database/postgres
1
0
Fork 0
mirror of https://github.com/postgres/postgres.git synced 2025-12-16 16:42:29 +03:00
Code Activity

Editing of more reference pages.

Browse Source
This commit is contained in:
Peter Eisentraut
2003-04-26 23:56:51 +00:00
parent 3a496c8af0
commit 20aae3047f
21 changed files with 1709 additions and 2737 deletions
Download Patch File Download Diff File Expand all files Collapse all files

257
doc/src/sgml/ref/notify.sgml
View File

@@ -1,5 +1,5 @@
<!--
$Header: /cvsroot/pgsql/doc/src/sgml/ref/notify.sgml,v 1.19 2002/09/21 18:32:54 petere Exp $
$Header: /cvsroot/pgsql/doc/src/sgml/ref/notify.sgml,v 1.20 2003/04/26 23:56:51 petere Exp $
PostgreSQL documentation
-->
@@ -8,113 +8,53 @@ PostgreSQL documentation
<refentrytitle id="sql-notify-title">NOTIFY</refentrytitle>
<refmiscinfo>SQL - Language Statements</refmiscinfo>
</refmeta>
<refnamediv>
<refname>
NOTIFY
</refname>
<refpurpose>
generate a notification
</refpurpose>
<refname>NOTIFY</refname>
<refpurpose>generate a notification</refpurpose>
</refnamediv>
<refsynopsisdiv>
<refsynopsisdivinfo>
<date>1999-07-20</date>
</refsynopsisdivinfo>
<synopsis>
<synopsis>
NOTIFY <replaceable class="PARAMETER">name</replaceable>
</synopsis>
<refsect2 id="R2-SQL-NOTIFY-1">
<refsect2info>
<date>1998-10-07</date>
</refsect2info>
<title>
Inputs
</title>
<para>
<variablelist>
<varlistentry>
<term><replaceable class="PARAMETER">notifyname</replaceable></term>
<listitem>
<para>
Notify condition to be signaled.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</refsect2>
<refsect2 id="R2-SQL-NOTIFY-2">
<refsect2info>
<date>1998-10-07</date>
</refsect2info>
<title>
Outputs
</title>
<para>
<variablelist>
<varlistentry>
<term><computeroutput>
NOTIFY
</computeroutput></term>
<listitem>
<para>
Acknowledgement that notify command has executed.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable>Notify events</replaceable></term>
<listitem>
<para>
Events are delivered to listening frontends; whether and how each frontend
application reacts depends on its programming.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</refsect2>
</synopsis>
</refsynopsisdiv>
<refsect1 id="R1-SQL-NOTIFY-1">
<refsect1info>
<date>1998-10-07</date>
</refsect1info>
<title>
Description
</title>
<refsect1>
<title>Description</title>
<para>
The <command>NOTIFY</command> command sends a notify event to each
frontend application that has previously executed
<command>LISTEN <replaceable class="parameter">notifyname</replaceable></command>
for the specified notify condition in the current database.
The <command>NOTIFY</command> command sends a notification event to each
client application that has previously executed
<command>LISTEN <replaceable class="parameter">name</replaceable></command>
for the specified notification name in the current database.
</para>
<para>
The information passed to the frontend for a notify event includes the notify
condition name and the notifying backend process's <acronym>PID</>. It is up to the
database designer to define the condition names that will be used in a given
The information passed to the client for a notifiation event includes the notification
name and the notifying session's server process <acronym>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.
</para>
<para>
Commonly, the notify condition name is the same as the name of some table in
the database, and the notify event essentially means <quote>I changed this table,
Commonly, the notification name is the same as the name of some table in
the database, and the notify event essentially means, <quote>I changed this table,
take a look at it to see what's new</quote>. But no such association is enforced by
the <command>NOTIFY</command> and <command>LISTEN</command> commands. For
example, a database designer could use several different condition names
example, a database designer could use several different notification names
to signal different sorts of changes to a single table.
</para>
<para>
<command>NOTIFY</command> provides a simple form of signal or
<acronym>IPC</> (interprocess communication) mechanism for a collection of processes
interprocess communication mechanism for a collection of processes
accessing the same <productname>PostgreSQL</productname> 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
pass additional data (beyond a mere notification name) from notifier to
listener(s).
</para>
<para>
When <command>NOTIFY</command> is used to signal the occurrence of changes
to a particular table, a useful programming technique is to put the
@@ -122,86 +62,91 @@ NOTIFY
In this way, notification happens automatically when the table is changed,
and the application programmer can't accidentally forget to do it.
</para>
<para>
<command>NOTIFY</command> interacts with SQL transactions in some important
ways. Firstly, if a <command>NOTIFY</command> 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
is aborted, all the commands within it have had no
effect, including <command>NOTIFY</command>. 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
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 notify were delivered within a transaction that was
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 backend cannot <quote>take back</quote> a notify once it has sent it to the frontend.
So notify events are only delivered between transactions. The upshot of this
the server cannot <quote>take back</quote> 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 <command>NOTIFY</command> for real-time signaling
should try to keep their transactions short.
</para>
<para>
<command>NOTIFY</command> 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
respect: if the same notification name is signaled multiple times in quick
succession, recipients may get only one notification event for several executions
of <command>NOTIFY</command>. So it is a bad idea to depend on the number
of notifies received. Instead, use <command>NOTIFY</command> to wake up
of notifications received. Instead, use <command>NOTIFY</command> 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.
</para>
<para>
It is common for a frontend that sends <command>NOTIFY</command> 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 <productname>PostgreSQL</productname> 6.4 and later, it is
possible to avoid such extra work by noticing whether the notifying backend
process's <acronym>PID</> (supplied in the notify event message) is the same as one's own
backend's <acronym>PID</> (available from <application>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.
<productname>PostgreSQL</productname> keeps self-notifies separate from notifies
arriving from other backends, so you cannot miss an outside notify by ignoring
your own notifies.)
</para>
<refsect2 id="R2-SQL-NOTIFY-3">
<refsect2info>
<date>1998-10-07</date>
</refsect2info>
<title>
Notes
</title>
<para>
<replaceable class="PARAMETER">name</replaceable>
can be any string valid as a name;
it need not correspond to the name of any actual table. If
<replaceable class="PARAMETER">name</replaceable>
is enclosed in double-quotes, it need not even be a syntactically
valid name, but can be any string up to 63 characters long.
</para>
<para>
In some previous releases of
<productname>PostgreSQL</productname>,
<replaceable class="PARAMETER">name</replaceable>
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.
</para>
<para>
In <productname>PostgreSQL</productname> releases prior to 6.4, the backend
<acronym>PID</> delivered in a notify message was always the <acronym>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.
</para>
</refsect2>
<para>
It is common for a client that executes <command>NOTIFY</command>
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 <acronym>PID</> (supplied in the
notification event message) is the same as one's own session's
<acronym>PID</> (available from <application>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.
<productname>PostgreSQL</productname> keeps self-notifiications
separate from notifications arriving from other sessions, so you
cannot miss an outside notification by ignoring your own
notifications.)
</para>
</refsect1>
<refsect1 id="R1-SQL-NOTIFY-2">
<title>
Usage
</title>
<refsect1>
<title>Parameters</title>
<variablelist>
<varlistentry>
<term><replaceable class="PARAMETER">name</replaceable></term>
<listitem>
<para>
Name of the notification to be signaled (any identifier).
</para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1>
<title>Diagnostics</title>
<variablelist>
<varlistentry>
<term><computeroutput>NOTIFY</computeroutput></term>
<listitem>
<para>
Message returned when the commmand has executed.
</para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1>
<title>Examples</title>
<para>
Configure and execute a listen/notify sequence from
<application>psql</application>:
@@ -214,23 +159,13 @@ Asynchronous NOTIFY 'virtual' from backend with pid '8448' received.
</para>
</refsect1>
<refsect1 id="R1-SQL-NOTIFY-3">
<title>
Compatibility
</title>
<refsect1>
<title>Compatibility</title>
<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>.
</para>
</refsect2>
<para>
There is no <command>NOTIFY</command> statement in the SQL
standard.
</para>
</refsect1>
</refentry>