235 lines
9.1 KiB
Plaintext
235 lines
9.1 KiB
Plaintext
<!--
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doc/src/sgml/ref/notify.sgml
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PostgreSQL documentation
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-->
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<refentry id="sql-notify">
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<indexterm zone="sql-notify">
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<primary>NOTIFY</primary>
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</indexterm>
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<refmeta>
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<refentrytitle>NOTIFY</refentrytitle>
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<manvolnum>7</manvolnum>
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<refmiscinfo>SQL - Language Statements</refmiscinfo>
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</refmeta>
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<refnamediv>
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<refname>NOTIFY</refname>
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<refpurpose>generate a notification</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<synopsis>
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NOTIFY <replaceable class="parameter">channel</replaceable> [ , <replaceable class="parameter">payload</replaceable> ]
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</synopsis>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para>
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The <command>NOTIFY</command> command sends a notification event together
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with an optional <quote>payload</quote> string to each client application that
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has previously executed
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<command>LISTEN <replaceable class="parameter">channel</replaceable></command>
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for the specified channel name in the current database.
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Notifications are visible to all users.
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</para>
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<para>
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<command>NOTIFY</command> provides a simple
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interprocess communication mechanism for a collection of processes
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accessing the same <productname>PostgreSQL</productname> database.
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A payload string can be sent along with the notification, and
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higher-level mechanisms for passing structured data can be built by using
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tables in the database to pass additional data from notifier to listener(s).
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</para>
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<para>
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The information passed to the client for a notification event includes the
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notification channel
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name, the notifying session's server process <acronym>PID</acronym>, and the
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payload string, which is an empty string if it has not been specified.
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</para>
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<para>
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It is up to the database designer to define the channel names that will
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be used in a given database and what each one means.
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Commonly, the channel name is the same as the name of some table in
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the database, and the notify event essentially means, <quote>I changed this table,
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take a look at it to see what's new</quote>. But no such association is enforced by
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the <command>NOTIFY</command> and <command>LISTEN</command> commands. For
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example, a database designer could use several different channel names
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to signal different sorts of changes to a single table. Alternatively,
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the payload string could be used to differentiate various cases.
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</para>
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<para>
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When <command>NOTIFY</command> is used to signal the occurrence of changes
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to a particular table, a useful programming technique is to put the
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<command>NOTIFY</command> in a statement trigger that is triggered by table updates.
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In this way, notification happens automatically when the table is changed,
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and the application programmer cannot accidentally forget to do it.
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</para>
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<para>
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<command>NOTIFY</command> interacts with SQL transactions in some important
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ways. Firstly, if a <command>NOTIFY</command> is executed inside a
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transaction, the notify events are not delivered until and unless the
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transaction is committed. This is appropriate, since if the transaction
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is aborted, all the commands within it have had no
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effect, including <command>NOTIFY</command>. But it can be disconcerting if one
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is expecting the notification events to be delivered immediately. Secondly, if
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a listening session receives a notification signal while it is within a transaction,
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the notification event will not be delivered to its connected client until just
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after the transaction is completed (either committed or aborted). Again, the
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reasoning is that if a notification were delivered within a transaction that was
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later aborted, one would want the notification to be undone somehow —
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but
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the server cannot <quote>take back</quote> a notification once it has sent it to the client.
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So notification events are only delivered between transactions. The upshot of this
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is that applications using <command>NOTIFY</command> for real-time signaling
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should try to keep their transactions short.
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</para>
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<para>
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If the same channel name is signaled multiple times with identical
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payload strings within the same transaction, only one instance of the
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notification event is delivered to listeners.
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On the other hand, notifications with distinct payload strings will
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always be delivered as distinct notifications. Similarly, notifications from
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different transactions will never get folded into one notification.
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Except for dropping later instances of duplicate notifications,
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<command>NOTIFY</command> guarantees that notifications from the same
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transaction get delivered in the order they were sent. It is also
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guaranteed that messages from different transactions are delivered in
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the order in which the transactions committed.
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</para>
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<para>
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It is common for a client that executes <command>NOTIFY</command>
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to be listening on the same notification channel itself. In that case
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it will get back a notification event, just like all the other
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listening sessions. Depending on the application logic, this could
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result in useless work, for example, reading a database table to
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find the same updates that that session just wrote out. It is
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possible to avoid such extra work by noticing whether the notifying
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session's server process <acronym>PID</acronym> (supplied in the
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notification event message) is the same as one's own session's
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<acronym>PID</acronym> (available from <application>libpq</application>). When they
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are the same, the notification event is one's own work bouncing
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back, and can be ignored.
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</para>
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</refsect1>
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<refsect1>
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<title>Parameters</title>
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<variablelist>
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<varlistentry>
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<term><replaceable class="parameter">channel</replaceable></term>
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<listitem>
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<para>
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Name of the notification channel to be signaled (any identifier).
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="parameter">payload</replaceable></term>
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<listitem>
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<para>
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The <quote>payload</quote> string to be communicated along with the
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notification. This must be specified as a simple string literal.
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In the default configuration it must be shorter than 8000 bytes.
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(If binary data or large amounts of information need to be communicated,
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it's best to put it in a database table and send the key of the record.)
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</refsect1>
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<refsect1>
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<title>Notes</title>
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<para>
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There is a queue that holds notifications that have been sent but not
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yet processed by all listening sessions. If this queue becomes full,
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transactions calling <command>NOTIFY</command> will fail at commit.
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The queue is quite large (8GB in a standard installation) and should be
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sufficiently sized for almost every use case. However, no cleanup can take
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place if a session executes <command>LISTEN</command> and then enters a
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transaction for a very long time. Once the queue is half full you will see
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warnings in the log file pointing you to the session that is preventing
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cleanup. In this case you should make sure that this session ends its
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current transaction so that cleanup can proceed.
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</para>
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<para>
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The function <function>pg_notification_queue_usage</function> returns the
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fraction of the queue that is currently occupied by pending notifications.
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See <xref linkend="functions-info"/> for more information.
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</para>
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<para>
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A transaction that has executed <command>NOTIFY</command> cannot be
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prepared for two-phase commit.
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</para>
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<refsect2>
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<title>pg_notify</title>
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<indexterm>
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<primary>pg_notify</primary>
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</indexterm>
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<para>
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To send a notification you can also use the function
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<literal><function>pg_notify</function>(<type>text</type>,
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<type>text</type>)</literal>. The function takes the channel name as the
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first argument and the payload as the second. The function is much easier
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to use than the <command>NOTIFY</command> command if you need to work with
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non-constant channel names and payloads.
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</para>
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</refsect2>
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</refsect1>
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<refsect1>
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<title>Examples</title>
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<para>
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Configure and execute a listen/notify sequence from
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<application>psql</application>:
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<programlisting>
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LISTEN virtual;
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NOTIFY virtual;
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Asynchronous notification "virtual" received from server process with PID 8448.
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NOTIFY virtual, 'This is the payload';
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Asynchronous notification "virtual" with payload "This is the payload" received from server process with PID 8448.
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LISTEN foo;
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SELECT pg_notify('fo' || 'o', 'pay' || 'load');
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Asynchronous notification "foo" with payload "payload" received from server process with PID 14728.
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</programlisting></para>
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</refsect1>
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<refsect1>
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<title>Compatibility</title>
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<para>
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There is no <command>NOTIFY</command> statement in the SQL
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standard.
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</para>
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</refsect1>
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<refsect1>
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<title>See Also</title>
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<simplelist type="inline">
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<member><xref linkend="sql-listen"/></member>
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<member><xref linkend="sql-unlisten"/></member>
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<member><xref linkend="guc-max-notify-queue-pages"/></member>
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</simplelist>
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</refsect1>
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</refentry>
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