TITLE>
<PARA>
<command>NOTIFY</command> is used to awaken all sessions which have
previously executed
<command>LISTEN <replaceable class="parameter">classname</replaceable></command>.
This can be used either within an instance-level rule
as part of the action body or from a normal query.
<para>
When used from within a normal query,
this can be thought of as interprocess communication (IPC). When
used from within a rule, this can be thought of as an alert mechanism.
<para>
Note that the mere fact that a <command>NOTIFY</command> has been
executed does not imply anything in particular about the state
of the class (e.g., that it has been updated),
nor does the notification protocol transmit any useful information
other than the class name.
Therefore, all <command>NOTIFY</command> does is indicate that some backend
wishes its peers to
examine <replaceable class="parameter">classname</replaceable>
in some application-specific way.
<para>
In fact, <replaceable class="parameter">classname</replaceable>
need not be the name of an SQL class at all.
It is best thought of as a condition name
that the application programmer selects.
<para>
This event notification is performed through the libpq protocol
and frontend application interface. The application program
must call the routine <function>PQnotifies</function>
in order to find out the name of the class to which a given
notification corresponds.
If this code is not included in the application,
the event notification will be
queued and never be processed.
<REFSECT2 ID="R2-SQL-NOTIFY-3">
<REFSECT2INFO>
<DATE>1998-09-24</DATE>
</REFSECT2INFO>
<TITLE>
Notes
</TITLE>
<PARA>
</PARA>
</REFSECT2>
<REFSECT1 ID="R1-SQL-NOTIFY-2">
<TITLE>
Usage
</TITLE>
<PARA>
</PARA>
<ProgramListing>
-- Configure and execute a listen/notify sequence
-- from psql
CREATE TABLE t (i int4);
<computeroutput>
LISTEN t;
</computeroutput>
NOTIFY t;
<computeroutput>
NOTIFY
ASYNC NOTIFY of 't' from backend pid '10949' received
</computeroutput>
</ProgramListing>
</REFSECT1>
<REFSECT1 ID="R1-SQL-NOTIFY-3">
<TITLE>
Compatibility
</TITLE>
<PARA>
<REFSECT2 ID="R2-SQL-NOTIFY-4">
<REFSECT2INFO>
<DATE>1998-09-24</DATE>
</REFSECT2INFO>
<TITLE>
SQL92
</TITLE>
<PARA>
There is no <command>NOTIFY</command> statement in <acronym>SQL92</acronym>.
</REFENTRY>
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