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$PostgreSQL: pgsql/doc/src/sgml/runtime.sgml,v 1.243 2004/03/01 17:58:39 neilc Exp $
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<Chapter Id="runtime">
 <Title>Server Run-time Environment</Title>

 <Para>
  This chapter discusses how to set up and run the database server
  and the interactions with the operating system.
 </para>

 <sect1 id="postgres-user">
  <title>The <productname>PostgreSQL</productname> User Account</title>

  <indexterm>
   <primary>postgres user</primary>
  </indexterm>

  <para>
   As with any other server daemon that is connected to outside world,
   it is advisable to run <productname>PostgreSQL</productname> under a
   separate user account. This user account should only own the data
   that is managed by the server, and should not be shared with other
   daemons. (For example, using the user <literal>nobody</literal> is a bad
   idea.) It is not advisable to install executables owned by
   this user because compromised systems could then modify their own 
   binaries.
  </para>

  <para>
   To add a Unix user account to your system, look for a command
   <command>useradd</command> or <command>adduser</command>. The user
   name <systemitem>postgres</systemitem> is often used but is by no
   means required.
  </para>
 </sect1>

 <sect1 id="creating-cluster">
  <title>Creating a Database Cluster</title>

  <indexterm>
   <primary>database cluster</primary>
  </indexterm>

  <indexterm>
   <primary>data area</primary>
   <see>database cluster</see>
  </indexterm>

  <para>
   Before you can do anything, you must initialize a database storage
   area on disk. We call this a <firstterm>database cluster</firstterm>.
   (<acronym>SQL</acronym> uses the term catalog cluster instead.) A
   database cluster is a collection of databases is accessible by a
   single instance of a running database server. After initialization, a
   database cluster will contain a database named
   <literal>template1</literal>. As the name suggests, this will be used
   as a template for subsequently created databases; it should not be
   used for actual work.  (See <xref linkend="managing-databases"> for information
   about creating databases.)
  </para>

  <para>
   In file system terms, a database cluster will be a single directory
   under which all data will be stored. We call this the <firstterm>data
   directory</firstterm> or <firstterm>data area</firstterm>. It is
   completely up to you where you choose to store your data.  There is no
   default, although locations such as
   <filename>/usr/local/pgsql/data</filename> or
   <filename>/var/lib/pgsql/data</filename> are popular. To initialize a
   database cluster, use the command <command>initdb</command>,<indexterm><primary>initdb</></> which is
   installed with <productname>PostgreSQL</productname>. The desired
   file system location of your database system is indicated by the
   <option>-D</option> option, for example
<screen>
<prompt>$</> <userinput>initdb -D /usr/local/pgsql/data</userinput>
</screen>
   Note that you must execute this command while logged into the
   <productname>PostgreSQL</productname> user account, which is
   described in the previous section.
  </para>

  <tip>
   <para>
    As an alternative to the <option>-D</option> option, you can set
    the environment variable <envar>PGDATA</envar>.
    <indexterm><primary><envar>PGDATA</envar></primary></indexterm>
   </para>
  </tip>

  <para>
   <command>initdb</command> will attempt to create the directory you
   specify if it does not already exist. It is likely that it will not
   have the permission to do so (if you followed our advice and created
   an unprivileged account). In that case you should create the
   directory yourself (as root) and change the owner to be the
   <productname>PostgreSQL</productname> user. Here is how this might
   be done:
<screen>
root# <userinput>mkdir /usr/local/pgsql/data</userinput>
root# <userinput>chown postgres /usr/local/pgsql/data</userinput>
root# <userinput>su postgres</userinput>
postgres$ <userinput>initdb -D /usr/local/pgsql/data</userinput>
</screen>
  </para>

  <para>
   <command>initdb</command> will refuse to run if the data directory
   looks like it it has already been initialized.</para>

  <para>
   Because the data directory contains all the data stored in the
   database, it is essential that it be secured from unauthorized
   access. <command>initdb</command> therefore revokes access
   permissions from everyone but the
   <productname>PostgreSQL</productname> user.
  </para>

  <para>
   However, while the directory contents are secure, the default
   client authentication setup allows any local user to connect to the
   database and even become the database superuser. If you do not
   trust other local users, we recommend you use
   <command>initdb</command>'s <option>-W</option> or
   <option>--pwprompt</option> option to assign a password to the
   database superuser.<indexterm><primary>password</><secondary>of the
   superuser</></indexterm> After <command>initdb</command>, modify
   the <filename>pg_hba.conf</filename> file to use <literal>md5</> or
   <literal>password</> instead of <literal>trust</> authentication
   <emphasis>before</> you start the server for the first time. (Other
   approaches include using <literal>ident</literal> authentication or
   file system permissions to restrict connections. See <xref
   linkend="client-authentication"> for more information.)
  </para>

  <para>
   <command>initdb</command> also initializes the default
   locale<indexterm><primary>locale</></> for the database cluster.
   Normally, it will just take the locale settings in the environment
   and apply them to the initialized database.  It is possible to
   specify a different locale for the database; more information about
   that can be found in <xref linkend="locale">.  The sort order used
   within a particular database cluster is set by
   <command>initdb</command> and cannot be changed later, short of
   dumping all data, rerunning <command>initdb</command>, and
   reloading the data. So it's important to make this choice correctly
   the first time.
  </para>
 </sect1>

 <sect1 id="postmaster-start">
  <title>Starting the Database Server</title>

  <para>
   Before anyone can access the database, you must start the database
   server. The database server program is called
   <command>postmaster</command>.<indexterm><primary>postmaster</></>
   The <command>postmaster</command> must know where to
   find the data it is supposed to use. This is done with the
   <option>-D</option> option. Thus, the simplest way to start the
   server is:
<screen>
$ <userinput>postmaster -D /usr/local/pgsql/data</userinput>
</screen>
   which will leave the server running in the foreground. This must be
   done while logged into the <productname>PostgreSQL</productname> user
   account. Without <option>-D</option>, the server will try to use
   the data directory in the environment variable <envar>PGDATA</envar>.
   If neither of these succeed, it will fail.
  </para>

  <para>
   To start the <command>postmaster</command> in the
   background, use the usual shell syntax:
<screen>
$ <userinput>postmaster -D /usr/local/pgsql/data &gt;logfile 2&gt;&amp;1 &amp;</userinput>
</screen>
   It is an important to store the server's <systemitem>stdout</> and
   <systemitem>stderr</> output somewhere, as shown above. It will help
   for auditing purposes and to diagnose problems. (See <xref
   linkend="logfile-maintenance"> for a more thorough discussion of log
   file handling.)
  </para>

  <para>
   The <command>postmaster</command> also takes a number of other
   command line options. For more information, see the reference page
   and <xref linkend="runtime-config"> below. In particular, in order
   for the server to accept
   TCP/IP<indexterm><primary>TCP/IP</primary></indexterm> connections
   (rather than just Unix-domain socket ones), you must specify the
   <option>-i</option> option.
  </para>

  <para>
   This shell syntax can get tedious quickly.  Therefore the shell
   script wrapper
   <command>pg_ctl</command><indexterm><primary>pg_ctl</primary></indexterm>
   is provided to simplify some tasks.  For example:
<programlisting>
pg_ctl start -l logfile
</programlisting>
   will start the server in the background and put the output into the
   named log file. The <option>-D</option> option has the same meaning
   here as in the <command>postmaster</command>. <command>pg_ctl</command> is also
   capable of stopping the server.
  </para>

  <para>
   Normally, you will want to start the database server when the
   computer boots.<indexterm><primary>booting</><secondary>starting
   the server during</></> Autostart scripts are operating
   system-specific.  There are a few distributed with
   <productname>PostgreSQL</productname> in the
   <filename>contrib/start-scripts</> directory. This may require root
   privileges.
  </para>

  <para>
   Different systems have different conventions for starting up daemons
   at boot time. Many systems have a file
   <filename>/etc/rc.local</filename> or
   <filename>/etc/rc.d/rc.local</filename>. Others use
   <filename>rc.d</> directories. Whatever you do, the server must be
   run by the <productname>PostgreSQL</productname> user account
   <emphasis>and not by root</emphasis> or any other user. Therefore you
   probably should form your commands using <literal>su -c '...'
   postgres</literal>.  For example:
<programlisting>
su -c 'pg_ctl start -D /usr/local/pgsql/data -l serverlog' postgres
</programlisting>
  </para>

  <para>
   Here are a few more operating system specific suggestions. (Always
   replace these with the proper installation directory and the user
   name.)

   <itemizedlist>
    <listitem>
     <para>
      For <productname>FreeBSD</productname>, look at the file
      <filename>contrib/start-scripts/freebsd</filename> in the
      <productname>PostgreSQL</productname> source distribution.
      <indexterm><primary>FreeBSD</><secondary>start script</secondary></>
     </para>
    </listitem>

    <listitem>
     <para>
      On <productname>OpenBSD</productname>, add the following lines
      to the file <filename>/etc/rc.local</filename>:
      <indexterm><primary>OpenBSD</><secondary>start script</secondary></>
<programlisting>
if [ -x /usr/local/pgsql/bin/pg_ctl -a -x /usr/local/pgsql/bin/postmaster ]; then
    su - -c '/usr/local/pgsql/bin/pg_ctl start -l /var/postgresql/log -s' postgres
    echo -n ' postgresql'
fi
</programlisting>
     </para>
    </listitem>

    <listitem>
     <para>
      On <productname>Linux</productname> systems either add
      <indexterm><primary>Linux</><secondary>start script</secondary></>
<programlisting>
/usr/local/pgsql/bin/pg_ctl start -l logfile -D /usr/local/pgsql/data
</programlisting>
      to <filename>/etc/rc.d/rc.local</filename> or look at the file
      <filename>contrib/start-scripts/linux</filename> in the
      <productname>PostgreSQL</productname> source distribution.
     </para>
    </listitem>

    <listitem>
     <para>
      On <productname>NetBSD</productname>, either use the
      <productname>FreeBSD</productname> or
      <productname>Linux</productname> start scripts, depending on
      preference. <indexterm><primary>NetBSD</><secondary>start script</secondary></>
     </para>
    </listitem>

    <listitem>
     <para>
      On <productname>Solaris</productname>, create a file called
      <filename>/etc/init.d/postgresql</filename> that contains
      the following line:
      <indexterm><primary>Solaris</><secondary>start script</secondary></>
<programlisting>
su - postgres -c "/usr/local/pgsql/bin/pg_ctl start -l logfile -D /usr/local/pgsql/data"
</programlisting>
      Then, create a symbolic link to it in <filename>/etc/rc3.d</> as
      <filename>S99postgresql</>.
     </para>
    </listitem>
   </itemizedlist>

  </para>

   <para>
    While the <command>postmaster</command> is running, its
    <acronym>PID</acronym> is stored in the file
    <filename>postmaster.pid</filename> in the data directory. This is
    used to prevent multiple <command>postmaster</command> processes
    running in the same data directory and can also be used for
    shutting down the <command>postmaster</command> process.
   </para>

   <sect2 id="postmaster-start-failures">
    <title>Server Start-up Failures</title>

    <para>
     There are several common reasons the server might fail to
     start. Check the server's log file, or start it by hand (without
     redirecting standard output or standard error) and see what error
     messages appear. Below we explain some of the most common error
     messages in more detail.
    </para>

    <para>
<screen>
LOG:  could not bind IPv4 socket: Address already in use
HINT:  Is another postmaster already running on port 5432? If not, wait a few seconds and retry.
FATAL:  could not create TCP/IP listen socket
</screen>
     This usually means just what it suggests: you tried to start
     another <command>postmaster</command> on the same port where one is already running.
     However, if the kernel error message is not <computeroutput>Address
     already in use</computeroutput> or some variant of that, there may
     be a different problem. For example, trying to start a <command>postmaster</command>
     on a reserved port number may draw something like:
<screen>
$ <userinput>postmaster -i -p 666</userinput>
LOG:  could not bind IPv4 socket: Permission denied
HINT:  Is another postmaster already running on port 666? If not, wait a few seconds and retry.
FATAL:  could not create TCP/IP listen socket
</screen>
    </para>

    <para>
     A message like
<screen>
FATAL:  could not create shared memory segment: Invalid argument
DETAIL:  Failed system call was shmget(key=5440001, size=4011376640, 03600).
</screen>
     probably means your kernel's limit on the size of shared memory is
     smaller than the work area <productname>PostgreSQL</productname>
     is trying to create (4011376640 bytes in this example). Or it could
     mean that you do not have System-V-style shared memory support
     configured into your kernel at all. As a temporary workaround, you
     can try starting the server with a smaller-than-normal number
     of buffers (<option>-B</option> switch). You will eventually want
     to reconfigure your kernel to increase the allowed shared memory
     size. You may also see this message when trying to start multiple
     servers on the same machine, if their total space requested
     exceeds the kernel limit.
    </para>

    <para>
     An error like
<screen>
FATAL:  could not create semaphores: No space left on device
DETAIL:  Failed system call was semget(5440126, 17, 03600).
</screen>
     does <emphasis>not</emphasis> mean you've run out of disk
     space. It means your kernel's limit on the number of <systemitem
     class="osname">System V</> semaphores is smaller than the number
     <productname>PostgreSQL</productname> wants to create. As above,
     you may be able to work around the problem by starting the
     server with a reduced number of allowed connections
     (<option>-N</option> switch), but you'll eventually want to
     increase the kernel limit.
    </para>

    <para>
     If you get an <quote>illegal system call</> error, it is likely that
     shared memory or semaphores are not supported in your kernel at
     all. In that case your only option is to reconfigure the kernel to
     enable these features.
    </para>

    <para>
     Details about configuring <systemitem class="osname">System V</>
     <acronym>IPC</> facilities are given in <xref linkend="sysvipc">.
    </para>
   </sect2>

   <sect2 id="client-connection-problems">
    <title>Client Connection Problems</title>

    <para>
     Although the error conditions possible on the client side are quite
     varied and application-dependent, a few of them might be directly
     related to how the server was started up. Conditions other than
     those shown below should be documented with the respective client
     application.
    </para>

    <para>
<screen>
psql: could not connect to server: Connection refused
        Is the server running on host "server.joe.com" and accepting
        TCP/IP connections on port 5432?
</screen>
     This is the generic <quote>I couldn't find a server to talk
     to</quote> failure. It looks like the above when TCP/IP
     communication is attempted. A common mistake is to forget to
     configure the server to allow TCP/IP connections.
    </para>

    <para>
     Alternatively, you'll get this when attempting Unix-domain socket
     communication to a local server:
<screen>
psql: could not connect to server: No such file or directory
        Is the server running locally and accepting
        connections on Unix domain socket "/tmp/.s.PGSQL.5432"?
</screen>
    </para>

    <para>
     The last line is useful in verifying that the client is trying to
     connect to the right place. If there is in fact no server
     running there, the kernel error message will typically be either
     <computeroutput>Connection refused</computeroutput> or
     <computeroutput>No such file or directory</computeroutput>, as
     illustrated. (It is important to realize that
     <computeroutput>Connection refused</computeroutput> in this context
     does <emphasis>not</emphasis> mean that the server got your
     connection request and rejected it. That case will produce a
     different message, as shown in <xref
     linkend="client-authentication-problems">.) Other error messages
     such as <computeroutput>Connection timed out</computeroutput> may
     indicate more fundamental problems, like lack of network
     connectivity.
    </para>
   </sect2>
  </sect1>

  <sect1 id="runtime-config">
   <Title>Run-time Configuration</Title>

   <indexterm>
    <primary>configuration</primary>
    <secondary>of the server</secondary>
   </indexterm>

   <para>
    There are a lot of configuration parameters that affect the
    behavior of the database system. In this subsection, we describe
    how to set configuration parameters; the following subsections
    discuss each parameter in detail.
   </para>

   <para>
    All parameter names are case-insensitive. Every parameter takes a
    value of one of the four types: boolean, integer, floating point,
    and string. Boolean values are <literal>ON</literal>,
    <literal>OFF</literal>, <literal>TRUE</literal>,
    <literal>FALSE</literal>, <literal>YES</literal>,
    <literal>NO</literal>, <literal>1</literal>, <literal>0</literal>
    (case-insensitive) or any non-ambiguous prefix of these.
   </para>

   <para>
    One way to set these parameters is to edit the file
    <filename>postgresql.conf</filename><indexterm><primary>postgresql.conf</></>
    in the data directory. (A default file is installed there.) An
    example of what this file might look like is:
<programlisting>
# This is a comment
log_connections = yes
syslog = 2
search_path = '$user, public'
</programlisting>
    One parameter is specified per line. The equal sign between name and
    value is optional. Whitespace is insignificant and blank lines are
    ignored. Hash marks (<literal>#</literal>) introduce comments
    anywhere.  Parameter values that are not simple identifiers or
    numbers should be single-quoted.
   </para>

   <para>
    <indexterm>
     <primary>SIGHUP</primary>
    </indexterm>
    The configuration file is reread whenever the
    <command>postmaster</command> process receives a
    <systemitem>SIGHUP</> signal (which is most easily sent by means
    of <literal>pg_ctl reload</>). The <command>postmaster</command>
    also propagates this signal to all currently running server
    processes so that existing sessions also get the new
    value. Alternatively, you can send the signal to a single server
    process directly.
   </para>

   <para>
    A second way to set these configuration parameters is to give them
    as a command line option to the <command>postmaster</command>, such as:
<programlisting>
postmaster -c log_connections=yes -c syslog=2
</programlisting>
    Command-line options override any conflicting settings in
    <filename>postgresql.conf</filename>.
   </para>

   <para>
    Occasionally it is also useful to give a command line option to
    one particular session only. The environment variable
    <envar>PGOPTIONS</envar> can be used for this purpose on the
    client side:
<programlisting>
env PGOPTIONS='-c geqo=off' psql
</programlisting>
    (This works for any <application>libpq</>-based client application, not just
    <application>psql</application>.) Note that this won't work for
    parameters that are fixed when the server is started, such as the port
    number.
   </para>

   <para>
    Furthermore, it is possible to assign a set of option settings to
    a user or a database.  Whenever a session is started, the default
    settings for the user and database involved are loaded.  The
    commands <command>ALTER DATABASE</command> and <command>ALTER
    USER</command>, respectively, are used to configure these
    settings.  Per-database settings override anything received
    from the <command>postmaster</command> command-line or the
    configuration file, and in turn are overridden by per-user
    settings; both are overridden by per-session options.
   </para>

   <para>
    Some parameters can be changed in individual <acronym>SQL</acronym>
    sessions with the <xref linkend="SQL-SET" endterm="SQL-SET-title">
    command, for example:
<screen>
SET ENABLE_SEQSCAN TO OFF;
</screen>
    If <command>SET</> is allowed, it overrides all other sources of
    values for the parameter.  Superusers are allowed to <command>SET</>
    more values than ordinary users.
   </para>

   <para>
    The <xref linkend="SQL-SHOW" endterm="SQL-SHOW-title">
    command allows inspection of the current values of all parameters.
   </para>

   <para>
    The virtual table <structname>pg_settings</structname>
    (described in <xref linkend="view-pg-settings">) also allows
    displaying and updating session run-time parameters.  It is equivalent
    to <command>SHOW</> and <command>SET</>, but can be more convenient
    to use because it can be joined with other tables, or selected from using
    any desired selection condition.
   </para>
    
   <sect2 id="runtime-config-connection">
    <title>Connections and Authentication</title>

    <sect3 id="runtime-config-connection-settings">
     <title>Connection Settings</title>

     <variablelist>
     
     <varlistentry>
      <term><varname>tcpip_socket</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        If this is true, then the server will accept TCP/IP connections.<indexterm><primary>TCP/IP</></>
        Otherwise only local Unix domain socket connections are
        accepted. It is off by default. This option can only be set at
        server start.
       </para>
      </listitem>
     </varlistentry>
  
     <varlistentry>
      <term><varname>max_connections</varname> (<type>integer</type>)</term>
      <listitem>
       <para>
        Determines the maximum number of concurrent connections to the
        database server. The default is typically 100, but may be less
        if your kernel settings will not support it (as determined
        during <application>initdb</>).  This parameter can only be
        set at server start.
       </para>

       <para>
        Increasing this parameter may cause <productname>PostgreSQL</>
        to request more <systemitem class="osname">System V</> shared
        memory or semaphores than your operating system's default configuration
        allows. See <xref linkend="sysvipc"> for information on how to
        adjust these parameters, if necessary.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry>
      <term><varname>superuser_reserved_connections</varname>
      (<type>integer</type>)</term>
      <listitem>
       <para>
        Determines the number of <quote>connection slots</quote> that
        are reserved for connections by <productname>PostgreSQL</>
        superusers.  At most <varname>max_connections</> connections can
        ever be active simultaneously.  Whenever the number of active
        concurrent connections is at least <varname>max_connections</> minus
        <varname>superuser_reserved_connections</varname>, new connections
        will be accepted only for superusers.
       </para>

       <para>
        The default value is 2. The value must be less than the value of
        <varname>max_connections</varname>. This parameter can only be
        set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>port</varname> (<type>integer</type>)</term>
      <indexterm><primary>port</></>
      <listitem>
       <para>
        The TCP port the server listens on; 5432 by default. This
        option can only be set at server start.
       </para>
      </listitem>
     </varlistentry>


     <varlistentry>
      <term><varname>unix_socket_directory</varname> (<type>string</type>)</term>
      <listitem>
       <para>
        Specifies the directory of the Unix-domain socket on which the
        server is to listen for
        connections from client applications.  The default is normally
        <filename>/tmp</filename>, but can be changed at build time.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>unix_socket_group</varname> (<type>string</type>)</term>
      <listitem>
       <para>
        Sets the group owner of the Unix domain socket.  (The owning
        user of the socket is always the user that starts the
        server.)  In combination with the option
        <varname>unix_socket_permissions</varname> this can be used as
        an additional access control mechanism for this socket type.
        By default this is the empty string, which uses the default
        group for the current user.  This option can only be set at
        server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>unix_socket_permissions</varname> (<type>integer</type>)</term>
      <listitem>
       <para>
        Sets the access permissions of the Unix domain socket.  Unix
        domain sockets use the usual Unix file system permission set.
        The option value is expected to be an numeric mode
        specification in the form accepted by the
        <function>chmod</function> and <function>umask</function>
        system calls.  (To use the customary octal format the number
        must start with a <literal>0</literal> (zero).)
       </para>

       <para>
        The default permissions are <literal>0777</literal>, meaning
        anyone can connect. Reasonable alternatives are
        <literal>0770</literal> (only user and group, see also under
        <varname>unix_socket_group</varname>) and <literal>0700</literal>
        (only user). (Note that actually for a Unix domain socket, only write
        permission matters and there is no point in setting or revoking
        read or execute permissions.)
       </para>

       <para>
        This access control mechanism is independent of the one
        described in <xref linkend="client-authentication">.
       </para>

       <para>
        This option can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>virtual_host</varname> (<type>string</type>)</term>
      <listitem>
       <para>
        Specifies the host name or IP address on which the server is
        to listen for connections from client applications. The
        default is to listen on all configured addresses (including
        <systemitem class="systemname">localhost</>).
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry>
      <term><varname>rendezvous_name</varname> (<type>string</type>)</term>
      <listitem>
       <para>
        Specifies the Rendezvous broadcast name.  By default, the
        computer name is used, specified as ''.
       </para>
      </listitem>
     </varlistentry>
     
     </variablelist>
     </sect3>
     <sect3 id="runtime-config-connection-security">
     <title>Security and Authentication</title>
     
     <variablelist>
     <varlistentry>
      <term><varname>authentication_timeout</varname> (<type>integer</type>)</term>
      <indexterm><primary>timeout</><secondary>client authentication</></indexterm>
      <indexterm><primary>client authentication</><secondary>timeout during</></indexterm>
      <listitem>
       <para>
        Maximum time to complete client authentication, in seconds. If a
        would-be client has not completed the authentication protocol in
        this much time, the server breaks the connection. This prevents
        hung clients from occupying a connection indefinitely. This
        option can only be set at server start or in the
        <filename>postgresql.conf</filename> file. The default is 60.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry>
      <indexterm>
       <primary>SSL</primary>
      </indexterm>

      <term><varname>ssl</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        Enables <acronym>SSL</> connections. Please read
        <xref linkend="ssl-tcp"> before using this. The default
        is off.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>password_encryption</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        When a password is specified in <command>CREATE USER</> or
        <command>ALTER USER</> without writing either <literal>ENCRYPTED</> or
        <literal>UNENCRYPTED</>, this option determines whether the password is to be
        encrypted. The default is on (encrypt the password).
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>krb_server_keyfile</varname> (<type>string</type>)</term>
      <listitem>
       <para>
        Sets the location of the Kerberos server key file. See
        <xref linkend="kerberos-auth"> for details.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>db_user_namespace</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        This allows per-database user names.  It is off by default.
       </para>

       <para>
        If this is on, you should create users as <literal>username@dbname</>.
        When <literal>username</> is passed by a connecting client,
        <literal>@</> and the database name is appended to the user
        name and that database-specific user name is looked up by the
        server. Note that when you create users with names containing
        <literal>@</> within the SQL environment, you will need to
        quote the user name.
       </para>

       <para>
        With this option enabled, you can still create ordinary global
        users.  Simply append <literal>@</> when specifying the user
        name in the client.  The <literal>@</> will be stripped off
        before the user name is looked up by the server.
       </para>

       <note>
        <para>
         This feature is intended as a temporary measure until a
         complete solution is found.  At that time, this option will
         be removed.
        </para>
       </note>
      </listitem>
     </varlistentry>

    </variablelist>
    </sect3>
   </sect2>

   <sect2 id="runtime-config-resource">
    <title>Resource Consumption</title>

    <sect3 id="runtime-config-resource-memory">
     <title>Memory</title>

     <variablelist>
     <varlistentry>
      <term><varname>shared_buffers</varname> (<type>integer</type>)</term>
      <listitem>
       <para>
        Sets the number of shared memory buffers used by the database
        server. The default is typically 1000, but may be less if your
        kernel settings will not support it (as determined during
        <application>initdb</>).  Each buffer is 8192 bytes, unless a
        different value of <symbol>BLCKSZ</symbol> was chosen when building
        the server.  This setting must be at least 16, as well as at
        least twice the value of <varname>max_connections</varname>;
        however, settings significantly higher than the minimum are
        usually needed for good performance.  Values of a few thousand
        are recommended for production installations.  This option can
        only be set at server start.
       </para>

       <para>
        Increasing this parameter may cause <productname>PostgreSQL</>
        to request more <systemitem class="osname">System V</> shared
        memory than your operating system's default configuration
        allows. See <xref linkend="sysvipc"> for information on how to
        adjust these parameters, if necessary.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>work_mem</varname> (<type>integer</type>)</term>
      <listitem>
       <para>
        Specifies the amount of memory to be used by internal sort operations
        and hash tables before switching to temporary disk files. The value is
        specified in kilobytes, and defaults to 1024 kilobytes (1 MB).
        Note that for a complex query, several sort or hash operations might be
        running in parallel; each one will be allowed to use as much memory
        as this value specifies before it starts to put data into temporary
        files. Also, several running sessions could be doing such operations
        concurrently.  So the total memory used could be many
        times the value of <varname>work_mem</varname>; it is necessary to
        keep this fact in mind when choosing the value. Sort operations are
        used for <literal>ORDER BY</>, <literal>DISTINCT</>, and
        merge joins.
        Hash tables are used in hash joins, hash-based aggregation, and
        hash-based processing of <literal>IN</> subqueries.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry>
      <term><varname>maintenance_work_mem</varname> (<type>integer</type>)</term>
      <listitem>
       <para>
        Specifies the maximum amount of memory to be used in maintenance
        operations, such as <command>VACUUM</command>, <command>CREATE
        INDEX</>, and <command>ALTER TABLE ADD FOREIGN KEY</>.
        The value is specified in kilobytes, and defaults to 16384 kilobytes
        (16 MB).  Since only one of these operations can be executed at 
        a time by a database session, and an installation normally doesn't
        have very many of them happening concurrently, it's safe to set this
        value significantly larger than <varname>work_mem</varname>.  Larger
        settings may improve performance for vacuuming and for restoring
        database dumps.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
     </sect3>
     <sect3 id="runtime-config-resource-fsm">
     <title>Free Space Map</title>

     <variablelist>
     <varlistentry>
      <term><varname>max_fsm_pages</varname> (<type>integer</type>)</term>
      <listitem>
       <para>
        Sets the maximum number of disk pages for which free space will
        be tracked in the shared free-space map.  Six bytes of shared memory
        are consumed for each page slot.  This setting must be more than
        16 * <varname>max_fsm_relations</varname>.  The default is 20000.
        This option can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>max_fsm_relations</varname> (<type>integer</type>)</term>
      <listitem>
       <para>
        Sets the maximum number of relations (tables and indexes) for which
        free space will be tracked in the shared free-space map.  Roughly
        fifty bytes of shared memory are consumed for each slot.
        The default is 1000.
        This option can only be set at server start.
       </para>
      </listitem>
     </varlistentry>
     
     </variablelist>
     </sect3>
     <sect3 id="runtime-config-resource-kernel">
     <title>Kernel Resource Usage</title>
     <variablelist>

     <varlistentry>
      <term><varname>max_files_per_process</varname> (<type>integer</type>)</term>
      <listitem>
       <para>
        Sets the maximum number of simultaneously open files allowed to each
        server subprocess. The default is 1000. If the kernel is enforcing
        a safe per-process limit, you don't need to worry about this setting.
        But on some platforms (notably, most BSD systems), the kernel will
        allow individual processes to open many more files than the system
        can really support when a large number of processes all try to open
        that many files. If you find yourself seeing <quote>Too many open
        files</> failures, try reducing this setting.
        This option can only be set at server start.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry>
      <term><varname>preload_libraries</varname> (<type>string</type>)</term>
      <indexterm><primary>preload_libraries</></>
      <listitem>
       <para>
        This variable specifies one or more shared libraries that are
        to be preloaded at server start. A parameterless
        initialization function can optionally be called for each
        library.  To specify that, add a colon and the name of the
        initialization function after the library name. For example
        <literal>'$libdir/mylib:mylib_init'</literal> would cause
        <literal>mylib</> to be preloaded and <literal>mylib_init</>
        to be executed. If more than one library is to be loaded,
        separate their names with commas.
       </para>

       <para>
        If <literal>mylib</> or <literal>mylib_init</> are not found, the
        server will fail to start.
       </para>

       <para>
        <productname>PostgreSQL</productname> procedural language
        libraries may be preloaded in this way, typically by using the
        syntax <literal>'$libdir/plXXX:plXXX_init'</literal> where
        <literal>XXX</literal> is <literal>pgsql</>, <literal>perl</>,
        <literal>tcl</>, or <literal>python</>.
       </para>

       <para>
        By preloading a shared library (and initializing it if
        applicable), the library startup time is avoided when the
        library is first used.  However, the time to start each new
        server process may increase, even if that process never
        uses the library.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
    </sect3>

    <sect3 id="runtime-config-resource-vacuum-cost">
     <title>Cost-Based Vacuum Delay</title>

     <para>
      During the execution of <command>VACUUM</command>
      and <command>ANALYZE</command> commands,
      the system maintains an internal counter that keeps track of the
      estimated cost of the various I/O operations that are performed.
      When the accumulated cost reaches a limit
      (specified by <varname>vacuum_cost_limit</varname>), the process
      performing the operation will sleep for a while (specified by
      <varname>vacuum_cost_naptime</varname>). Then it will reset the
      counter and continue execution.
     </para>

     <para>
      The intent of this feature is to allow administrators to reduce
      the I/O impact of these commands on concurrent database
      activity. There are some situations in which it is not very
      important that maintenance commands like
      <command>VACUUM</command> and <command>ANALYZE</command> finish
      quickly; however, it is usually very important that these
      commands do not significantly interfere with the ability of the
      system to perform other database operations. Cost-based vacuum
      delay provides a way for administrators to achieve this.
     </para>

     <para>
      This feature is disabled by default. To enable it, set the
      <varname>vacuum_cost_naptime</varname> variable to a nonzero
      value.
     </para>

     <variablelist>
      <varlistentry>
       <term><varname>vacuum_cost_page_hit</varname> (<type>integer</type>)</term>
       <listitem>
        <para>
         The estimated cost for vacuuming a buffer found in the shared buffer
         cache. It represents the cost to lock the buffer pool, lookup
         the shared hash table and scan the content of the page. The
         default value is 1.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><varname>vacuum_cost_page_miss</varname> (<type>integer</type>)</term>
       <listitem>
        <para>
         The estimated cost for vacuuming a buffer that has to be read from
         disk.  This represents the effort to lock the buffer pool,
         lookup the shared hash table, read the desired block in from
         the disk and scan its content. The default value is 10.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><varname>vacuum_cost_page_dirty</varname> (<type>integer</type>)</term>
       <listitem>
        <para>
         The estimated cost charged when vacuum modifies a block that was
         previously clean. It represents the extra I/O required to
         flush the dirty block out to disk again. The default value is
         20.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><varname>vacuum_cost_limit</varname> (<type>integer</type>)</term>
       <listitem>
        <para>
         The accumulated cost that will cause the vacuuming process to briefly
         nap. The default value is 200.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><varname>vacuum_cost_naptime</varname> (<type>integer</type>)</term>
       <listitem>
        <para>
         The length of time, in milliseconds, that the process will nap
         when the cost limit has been exceeded.
         The default value is 0, which disables the cost-based vacuum
         delay feature.  Positive values enable cost-based vacuuming.
         Note however that on many systems, the effective resolution
         of sleep delays is 10 milliseconds; setting
         <varname>vacuum_cost_naptime</varname> to a value that is
         not a multiple of 10 may have the same results as setting it
         to the next higher multiple of 10.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>

     <note>
      <para>
       There are certain bulk operations that hold critical locks and should
       therefore complete as quickly as possible.  Cost-based vacuum
       delays do not occur during such operations.  Therefore it is
       possible that the cost accumulates far higher than the specified
       limit.  To avoid uselessly long delays in such cases, the actual
       naptime is calculated as <varname>vacuum_cost_naptime</varname> *
       <varname>accumulated_balance</varname> /
       <varname>vacuum_cost_limit</varname> with a maximum of
       <varname>vacuum_cost_naptime</varname> * 4.
      </para>
     </note>

    </sect3>
   </sect2>

   <sect2 id="runtime-config-wal">
    <title>Write Ahead Log</title>

   <para>
    See also <xref linkend="wal-configuration"> for details on WAL
    tuning.
   </para>

    <sect3 id="runtime-config-wal-settings">
     <title>Settings</title>
     <variablelist>
     
     <varlistentry>
      <indexterm>
       <primary>fsync</primary>
      </indexterm>

      <term><varname>fsync</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        If this option is on, the <productname>PostgreSQL</> server
        will use the <function>fsync()</> system call in several places
        to make sure that updates are physically written to disk. This
        insures that a database cluster will recover to a
        consistent state after an operating system or hardware crash.
        (Crashes of the database server itself are <emphasis>not</>
        related to this.)
       </para>

       <para>
        However, using <function>fsync()</function> results in a
        performance penalty: when a transaction is committed,
        <productname>PostgreSQL</productname> must wait for the
        operating system to flush the write-ahead log to disk.  When
        <varname>fsync</varname> is disabled, the operating system is
        allowed to do its best in buffering, ordering, and delaying
        writes. This can result in significantly improved performance.
        However, if the system crashes, the results of the last few
        committed transactions may be lost in part or whole. In the
        worst case, unrecoverable data corruption may occur.
       </para>

       <para>
        Due to the risks involved, there is no universally correct
        setting for <varname>fsync</varname>. Some administrators
        always disable <varname>fsync</varname>, while others only
        turn it off for bulk loads, where there is a clear restart
        point if something goes wrong, whereas some administrators
        always leave <varname>fsync</varname> enabled. The default is
        to enable <varname>fsync</varname>, for maximum reliability.
        If you trust your operating system, your hardware, and your
        utility company (or your battery backup), you can consider
        disabling <varname>fsync</varname>.
       </para>

       <para>
        This option can only be set at server start or in the
        <filename>postgresql.conf</filename> file.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry>
      <term><varname>wal_sync_method</varname> (<type>string</type>)</term>
      <listitem>
       <para>
        Method used for forcing WAL updates out to disk.  Possible
        values are
        <literal>fsync</> (call <function>fsync()</> at each commit),
        <literal>fdatasync</> (call <function>fdatasync()</> at each commit),
        <literal>open_sync</> (write WAL files with <function>open()</> option <symbol>O_SYNC</>), and
        <literal>open_datasync</> (write WAL files with <function>open()</> option <symbol>O_DSYNC</>).
        Not all of these choices are available on all platforms.
        This option can only be set at server start or in the
        <filename>postgresql.conf</filename> file.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry>
      <term><varname>wal_buffers</varname> (<type>integer</type>)</term>
      <listitem>
       <para>
        Number of disk-page buffers in shared memory for WAL
        logging. The default is 8. This option can only be set at
        server start.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
     </sect3>
     <sect3 id="runtime-config-wal-checkpoints">
     <title>Checkpoints</title>

    <variablelist>
     <varlistentry>
      <term><varname>checkpoint_segments</varname> (<type>integer</type>)</term>
      <listitem>
       <para>
        Maximum distance between automatic WAL checkpoints, in log
        file segments (each segment is normally 16 megabytes). The
        default is three.  This option can only be set at server start
        or in the <filename>postgresql.conf</filename> file.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>checkpoint_timeout</varname> (<type>integer</type>)</term>
      <listitem>
       <para>
        Maximum time between automatic WAL checkpoints, in
        seconds. The default is 300 seconds.  This option can only be
        set at server start or in the <filename>postgresql.conf</>
        file.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>checkpoint_warning</varname> (<type>integer</type>)</term>
      <listitem>
       <para>
        Write a message to the server logs if checkpoints caused by
        the filling of checkpoint segment files happens more
        frequently than this number of seconds.  The default is 30
        seconds.  Zero turns off the warning.
       </para>
      </listitem>
     </varlistentry>
                
                
     <varlistentry>
      <term><varname>commit_delay</varname> (<type>integer</type>)</term>
      <listitem>
       <para>
        Time delay between writing a commit record to the WAL buffer
        and flushing the buffer out to disk, in microseconds. A
        nonzero delay allows multiple transactions to be committed
        with only one <function>fsync()</function> system call, if
        system load is high enough additional transactions may become
        ready to commit within the given interval. But the delay is
        just wasted if no other transactions become ready to
        commit. Therefore, the delay is only performed if at least
        <varname>commit_siblings</varname> other transactions are
        active at the instant that a server process has written its
        commit record. The default is zero (no delay).
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>commit_siblings</varname> (<type>integer</type>)</term>
      <listitem>
       <para>
        Minimum number of concurrent open transactions to require
        before performing the <varname>commit_delay</> delay. A larger
        value makes it more probable that at least one other
        transaction will become ready to commit during the delay
        interval. The default is five.
       </para>
      </listitem>
     </varlistentry>
     
     </variablelist>
    </sect3>
   </sect2>

   <sect2 id="runtime-config-query">
    <title>Query Planning</title>

    <sect3 id="runtime-config-query-enable">
     <title>Planner Method Configuration</title>

     <note>
      <para>
       These configuration parameters provide a crude method for
       influencing the query plans chosen by the query optimizer. If
       the default plan chosen by the optimizer for a particular query
       is not optimal, a temporary solution may be found by using one
       of these configuration parameters to force the optimizer to
       choose a better plan. Other ways to improve the quality of the
       plans chosen by the optimizer include configuring the <xref
       linkend="runtime-config-query-constants"
       endterm="runtime-config-query-constants-title">, running
       <command>ANALYZE</command> more frequently, and increasing the
       amount of statistics collected for a particular column using
       <command>ALTER TABLE SET STATISTICS</command>.
      </para>
     </note>

     <variablelist>
     <varlistentry>
      <term><varname>enable_hashagg</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        Enables or disables the query planner's use of hashed
        aggregation plan types. The default is on. This is used for
        debugging the query planner.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>enable_hashjoin</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        Enables or disables the query planner's use of hash-join plan
        types. The default is on. This is used for debugging the query
        planner.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <indexterm>
       <primary>index scan</primary>
      </indexterm>

      <term><varname>enable_indexscan</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        Enables or disables the query planner's use of index-scan plan
        types. The default is on. This is used for debugging the query
        planner.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>enable_mergejoin</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        Enables or disables the query planner's use of merge-join plan
        types. The default is on. This is used for debugging the query
        planner.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>enable_nestloop</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        Enables or disables the query planner's use of nested-loop join
        plans. It's not possible to suppress nested-loop joins entirely,
        but turning this variable off discourages the planner from using
        one if there are other methods available. The default is
        on. This is used for debugging the query planner.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <indexterm>
       <primary>sequential scan</primary>
      </indexterm>

      <term><varname>enable_seqscan</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        Enables or disables the query planner's use of sequential scan
        plan types. It's not possible to suppress sequential scans
        entirely, but turning this variable off discourages the planner
        from using one if there are other methods available. The
        default is on. This is used for debugging the query planner.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>enable_sort</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        Enables or disables the query planner's use of explicit sort
        steps. It's not possible to suppress explicit sorts entirely,
        but turning this variable off discourages the planner from
        using one if there are other methods available. The default
        is on. This is used for debugging the query planner.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>enable_tidscan</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        Enables or disables the query planner's use of <acronym>TID</>
        scan plan types. The default is on. This is used for debugging
        the query planner.
       </para>
      </listitem>
     </varlistentry>
     
     </variablelist>
     </sect3>
     <sect3 id="runtime-config-query-constants">
     <title id="runtime-config-query-constants-title">
      Planner Cost Constants
     </title>

   <note>
    <para>
     Unfortunately, there is no well-defined method for determining
     ideal values for the family of <quote>cost</quote> variables that
     appear below. You are encouraged to experiment and share
     your findings.
    </para>
   </note>

     <variablelist>
     
     <varlistentry>
      <term><varname>effective_cache_size</varname> (<type>floating point</type>)</term>
      <listitem>
       <para>
        Sets the planner's assumption about the effective size of the
        disk cache (that is, the portion of the kernel's disk cache
        that will be used for <productname>PostgreSQL</productname>
        data files). This is measured in disk pages, which are
        normally 8192 bytes each. The default is 1000.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>random_page_cost</varname> (<type>floating point</type>)</term>
      <listitem>
       <para>
        Sets the query planner's estimate of the cost of a
        nonsequentially fetched disk page. This is measured as a
        multiple of the cost of a sequential page fetch. A higher
        value makes it more likely a sequential scan will be used, a
        lower value makes it more likely an index scan will be
        used. The default is four.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>cpu_tuple_cost</varname> (<type>floating point</type>)</term>
      <listitem>
       <para>
        Sets the query planner's estimate of the cost of processing
        each row during a query. This is measured as a fraction of
        the cost of a sequential page fetch. The default is 0.01.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>cpu_index_tuple_cost</varname> (<type>floating point</type>)</term>
      <listitem>
       <para>
        Sets the query planner's estimate of the cost of processing
        each index row during an index scan. This is measured as a
        fraction of the cost of a sequential page fetch. The default
        is 0.001.
       </para>
      </listitem>
     </varlistentry>
    
     <varlistentry>
      <term><varname>cpu_operator_cost</varname> (<type>floating point</type>)</term>
      <listitem>
       <para>
        Sets the planner's estimate of the cost of processing each
        operator in a <literal>WHERE</> clause. This is measured as a fraction of
        the cost of a sequential page fetch. The default is 0.0025.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>

    </sect3>
     <sect3 id="runtime-config-query-geqo">
     <title>Genetic Query Optimizer</title>

     <variablelist>

     <varlistentry>
      <indexterm>
       <primary>genetic query optimization</primary>
      </indexterm>
      <indexterm>
       <primary>GEQO</primary>
       <see>genetic query optimization</see>
      </indexterm>
      <term><varname>geqo</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        Enables or disables genetic query optimization, which is an
        algorithm that attempts to do query planning without exhaustive
        searching. This is on by default. See also the various other
        <varname>geqo_</varname> settings.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>geqo_threshold</varname> (<type>integer</type>)</term>
      <listitem>
       <para>
        Use genetic query optimization to plan queries with at least
        this many <literal>FROM</> items involved. (Note that an outer
        <literal>JOIN</> construct counts as only one <literal>FROM</>
        item.) The default is 12. For simpler queries it is usually best
        to use the deterministic, exhaustive planner, but for queries with
        many tables the deterministic planner takes too long.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>geqo_effort</varname> (<type>integer</type>)</term>
      <term><varname>geqo_pool_size</varname> (<type>integer</type>)</term>
      <term><varname>geqo_generations</varname> (<type>integer</type>)</term>
      <term><varname>geqo_selection_bias</varname> (<type>floating point</type>)</term>
      <listitem>
       <para>
        Various tuning parameters for the genetic query optimization
        algorithm.  The recommended one to modify is
        <varname>geqo_effort</varname>, which can range from 1 to 10 with
        a default of 5.  Larger values increase the time spent in planning
        but make it more likely that a good plan will be found.
        <varname>geqo_effort</varname> doesn't actually do anything directly,
        it is just used to compute the default values for the other
        parameters.  If you prefer, you can set the other parameters by hand
        instead.
        The pool size is the number of individuals in the genetic population.
        It must be at least two, and useful values are typically 100 to 1000.
        If it is set to zero (the default setting) then a suitable default
        is chosen based on <varname>geqo_effort</varname> and the number of
        tables in the query.
        Generations specifies the number of iterations of the algorithm.
        It must be at least one, and useful values are in the same range
        as the pool size.
        If it is set to zero (the default setting) then a suitable default
        is chosen based on the pool size.
        The run time of the algorithm is roughly proportional to the sum of
        pool size and generations.
        The selection bias is the selective pressure within the
        population. Values can be from 1.50 to 2.00; the latter is the
        default.
       </para>
      </listitem>
     </varlistentry>
     
     </variablelist>
    </sect3>
     <sect3 id="runtime-config-query-other">
     <title>Other Planner Options</title>

     <variablelist>

     <varlistentry>
      <term><varname>default_statistics_target</varname> (<type>integer</type>)</term>
      <listitem>
       <para>
        Sets the default statistics target for table columns that have
        not had a column-specific target set via <command>ALTER TABLE
        SET STATISTICS</>.  Larger values increase the time needed to
        do <command>ANALYZE</>, but may improve the quality of the
        planner's estimates. The default is 10. For more information
        on the use of statistics by the <productname>PostgreSQL</>
        query planner, refer to <xref linkend="planner-stats">.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>from_collapse_limit</varname> (<type>integer</type>)</term>
      <listitem>
       <para>
        The planner will merge sub-queries into upper queries if the
        resulting <literal>FROM</literal> list would have no more than
        this many items.  Smaller values reduce planning time but may
        yield inferior query plans.  The default is 8.  It is usually
        wise to keep this less than <varname>geqo_threshold</varname>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>join_collapse_limit</varname> (<type>integer</type>)</term>
      <listitem>
       <para>
        The planner will rewrite explicit inner <literal>JOIN</>
        constructs into lists of <literal>FROM</> items whenever a
        list of no more than this many items in total would
        result. Prior to <productname>PostgreSQL</> 7.4, joins
        specified via the <literal>JOIN</literal> construct would
        never be reordered by the query planner. The query planner has
        subsequently been improved so that inner joins written in this
        form can be reordered; this configuration variable controls
        the extent to which this reordering is performed.
        <note>
         <para>
          At present, the order of outer joins specified via the
          <literal>JOIN</> construct is never adjusted by the query
          planner; therefore, <varname>join_collapse_limit</> has no
          effect on this behavior. The planner may be improved to
          reorder some classes of outer joins in a future release of
          <productname>PostgreSQL</productname>.
         </para>
        </note>
       </para>

       <para>
        By default, this variable is set to
        <varname>from_collapse_limit</varname>, which is appropriate
        for most uses. Setting it to 1 prevents any reordering of
        inner <literal>JOIN</>s. Thus, the explicit join order
        specified in the query will be the actual order in which the
        relations are joined. The query planner does not always choose
        the optimal join order; advanced users may elect to
        temporarily set this variable to 1, and then specify the join
        order they desire explicitly. Another consequence of setting
        this variable to 1 is that the query planner will behave more
        like the <productname>PostgreSQL</productname> 7.3 query
        planner, which some users might find useful for backward
        compatibility reasons.
       </para>

       <para>
        Setting this variable to a value between 1 and
        <varname>from_collapse_limit</varname> might be useful to
        trade off planning time against the quality of the chosen plan
        (higher values produce better plans).
       </para>
      </listitem>
     </varlistentry>
     
     </variablelist>
    </sect3>
   </sect2>

   <sect2 id="runtime-config-logging">
    <title>Error Reporting and Logging</title>

    <indexterm zone="runtime-config-logging">
     <primary>server log</primary>
    </indexterm>

    <sect3 id="runtime-config-logging-syslog">
     <title>Syslog</title>

     <indexterm zone="runtime-config-logging-syslog">
      <primary>syslog</primary>
     </indexterm>

     <variablelist>

     <varlistentry>
      <term><varname>syslog</varname> (<type>integer</type>)</term>
      <listitem>
       <para>
        <productname>PostgreSQL</productname> allows the use of
        <systemitem>syslog</systemitem> for logging. If this option is
        set to 1, messages go both to <systemitem>syslog</> and the
        standard output. A setting of 2 sends output only to
        <systemitem>syslog</>. (Some messages will still go to the
        standard output/error.) The default is 0, which means
        <systemitem>syslog</> is off. This option must be set at server
        start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>syslog_facility</varname> (<type>string</type>)</term>
       <listitem>
        <para>
          This option determines the <application>syslog</application>
          <quote>facility</quote> to be used when logging via
          <application>syslog</application> is enabled. You may choose
          from <literal>LOCAL0</>, <literal>LOCAL1</>,
          <literal>LOCAL2</>, <literal>LOCAL3</>, <literal>LOCAL4</>,
          <literal>LOCAL5</>, <literal>LOCAL6</>, <literal>LOCAL7</>;
          the default is <literal>LOCAL0</>. See also the
          documentation of your system's
          <application>syslog</application>.
        </para>
       </listitem>
     </varlistentry>
     
     <varlistentry>
      <term><varname>syslog_ident</varname> (<type>string</type>)</term>
       <listitem>
        <para>
         If logging to <application>syslog</> is enabled, this option
         determines the program name used to identify
         <productname>PostgreSQL</productname> messages in
         <application>syslog</application> log messages. The default is
         <literal>postgres</literal>.
        </para>
       </listitem>
      </varlistentry>
      
      </variablelist>
    </sect3>
     <sect3 id="runtime-config-logging-when">
     <title>When To Log</title>

     <variablelist>

     <varlistentry>
      <term><varname>client_min_messages</varname> (<type>string</type>)</term>
      <listitem>
       <para>
        Controls which message levels are sent to the client.
        Valid values are <literal>DEBUG5</>,
        <literal>DEBUG4</>, <literal>DEBUG3</>, <literal>DEBUG2</>,
        <literal>DEBUG1</>, <literal>LOG</>, <literal>NOTICE</>,
        <literal>WARNING</>, and <literal>ERROR</>.  Each level
        includes all the levels that follow it.  The later the level,
        the fewer messages are sent.  The default is
        <literal>NOTICE</>.  Note that <literal>LOG</> has a different
        rank here than in <varname>log_min_messages</>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>log_min_messages</varname> (<type>string</type>)</term>
      <listitem>
       <para>
        Controls which message levels are written to the server log.
        Valid values are <literal>DEBUG5</>, <literal>DEBUG4</>,
        <literal>DEBUG3</>, <literal>DEBUG2</>, <literal>DEBUG1</>,
        <literal>INFO</>, <literal>NOTICE</>, <literal>WARNING</>,
        <literal>ERROR</>, <literal>LOG</>, <literal>FATAL</>, and
        <literal>PANIC</>.  Each level includes all the levels that
        follow it.  The later the level, the fewer messages are sent
        to the log.  The default is <literal>NOTICE</>.  Note that
        <literal>LOG</> has a different rank here than in
        <varname>client_min_messages</>.
        Only superusers can increase this option.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>log_error_verbosity</varname> (<type>string</type>)</term>
      <listitem>
       <para>
        Controls the amount of detail written in the server log for each
        message that is logged.  Valid values are <literal>TERSE</>,
        <literal>DEFAULT</>, and <literal>VERBOSE</>, each adding more
        fields to displayed messages.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>log_min_error_statement</varname> (<type>string</type>)</term>
      <listitem>
       <para>
        Controls whether or not the SQL statement that causes an error
        condition will also be recorded in the server log. All SQL
        statements that cause an error of the specified level, or a
        higher level, are logged.  The default is
        <literal>PANIC</literal> (effectively turning this feature
        off for normal use). Valid values are <literal>DEBUG5</literal>,
        <literal>DEBUG4</literal>, <literal>DEBUG3</literal>,
        <literal>DEBUG2</literal>, <literal>DEBUG1</literal>,
        <literal>INFO</literal>, <literal>NOTICE</literal>,
        <literal>WARNING</literal>, <literal>ERROR</literal>,
        <literal>FATAL</literal>, and <literal>PANIC</literal>.  For
        example, if you set this to <literal>ERROR</literal> then all
        SQL statements causing errors, fatal errors, or panics will be
        logged. Enabling this option can be helpful in tracking down
        the source of any errors that appear in the server log.
        Only superusers can increase this option.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry>
      <term><varname>log_min_duration_statement</varname> (<type>integer</type>)</term>
       <listitem>
        <para>
         Sets a minimum statement execution time (in milliseconds)
         for statement to be logged.  All SQL statements
         that run in the time specified or longer will be logged with
         their duration.  Setting this to zero will print
         all queries and their durations.  Minus-one (the default)
         disables this.  For example, if you set it to 
         <literal>250</literal> then all SQL statements that run 250ms 
         or longer will be logged.  Enabling this option can be
         useful in tracking down unoptimized queries in your applications.
         Only superusers can increase this or set it to minus-one if this 
         option is set by the administrator.
        </para>
       </listitem>
      </varlistentry>

     <varlistentry>
      <term><varname>silent_mode</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        Runs the server silently. If this option is set, the server
        will automatically run in background and any controlling terminals
        are disassociated.  Thus, no messages are written to standard
        output or standard error (same effect as <command>postmaster</>'s <option>-S</option>
        option). Unless
        <application>syslog</> logging is enabled, using this option is
        discouraged since it makes it impossible to see error messages.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>

       <para>
        Here is a list of the various message severity levels used in
        these settings:
        <variablelist>
         <varlistentry>
          <term><literal>DEBUG[1-5]</literal></term>
          <listitem>
           <para>
            Provides information for use by developers.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>INFO</literal></term>
          <listitem>
           <para>
            Provides information implicitly requested by the user,
            e.g., during <command>VACUUM VERBOSE</>.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>NOTICE</literal></term>
          <listitem>
           <para>
            Provides information that may be helpful to users, e.g.,
            truncation of long identifiers and the creation of indexes as part
            of primary keys.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>WARNING</literal></term>
          <listitem>
           <para>
            Provides warnings to the user, e.g., <command>COMMIT</>
            outside a transaction block.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>ERROR</literal></term>
          <listitem>
           <para>
            Reports an error that caused the current transaction to abort.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>LOG</literal></term>
          <listitem>
           <para>
            Reports information of interest to administrators, e.g.,
            checkpoint activity.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>FATAL</literal></term>
          <listitem>
           <para>
            Reports an error that caused the current session to abort.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>PANIC</literal></term>
          <listitem>
           <para>
            Reports an error that caused all sessions to abort.
           </para>
          </listitem>
         </varlistentry>
        </variablelist>
       </para>

    </sect3>
     <sect3 id="runtime-config-logging-what">
     <title>What To Log</title>
     
     <variablelist>
     
     <varlistentry>
      <term><varname>debug_print_parse</varname> (<type>boolean</type>)</term>
      <term><varname>debug_print_rewritten</varname> (<type>boolean</type>)</term>
      <term><varname>debug_print_plan</varname> (<type>boolean</type>)</term>
      <term><varname>debug_pretty_print</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        These options enable various debugging output to be sent to
        the client or server log. For each executed query, they print
        the resulting parse tree, the query rewriter output, or the
        execution plan.  <varname>debug_pretty_print</varname> indents
        these displays to produce a more readable but much longer
        output format.  <varname>client_min_messages</varname> or
        <varname>log_min_messages</varname> must be
        <literal>DEBUG1</literal> or lower to send output to the
        client or server logs. These options are off by default.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>log_connections</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        This outputs a line to the server logs detailing each successful
        connection. This is off by default, although it is probably very
        useful. This option can only be set at server start or in the
        <filename>postgresql.conf</filename> configuration file.
       </para>
      </listitem>
     </varlistentry>


     <varlistentry>
      <term><varname>log_disconnections</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        This outputs a line in the server logs similar to
        <varname>log_connections</varname> but at session termination,
        and includes the duration of the session.  This is off by
        default. This option can only be set at server start or in the
        <filename>postgresql.conf</filename> configuration file.
       </para>
      </listitem>
     </varlistentry>


     <varlistentry>
      <term><varname>log_duration</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        Causes the duration of every completed statement to be logged.
        To use this option, enable <varname>log_statement</> and
        <varname>log_pid</> so you can link the statement to the
        duration using the process ID. The default is off.
        Only superusers can turn off this option if it is enabled by
        the administrator.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry>
      <term><varname>log_pid</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        Prefixes each message in the server log file with the process ID of
        the server process. This is useful to sort out which messages
        pertain to which connection. The default is off.  This parameter
        does not affect messages logged via <application>syslog</>, which 
        always contain the process ID.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>log_statement</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        Causes each SQL statement to be logged. The default is off.
        <command>EXECUTE</> only displays the plan name, not the
        prepared query. Server-side languages like
        <application>PL/pgSQL</> that store functions in a cache only
        display their queries on first function call. Only superusers can
        turn off this option if it is enabled by the administrator.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>log_timestamp</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        Prefixes each server log message with a time stamp. The default
        is off.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>log_hostname</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        By default, connection logs only show the IP address of the
        connecting host. If you want it to show the host name you can
        turn this on, but depending on your host name resolution setup
        it might impose a non-negligible performance penalty. This
        option can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>log_source_port</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        Shows the outgoing port number of the connecting host in the
        connection log messages. You could trace back the port number
        to find out what user initiated the connection. Other than
        that, it's pretty useless and therefore off by default. This
        option can only be set at server start.
       </para>
      </listitem>
     </varlistentry>
     
     </variablelist>
    </sect3>
   </sect2>

   <sect2 id="runtime-config-statistics">
    <title>Runtime Statistics</title>

    <sect3 id="runtime-config-statistics-monitor">
     <title>Statistics Monitoring</title>
     <variablelist>

     <varlistentry>
      <term><varname>log_statement_stats</varname> (<type>boolean</type>)</term>
      <term><varname>log_parser_stats</varname> (<type>boolean</type>)</term>
      <term><varname>log_planner_stats</varname> (<type>boolean</type>)</term>
      <term><varname>log_executor_stats</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        For each query, write performance statistics of the respective
        module to the server log. This is a crude profiling
        instrument. All of these options are disabled by default.
        Only superusers can turn off any of these options if they have
        been enabled by the administrator.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>

    </sect3>
    <sect3 id="runtime-config-statistics-collector">
     <title>Query and Index Statistics Collector</title>
     <variablelist>

     <varlistentry>
      <term><varname>stats_start_collector</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        Controls whether the server should start the
        statistics-collection subprocess.  This is on by default, but
        may be turned off if you know you have no interest in
        collecting statistics.  This option can only be set at server
        start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>stats_command_string</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        Enables the collection of statistics on the currently
        executing command of each session, along with the time at
        which that command began execution. This option is off by
        default. Note that even when enabled, this information is not
        visible to all users, only to superusers and the user owning
        the session being reported on; so it should not represent a
        security risk. This data can be accessed via the
        <structname>pg_stat_activity</structname> system view; refer
        to <xref linkend="monitoring"> for more information.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>stats_block_level</varname> (<type>boolean</type>)</term>
      <term><varname>stats_row_level</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        These enable the collection of block-level and row-level statistics
        on database activity, respectively. These options are off by
        default. This data can be accessed via the
        <structname>pg_stat</structname> and
        <structname>pg_statio</structname> family of system views;
        refer to <xref linkend="monitoring"> for more information.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>stats_reset_on_server_start</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        If on, collected statistics are zeroed out whenever the server
        is restarted. If off, statistics are accumulated across server
        restarts. The default is on. This option can only be set at
        server start.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
    </sect3>
   </sect2>

   <sect2 id="runtime-config-client">
    <title>Client Connection Defaults</title>

    <sect3 id="runtime-config-client-statement">
     <title>Statement Behavior</title>
     <variablelist>

     <varlistentry>
      <term><varname>search_path</varname> (<type>string</type>)</term>
      <indexterm><primary>search_path</></>
      <indexterm><primary>path</><secondary>for schemas</></>
      <listitem>
       <para>
        This variable specifies the order in which schemas are searched
        when an object (table, data type, function, etc.) is referenced by a
        simple name with no schema component.  When there are objects of
        identical names in different schemas, the one found first
        in the search path is used.  An object that is not in any of the
        schemas in the search path can only be referenced by specifying
        its containing schema with a qualified (dotted) name.
       </para>

       <para>
        The value for <varname>search_path</varname> has to be a comma-separated
        list of schema names.  If one of the list items is
        the special value <literal>$user</literal>, then the schema
        having the name returned by <function>SESSION_USER</> is substituted, if there
        is such a schema.  (If not, <literal>$user</literal> is ignored.)
       </para>

       <para>
        The system catalog schema, <literal>pg_catalog</>, is always
        searched, whether it is mentioned in the path or not.  If it is
        mentioned in the path then it will be searched in the specified
        order.  If <literal>pg_catalog</> is not in the path then it will
        be searched <emphasis>before</> searching any of the path items.
        It should also be noted that the temporary-table schema,
        <literal>pg_temp_<replaceable>nnn</></>, is implicitly searched before any of
        these.
       </para>

       <para>
        When objects are created without specifying a particular target
        schema, they will be placed in the first schema listed
        in the search path.  An error is reported if the search path is
        empty.
       </para>

       <para>
        The default value for this parameter is
        <literal>'$user, public'</literal> (where the second part will be
        ignored if there is no schema named <literal>public</>).
        This supports shared use of a database (where no users
        have private schemas, and all share use of <literal>public</>),
        private per-user schemas, and combinations of these.  Other
        effects can be obtained by altering the default search path
        setting, either globally or per-user.
       </para>

       <para>
        The current effective value of the search path can be examined
        via the <acronym>SQL</acronym> function
        <function>current_schemas()</>.  This is not quite the same as
        examining the value of <varname>search_path</varname>, since
        <function>current_schemas()</> shows how the requests
        appearing in <varname>search_path</varname> were resolved.
       </para>

       <para>
        For more information on schema handling, see <xref linkend="ddl-schemas">.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>check_function_bodies</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        This parameter is normally true.  When set false, it disables
        validation of the function body string in <command>CREATE FUNCTION</>.
        Disabling validation is occasionally useful to avoid problems such as
        forward references when restoring function definitions from a dump.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <indexterm>
       <primary>transaction isolation level</primary>
      </indexterm>

      <term><varname>default_transaction_isolation</varname> (<type>string</type>)</term>
      <listitem>
       <para>
        Each SQL transaction has an isolation level, which can be
        either <quote>read uncommitted</quote>, <quote>read
        committed</quote>, <quote>repeatable read</quote>, or
        <quote>serializable</quote>.  This parameter controls the
        default isolation level of each new transaction. The default
        is <quote>read committed</quote>.
       </para>

       <para>
        Consult <xref linkend="mvcc"> and <xref linkend="sql-set-transaction"> for more
        information.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <indexterm>
       <primary>read-only transaction</primary>
      </indexterm>

      <term><varname>default_transaction_read_only</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        A read-only SQL transaction cannot alter non-temporary tables.
        This parameter controls the default read-only status of each new
        transaction. The default is false (read/write).
       </para>

       <para>
        Consult <xref linkend="sql-set-transaction"> for more information.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry>
      <term><varname>statement_timeout</varname> (<type>integer</type>)</term>
      <listitem>
       <para>
        Aborts any statement that takes over the specified number of
        milliseconds.  A value of zero turns off the timer, which is
        the default value.
       </para>
      </listitem>
     </varlistentry>
     
     </variablelist>
    </sect3>
     <sect3 id="runtime-config-client-format">
     <title>Locale and Formatting</title>

     <variablelist>

     <varlistentry>
      <term><varname>datestyle</varname> (<type>string</type>)</term>
      <indexterm><primary>date style</></>
      <listitem>
       <para>
        Sets the display format for date and time values, as well as
        the rules for interpreting ambiguous date input values.
        For historical reasons, this variable contains two independent
        components: the output format specification (<literal>ISO</>,
        <literal>Postgres</>, <literal>SQL</>, or <literal>German</>) and
        the date field order specification (<literal>DMY</>, <literal>MDY</>,
        or <literal>YMD</>).  These can be set separately or together.
        The keywords <literal>Euro</> and <literal>European</> are synonyms
        for <literal>DMY</>; the keywords <literal>US</>, <literal>NonEuro</>,
        and <literal>NonEuropean</> are synonyms for <literal>MDY</>.
        See <xref linkend="datatype-datetime"> for more information.  The
        default is <literal>ISO, MDY</>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>timezone</varname> (<type>string</type>)</term>
      <indexterm><primary>time zone</></>
      <listitem>
       <para>
        Sets the time zone for displaying and interpreting time
        stamps.  The default is to use whatever the system environment
        specifies as the time zone.  See <xref
        linkend="datatype-datetime"> for more information.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>australian_timezones</varname> (<type>boolean</type>)</term>
      <indexterm><primary>time zone</><secondary>Australian</></>
      <listitem>
       <para>
        If set to true, <literal>ACST</literal>,
        <literal>CST</literal>, <literal>EST</literal>, and
        <literal>SAT</literal> are interpreted as Australian time
        zones rather than as North/South American time zones and
        Saturday. The default is false.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <indexterm>
       <primary>significant digits</primary>
      </indexterm>
      <indexterm>
       <primary>floating-point</primary>
       <secondary>display</secondary>
      </indexterm>

      <term><varname>extra_float_digits</varname> (<type>integer</type>)</term>
      <listitem>
       <para>
        This parameter adjusts the number of digits displayed for
        floating-point values, including <type>float4</>, <type>float8</>,
        and geometric data types.  The parameter value is added to the
        standard number of digits (<literal>FLT_DIG</> or <literal>DBL_DIG</>
        as appropriate).  The value can be set as high as 2, to include
        partially-significant digits; this is especially useful for dumping
        float data that needs to be restored exactly.  Or it can be set
        negative to suppress unwanted digits.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>client_encoding</varname> (<type>string</type>)</term>
      <indexterm><primary>character set</></>
      <listitem>
       <para>
        Sets the client-side encoding (character set).
        The default is to use the database encoding.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>lc_messages</varname> (<type>string</type>)</term>
      <listitem>
       <para>
        Sets the language in which messages are displayed.  Acceptable
        values are system-dependent; see <xref linkend="locale"> for
        more information.  If this variable is set to the empty string
        (which is the default) then the value is inherited from the
        execution environment of the server in a system-dependent way.
       </para>

       <para>
        On some systems, this locale category does not exist.  Setting
        this variable will still work, but there will be no effect.
        Also, there is a chance that no translated messages for the
        desired language exist.  In that case you will continue to see
        the English messages.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>lc_monetary</varname> (<type>string</type>)</term>
      <listitem>
       <para>
        Sets the locale to use for formatting monetary amounts, for
        example with the <function>to_char</function> family of
        functions.  Acceptable values are system-dependent; see <xref
        linkend="locale"> for more information.  If this variable is
        set to the empty string (which is the default) then the value
        is inherited from the execution environment of the server in a
        system-dependent way.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>lc_numeric</varname> (<type>string</type>)</term>
      <listitem>
       <para>
        Sets the locale to use for formatting numbers, for example
        with the <function>to_char()</function> family of
        functions. Acceptable values are system-dependent; see <xref
        linkend="locale"> for more information.  If this variable is
        set to the empty string (which is the default) then the value
        is inherited from the execution environment of the server in a
        system-dependent way.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>lc_time</varname> (<type>string</type>)</term>
      <listitem>
       <para>
        Sets the locale to use for formatting date and time values.
        (Currently, this setting does nothing, but it may in the
        future.)  Acceptable values are system-dependent; see <xref
        linkend="locale"> for more information.  If this variable is
        set to the empty string (which is the default) then the value
        is inherited from the execution environment of the server in a
        system-dependent way.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>

    </sect3>
     <sect3 id="runtime-config-client-other">
     <title>Other Defaults</title>

     <variablelist>

     <varlistentry>
      <term><varname>explain_pretty_print</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        Determines whether <command>EXPLAIN VERBOSE</> uses the
        indented or non-indented format for displaying detailed
        query-tree dumps. The default is on.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>dynamic_library_path</varname> (<type>string</type>)</term>
      <indexterm><primary>dynamic_library_path</></>
      <indexterm><primary>dynamic loading</></>
      <listitem>
       <para>
        If a dynamically loadable module needs to be opened and the
        specified name does not have a directory component (i.e. the
        name does not contain a slash), the system will search this
        path for the specified file.  (The name that is used is the
        name specified in the <command>CREATE FUNCTION</command> or
        <command>LOAD</command> command.)
       </para>

       <para>
        The value for <varname>dynamic_library_path</varname> has to be a colon-separated
        list of absolute directory names. If a directory name starts
        with the special value <literal>$libdir</literal>, the
        compiled-in <productname>PostgreSQL</productname> package
        library directory is substituted. This where the modules
        provided by the <productname>PostgreSQL</productname>
        distribution are installed. (Use <literal>pg_config
        --pkglibdir</literal> to print the name of this directory.) For
        example:
<programlisting>
dynamic_library_path = '/usr/local/lib/postgresql:/home/my_project/lib:$libdir'
</programlisting>
       </para>

       <para>
        The default value for this parameter is
        <literal>'$libdir'</literal>. If the value is set to an empty
        string, the automatic path search is turned off.
       </para>

       <para>
        This parameter can be changed at run time by superusers, but a
        setting done that way will only persist until the end of the
        client connection, so this method should be reserved for
        development purposes. The recommended way to set this parameter
        is in the <filename>postgresql.conf</filename> configuration
        file.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>max_expr_depth</varname> (<type>integer</type>)</term>
      <listitem>
       <para>
        Sets the maximum expression nesting depth of the parser. The
        default value of 10000 is high enough for any normal query,
        but you can raise it if needed. (But if you raise it too high,
        you run the risk of server crashes due to stack overflow.)
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
    </sect3>
   </sect2>

   <sect2 id="runtime-config-locks">
    <title>Lock Management</title>

     <variablelist>

     <varlistentry>
      <indexterm>
       <primary>deadlock</primary>
       <secondary>timeout during</secondary>
      </indexterm>
      <indexterm>
       <primary>timeout</primary>
       <secondary>deadlock</secondary>
      </indexterm>

      <term><varname>deadlock_timeout</varname> (<type>integer</type>)</term>
      <listitem>
       <para>
        This is the amount of time, in milliseconds, to wait on a lock
        before checking to see if there is a deadlock condition. The
        check for deadlock is relatively slow, so the server doesn't run
        it every time it waits for a lock. We (optimistically?) assume
        that deadlocks are not common in production applications and
        just wait on the lock for a while before starting the check for a
        deadlock. Increasing this value reduces the amount of time
        wasted in needless deadlock checks, but slows down reporting of
        real deadlock errors. The default is 1000 (i.e., one second),
        which is probably about the smallest value you would want in
        practice. On a heavily loaded server you might want to raise it.
        Ideally the setting should exceed your typical transaction time,
        so as to improve the odds that a lock will be released before
        the waiter decides to check for deadlock.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>max_locks_per_transaction</varname> (<type>integer</type>)</term>
      <listitem>
       <para>
        The shared lock table is sized on the assumption that at most
        <varname>max_locks_per_transaction</varname> *
        <varname>max_connections</varname> distinct objects will need to
        be locked at any one time. The default, 64, has historically
        proven sufficient, but you might need to raise this value if you
        have clients that touch many different tables in a single
        transaction. This option can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
   </sect2>

   <sect2 id="runtime-config-compatible">
    <title>Version and Platform Compatibility</title>

    <sect3 id="runtime-config-compatible-version">
     <title>Previous PostgreSQL Versions</title>
     <variablelist>

     <varlistentry>
      <term><varname>add_missing_from</varname> (<type>boolean</type>)</term>
      <indexterm><primary>FROM</><secondary>missing</></>
      <listitem>
       <para>
        When <literal>true</>, tables that are referenced by a query will be
        automatically added to the <literal>FROM</> clause if not already
        present.  The default is <literal>true</> for compatibility with
        previous releases of <productname>PostgreSQL</>.  However, this
        behavior is not SQL-standard, and many people dislike it because it
        can mask mistakes.  Set to <literal>false</> for the SQL-standard
        behavior of rejecting references to tables that are not listed in
        <literal>FROM</>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>regex_flavor</varname> (<type>string</type>)</term>
      <indexterm><primary>regular expressions</></>
      <listitem>
       <para>
        The regular expression <quote>flavor</> can be set to
        <literal>advanced</>, <literal>extended</>, or <literal>basic</>.
        The default is <literal>advanced</>.  The <literal>extended</>
        setting may be useful for exact backwards compatibility with
        pre-7.4 releases of <productname>PostgreSQL</>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>sql_inheritance</varname> (<type>boolean</type>)</term>
      <indexterm><primary>inheritance</></>
      <listitem>
       <para>
        This controls the inheritance semantics, in particular whether
        subtables are included by various commands by default. They were
        not included in versions prior to 7.1. If you need the old
        behavior you can set this variable to off, but in the long run
        you are encouraged to change your applications to use the
        <literal>ONLY</literal> key word to exclude subtables. See
        <xref linkend="ddl-inherit"> for more information about inheritance.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>default_with_oids</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        This controls whether <command>CREATE TABLE</command>
        and <command>CREATE TABLE AS</command> will include OIDs in
        newly-created tables, if neither <literal>WITH OIDS</literal>
        or <literal>WITHOUT OIDS</literal> have been specified. It
        also determines whether OIDs will be included in the table
        created by <command>SELECT INTO</command>. In
        <productname>PostgreSQL</productname> &version;
        <varname>default_with_oids</varname> defaults to
        true. This is also the behavior of previous versions
        of <productname>PostgreSQL</productname>. However, assuming
        that tables will contain OIDs by default is not
        encouraged. Therefore, this option will default to false in a
        future release of <productname>PostgreSQL</productname>.
       </para>

       <para>
        To ease compatibility with applications that make use of OIDs,
        this option should left enabled. To ease compatibility with
        future versions of <productname>PostgreSQL</productname>, this
        option should be disabled, and applications that require OIDs
        on certain tables should explictely specify <literal>WITH
        OIDS</literal> when issuing the <command>CREATE
        TABLE</command> statements for the tables in question.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
    </sect3>
    <sect3 id="runtime-config-compatible-clients">
     <title>Platform and Client Compatibility</title>
     <variablelist>

     <varlistentry>
      <term><varname>transform_null_equals</varname> (<type>boolean</type>)</term>
      <indexterm><primary>IS NULL</></>
      <listitem>
       <para>
        When turned on, expressions of the form
        <literal><replaceable>expr</> = NULL</literal> (or <literal>NULL
        = <replaceable>expr</></literal>) are treated as
        <literal><replaceable>expr</> IS NULL</literal>, that is, they
        return true if <replaceable>expr</> evaluates to the null value,
        and false otherwise. The correct behavior of
        <literal><replaceable>expr</> = NULL</literal> is to always
        return null (unknown). Therefore this option defaults to off.
       </para>

       <para>
        However, filtered forms in <productname>Microsoft
        Access</productname> generate queries that appear to use
        <literal><replaceable>expr</> = NULL</literal> to test for
        null values, so if you use that interface to access the database you
        might want to turn this option on.  Since expressions of the
        form <literal><replaceable>expr</> = NULL</literal> always
        return the null value (using the correct interpretation) they are not
        very useful and do not appear often in normal applications, so
        this option does little harm in practice.  But new users are
        frequently confused about the semantics of expressions
        involving null values, so this option is not on by default.
       </para>

       <para>
        Note that this option only affects the literal <literal>=</>
        operator, not other comparison operators or other expressions
        that are computationally equivalent to some expression
        involving the equals operator (such as <literal>IN</literal>).
        Thus, this option is not a general fix for bad programming.
       </para>

       <para>
        Refer to <xref linkend="functions-comparison"> for related information.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
    </sect3>
   </sect2>

   <sect2 id="runtime-config-compiler">
    <title>Compiled-in Options</title>

    <para>
     The following options are available read-only, and are determined
     at source code compile time.  As such, they have been excluded from the
     sample <filename>postgresql.conf</> file.  They determine various aspects
     of <productname>PostgreSQL</productname> behavior that may be of interest
     to certain applications, particularly administrative front-ends.
    </para>

    <variablelist>

     <varlistentry>
      <term><varname>block_size</varname> (<type>integer</type>)</term>
      <listitem>
       <para>
        Shows the size of a disk block.  It is determined by the value
        of <literal>BLCKSZ</> when building the server. The default
        value is 8192 bytes.  The <varname>shared_buffers</varname> setting is
        influenced by <varname>block_size</varname>.  See
        <xref linkend="runtime-config-resource"> for information.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>integer_datetimes</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        Shows <literal>on</literal> if <productname>PostgreSQL</productname>
        was built with support for 64-bit integer dates and times.  It is
        set by configuring with <literal>--enable-integer-datetimes</literal>.
        The default value is <literal>off</literal>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>max_function_args</varname> (<type>integer</type>)</term>
      <listitem>
       <para>
        Shows the maximum number of function arguments. It is determined by
        the value of <literal>FUNC_MAX_ARGS</> when building the server. The
        default value is 32.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>max_identifier_length</varname> (<type>integer</type>)</term>
      <listitem>
       <para>
        Shows the maximum identifier length. It is determined as one
        less than the value of <literal>NAMEDATALEN</> when building
        the server. The default value of <literal>NAMEDATALEN</> is
        64; therefore the default
        <varname>max_identifier_length</varname> is 63.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>max_index_keys</varname> (<type>integer</type>)</term>
      <listitem>
       <para>
        Shows the maximum number of index keys. It is determined by
        the value of <literal>INDEX_MAX_KEYS</> when building the server. The
        default value is 32.
       </para>
      </listitem>
     </varlistentry>

    </variablelist>
   </sect2>

   <sect2 id="runtime-config-developer">
    <title>Developer Options</title>

    <para>
     The following options are intended for work on the
     <productname>PostgreSQL</productname> source, and in some cases
     to assist with recovery of severely damaged databases.  There
     should be no reason to use them in a production database setup.
     As such, they have been excluded from the sample
     <filename>postgresql.conf</> file.  Note that many of these
     options require special source compilation flags to work at all.
    </para>

    <variablelist>
     <varlistentry>
      <term><varname>debug_assertions</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        Turns on various assertion checks. This is a debugging aid. If
        you are experiencing strange problems or crashes you might want
        to turn this on, as it might expose programming mistakes. To use
        this option, the macro <symbol>USE_ASSERT_CHECKING</symbol>
        must be defined when <productname>PostgreSQL</productname> is
        built (accomplished by the <command>configure</command> option
        <option>--enable-cassert</option>). Note that
        <varname>debug_assertions</varname> defaults to on if
        <productname>PostgreSQL</productname> has been built with
        assertions enabled.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>pre_auth_delay</varname> (<type>integer</type>)</term>
      <listitem>
       <para>
        If nonzero, a delay of this many seconds occurs just after a new
        server process is forked, before it conducts the authentication
        process.  This is intended to give an opportunity to attach to the
        server process with a debugger to trace down misbehavior in
        authentication.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>trace_notify</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        Generates a great amount of debugging output for the
        <command>LISTEN</command> and <command>NOTIFY</command>
        commands.  <varname>client_min_messages</varname> or
        <varname>log_min_messages</varname> must be
        <literal>DEBUG1</literal> or lower to send this output to the
        client or server log, respectively.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>trace_locks</varname> (<type>boolean</type>)</term>
      <term><varname>trace_lwlocks</varname> (<type>boolean</type>)</term>
      <term><varname>trace_userlocks</varname> (<type>boolean</type>)</term>
      <term><varname>trace_lock_oidmin</varname> (<type>boolean</type>)</term>
      <term><varname>trace_lock_table</varname> (<type>boolean</type>)</term>
      <term><varname>debug_deadlocks</varname> (<type>boolean</type>)</term>
      <term><varname>log_btree_build_stats</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        Various other code tracing and debugging options.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>wal_debug</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        If true, emit WAL-related debugging output. This option is
        only available if the <symbol>WAL_DEBUG</symbol> macro was
        defined when <productname>PostgreSQL</productname> was
        compiled.
       </para>
      </listitem>
     </varlistentry>

    <varlistentry>
      <term><varname>zero_damaged_pages</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        Detection of a damaged page header normally causes
        <productname>PostgreSQL</> to report an error, aborting the current
        transaction.  Setting <varname>zero_damaged_pages</> to true causes
        the system to instead report a warning, zero out the damaged page,
        and continue processing.  This behavior <emphasis>will destroy data</>,
        namely all the rows on the damaged page.  But it allows you to get
        past the error and retrieve rows from any undamaged pages that may
        be present in the table.  So it is useful for recovering data if
        corruption has occurred due to hardware or software error.  You should
        generally not set this true until you have given up hope of recovering
        data from the damaged page(s) of a table.  The
        default setting is off, and it can only be changed by a superuser.
       </para>
      </listitem>
     </varlistentry>
   </variablelist>
  </sect2>
  <sect2 id="runtime-config-short">
   <title>Short Options</title>

   <para>
    For convenience there are also single letter command-line option switches
    available for some parameters. They are described in <xref
    linkend="runtime-config-short-table">.
   </para>

    <table id="runtime-config-short-table">
     <title>Short option key</title>
     <tgroup cols="2">
      <thead>
       <row>
        <entry>Short option</entry>
        <entry>Equivalent</entry>
       </row>
      </thead>

      <tbody>
       <row>
        <entry><option>-B <replaceable>x</replaceable></option></entry>
        <entry><literal>shared_buffers = <replaceable>x</replaceable></></entry>
       </row>
       <row>
        <entry><option>-d <replaceable>x</replaceable></option></entry>
        <entry><literal>log_min_messages = DEBUG<replaceable>x</replaceable></></entry>
       </row>
       <row>
        <entry><option>-F</option></entry>
        <entry><literal>fsync = off</></entry>
       </row>
       <row>
        <entry><option>-h <replaceable>x</replaceable></option></entry>
        <entry><literal>virtual_host = <replaceable>x</replaceable></></entry>
       </row>
       <row>
        <entry><option>-i</option></entry>
        <entry><literal>tcpip_socket = on</></entry>
       </row>
       <row>
        <entry><option>-k <replaceable>x</replaceable></option></entry>
        <entry><literal>unix_socket_directory = <replaceable>x</replaceable></></entry>
       </row>
       <row>
        <entry><option>-l</option></entry>
        <entry><literal>ssl = on</></entry>
       </row>
       <row>
        <entry><option>-N <replaceable>x</replaceable></option></entry>
        <entry><literal>max_connections = <replaceable>x</replaceable></></entry>
       </row>
       <row>
        <entry><option>-p <replaceable>x</replaceable></option></entry>
        <entry><literal>port = <replaceable>x</replaceable></></entry>
       </row>

       <row>
        <entry>
          <option>-fi</option>, <option>-fh</option>,
          <option>-fm</option>, <option>-fn</option>,
          <option>-fs</option>, <option>-ft</option><footnote
          id="fn.runtime-config-short">
           <para>
            For historical reasons, these options must be passed to
            the individual server process via the <option>-o</option>
            <command>postmaster</command> option, for example,
<screen>
$ <userinput>postmaster -o '-S 1024 -s'</userinput>
</screen>
            or via <envar>PGOPTIONS</envar> from the client side, as
            explained above.
           </para>
          </footnote>
         </entry>
         <entry>
          <literal>enable_indexscan=off</>,
          <literal>enable_hashjoin=off</>,
          <literal>enable_mergejoin=off</>,
          <literal>enable_nestloop=off</>,
          <literal>enable_seqscan=off</>,
          <literal>enable_tidscan=off</>
         </entry>
       </row>

       <row>
        <entry><option>-s</option><footnoteref linkend="fn.runtime-config-short"></entry>
        <entry><literal>log_statement_stats = on</></entry>
       </row>

       <row>
        <entry><option>-S <replaceable>x</replaceable></option><footnoteref linkend="fn.runtime-config-short">
        </entry>
        <entry><literal>work_mem = <replaceable>x</replaceable></></entry>
       </row>

       <row>
        <entry><option>-tpa</option>, <option>-tpl</option>, <option>-te</option><footnoteref linkend="fn.runtime-config-short"></entry>
        <entry><literal>log_parser_stats=on</>,
        <literal>log_planner_stats=on</>, 
        <literal>log_executor_stats=on</></entry>
       </row>
      </tbody>
     </tgroup>
    </table>

  </sect2>
 </sect1>


 <sect1 id="kernel-resources">
  <title>Managing Kernel Resources</title>

  <para>
   A large <productname>PostgreSQL</> installation can quickly exhaust
   various operating system resource limits. (On some systems, the
   factory defaults are so low that you don't even need a really
   <quote>large</> installation.) If you have encountered this kind of
   problem, keep reading.
  </para>

  <sect2 id="sysvipc">
   <title>Shared Memory and Semaphores</title>

   <indexterm zone="sysvipc">
    <primary>shared memory</primary>
   </indexterm>

   <indexterm zone="sysvipc">
    <primary>semaphores</primary>
   </indexterm>

   <para>
    Shared memory and semaphores are collectively referred to as
    <quote><systemitem class="osname">System V</>
    <acronym>IPC</></quote> (together with message queues, which are not
    relevant for <productname>PostgreSQL</>). Almost all modern
    operating systems provide these features, but not all of them have
    them turned on or sufficiently sized by default, especially systems
    with BSD heritage. (For the <systemitem class="osname">QNX</> and
    <systemitem class="osname">BeOS</> ports, <productname>PostgreSQL</>
    provides its own replacement implementation of these facilities.)
   </para>

   <para>
    The complete lack of these facilities is usually manifested by an
    <errorname>Illegal system call</> error upon server start. In
    that case there's nothing left to do but to reconfigure your
    kernel.  <productname>PostgreSQL</> won't work without them.
   </para>

   <para>
    When <productname>PostgreSQL</> exceeds one of the various hard
    <acronym>IPC</> limits, the server will refuse to start and
    should leave an instructive error message describing the problem
    encountered and what to do about it. (See also <xref
    linkend="postmaster-start-failures">.) The relevant kernel
    parameters are named consistently across different systems; <xref
    linkend="sysvipc-parameters"> gives an overview. The methods to set
    them, however, vary. Suggestions for some platforms are given below.
    Be warned that it is often necessary to reboot your machine, and
    possibly even recompile the kernel, to change these settings.
   </para>


   <table id="sysvipc-parameters">
    <title><systemitem class="osname">System V</> <acronym>IPC</> parameters</>

    <tgroup cols="3">
     <thead>
      <row>
       <entry>Name</>
       <entry>Description</>
       <entry>Reasonable values</>
      </row>
     </thead>

     <tbody>
      <row>
       <entry><varname>SHMMAX</></>
       <entry>Maximum size of shared memory segment (bytes)</>
       <entry>250 kB + 8.2 kB * <varname>shared_buffers</> + 14.2 kB * <varname>max_connections</> up to infinity</entry>
      </row>

      <row>
       <entry><varname>SHMMIN</></>
       <entry>Minimum size of shared memory segment (bytes)</>
       <entry>1</>
      </row>

      <row>
       <entry><varname>SHMALL</></>
       <entry>Total amount of shared memory available (bytes or pages)</>
       <entry>if bytes, same as <varname>SHMMAX</varname>; if pages, <literal>ceil(SHMMAX/PAGE_SIZE)</literal></>
      </row>

      <row>
       <entry><varname>SHMSEG</></>
       <entry>Maximum number of shared memory segments per process</>
       <entry>only 1 segment is needed, but the default is much higher</>
      </row>

       <row>
        <entry><varname>SHMMNI</></>
        <entry>Maximum number of shared memory segments system-wide</>
        <entry>like <varname>SHMSEG</> plus room for other applications</>
       </row>

       <row>
        <entry><varname>SEMMNI</></>
        <entry>Maximum number of semaphore identifiers (i.e., sets)</>
        <entry>at least <literal>ceil(max_connections / 16)</literal></>
       </row>

       <row>
        <entry><varname>SEMMNS</></>
        <entry>Maximum number of semaphores system-wide</>
        <entry><literal>ceil(max_connections / 16) * 17</literal> plus room for other applications</>
       </row>

       <row>
        <entry><varname>SEMMSL</></>
        <entry>Maximum number of semaphores per set</>
        <entry>at least 17</>
       </row>

       <row>
        <entry><varname>SEMMAP</></>
        <entry>Number of entries in semaphore map</>
        <entry>see text</>
       </row>

       <row>
        <entry><varname>SEMVMX</></>
        <entry>Maximum value of semaphore</>
        <entry>at least 1000 (The default is often 32767, don't change unless asked to.)</>
       </row>

     </tbody>
    </tgroup>
   </table>


   <para>
    <indexterm><primary>SHMMAX</primary></indexterm> The most important
    shared memory parameter is <varname>SHMMAX</>, the maximum size, in
    bytes, of a shared memory segment. If you get an error message from
    <function>shmget</> like <errorname>Invalid argument</>, it is
    possible that this limit has been exceeded. The size of the required
    shared memory segment varies both with the number of requested
    buffers (<option>-B</> option) and the number of allowed connections
    (<option>-N</> option), although the former is the most significant.
    (You can, as a temporary solution, lower these settings to eliminate
    the failure.) As a rough approximation, you can estimate the
    required segment size by multiplying the number of buffers and the
    block size (8 kB by default) plus ample overhead (at least half a
    megabyte). Any error message you might get will contain the size of
    the failed allocation request.
   </para>

   <para>
    Less likely to cause problems is the minimum size for shared
    memory segments (<varname>SHMMIN</>), which should be at most
    approximately 256 kB for <productname>PostgreSQL</> (it is
    usually just 1). The maximum number of segments system-wide
    (<varname>SHMMNI</>) or per-process (<varname>SHMSEG</>) should
    not cause a problem unless your system has them set to zero. Some
    systems also have a limit on the total amount of shared memory in
    the system; see the platform-specific instructions below.
   </para>

   <para>
    <productname>PostgreSQL</> uses one semaphore per allowed connection
    (<option>-N</> option), in sets of 16.  Each such set will also
    contain a 17th semaphore which contains a <quote>magic
    number</quote>, to detect collision with semaphore sets used by
    other applications. The maximum number of semaphores in the system
    is set by <varname>SEMMNS</>, which consequently must be at least
    as high as <varname>max_connections</> plus one extra for each 16
    allowed connections (see the formula in <xref
    linkend="sysvipc-parameters">).  The parameter <varname>SEMMNI</>
    determines the limit on the number of semaphore sets that can
    exist on the system at one time.  Hence this parameter must be at
    least <literal>ceil(max_connections / 16)</>. Lowering the number
    of allowed connections is a temporary workaround for failures,
    which are usually confusingly worded <errorname>No space
    left on device</>, from the function <function>semget</>.
   </para>

   <para>
    In some cases it might also be necessary to increase
    <varname>SEMMAP</> to be at least on the order of
    <varname>SEMMNS</>. This parameter defines the size of the semaphore
    resource map, in which each contiguous block of available semaphores
    needs an entry. When a semaphore set is freed it is either added to
    an existing entry that is adjacent to the freed block or it is
    registered under a new map entry. If the map is full, the freed
    semaphores get lost (until reboot). Fragmentation of the semaphore
    space could over time lead to fewer available semaphores than there
    should be.
   </para>

   <para>
    The <varname>SEMMSL</> parameter, which determines how many
    semaphores can be in a set, must be at least 17 for
    <productname>PostgreSQL</>.
   </para>

   <para>
    Various other settings related to <quote>semaphore undo</>, such as
    <varname>SEMMNU</> and <varname>SEMUME</>, are not of concern
    for <productname>PostgreSQL</>.
   </para>

    <variablelist>

     <varlistentry>
      <term><systemitem class="osname">BSD/OS</></term>
      <indexterm><primary>BSD/OS</><secondary>IPC configuration</></>
      <listitem>
       <formalpara>
        <title>Shared Memory</>
        <para>
         By default, only 4 MB of shared memory is supported. Keep in
         mind that shared memory is not pageable; it is locked in RAM.
         To increase the amount of shared memory supported by your
         system, add the following to your kernel configuration
         file. A <varname>SHMALL</> value of 1024 represents 4 MB of
         shared memory. The following increases the maximum shared
         memory area to 32 MB:
<programlisting>
options "SHMALL=8192"
options "SHMMAX=\(SHMALL*PAGE_SIZE\)"
</programlisting>
         For those running 4.3 or later, you will probably need to increase
         <varname>KERNEL_VIRTUAL_MB</> above the default <literal>248</>.
         Once all changes have been made, recompile the kernel, and reboot.
        </para>
       </formalpara>

       <para>
        For those running 4.0 and earlier releases, use <command>bpatch</>
        to find the <varname>sysptsize</> value in the current
        kernel. This is computed dynamically at boot time.
<screen>
$ <userinput>bpatch -r sysptsize</>
<computeroutput>0x9 = 9</>
</screen>
        Next, add <varname>SYSPTSIZE</> as a hard-coded value in the
        kernel configuration file. Increase the value you found using
        <command>bpatch</>. Add 1 for every additional 4 MB of
        shared memory you desire.
<programlisting>
options "SYSPTSIZE=16"
</programlisting>
        <varname>sysptsize</> cannot be changed by <command>sysctl</command>.
       </para>

       <formalpara>
        <title>Semaphores</>
        <para>
         You may need to increase the number of semaphores. By
         default, <productname>PostgreSQL</> allocates 34 semaphores,
         which is over half the default system total of 60.  Set the
         values you want in your kernel configuration file, e.g.:
<programlisting>
options "SEMMNI=40"
options "SEMMNS=240"
</programlisting>
        </para>
       </formalpara>
      </listitem>
     </varlistentry>


     <varlistentry>
      <term><systemitem class="osname">FreeBSD</></term>
      <term><systemitem class="osname">NetBSD</></term>
      <term><systemitem class="osname">OpenBSD</></term>
      <indexterm><primary>FreeBSD</><secondary>IPC configuration</></>
      <indexterm><primary>NetBSD</><secondary>IPC configuration</></>
      <indexterm><primary>OpenBSD</><secondary>IPC configuration</></>
      <listitem>
       <para>
        The options <varname>SYSVSHM</> and <varname>SYSVSEM</> need
        to be enabled when the kernel is compiled. (They are by
        default.) The maximum size of shared memory is determined by
        the option <varname>SHMMAXPGS</> (in pages). The following
        shows an example of how to set the various parameters:
<programlisting>
options         SYSVSHM
options         SHMMAXPGS=4096
options         SHMSEG=256

options         SYSVSEM
options         SEMMNI=256
options         SEMMNS=512
options         SEMMNU=256
options         SEMMAP=256
</programlisting>
        (On <systemitem class="osname">NetBSD</> and <systemitem
        class="osname">OpenBSD</> the key word is actually
        <literal>option</literal> singular.)
       </para>
       <para>
        You might also want to configure your kernel to lock shared
        memory into RAM and prevent it from being paged out to swap.
        Use the <command>sysctl</> setting
        <literal>kern.ipc.shm_use_phys</>.
       </para>
      </listitem>
     </varlistentry>


     <varlistentry>
      <term><systemitem class="osname">HP-UX</></term>
      <indexterm><primary>HP-UX</><secondary>IPC configuration</></>
      <listitem>
       <para>
        The default settings tend to suffice for normal installations.
        On <productname>HP-UX</> 10, the factory default for
        <varname>SEMMNS</> is 128, which might be too low for larger
        database sites.
       </para>
       <para>
        <acronym>IPC</> parameters can be set in the <application>System
        Administration Manager</> (<acronym>SAM</>) under
        <menuchoice><guimenu>Kernel
        Configuration</><guimenuitem>Configurable Parameters</></>. Hit
        <guibutton>Create A New Kernel</> when you're done.
       </para>
      </listitem>
     </varlistentry>


     <varlistentry>
      <term><systemitem class="osname">Linux</></term>
      <indexterm><primary>Linux</><secondary>IPC configuration</></>
      <listitem>
       <para>
        The default shared memory limit (both
        <varname>SHMMAX</varname> and <varname>SHMALL</varname>) is 32
        MB in 2.2 kernels, but it can be changed in the
        <filename>proc</filename> file system (without reboot).  For
        example, to allow 128 MB:
<screen>
<prompt>$</prompt> <userinput>echo 134217728 >/proc/sys/kernel/shmall</userinput>
<prompt>$</prompt> <userinput>echo 134217728 >/proc/sys/kernel/shmmax</userinput>
</screen>
        You could put these commands into a script run at boot-time.
       </para>

       <para>
        Alternatively, you can use <command>sysctl</command>, if
        available, to control these parameters.  Look for a file
        called <filename>/etc/sysctl.conf</filename> and add lines
        like the following to it:
<programlisting>
kernel.shmall = 134217728
kernel.shmmax = 134217728
</programlisting>
        This file is usually processed at boot time, but
        <command>sysctl</command> can also be called
        explicitly later.
       </para>

       <para>
        Other parameters are sufficiently sized for any application. If
        you want to see for yourself look in
        <filename>/usr/src/linux/include/asm-<replaceable>xxx</>/shmpara
        m.h</> and <filename>/usr/src/linux/include/linux/sem.h</>.
       </para>
      </listitem>
     </varlistentry>


     <varlistentry>
      <term><systemitem class="osname">MacOS X</></term>
      <indexterm><primary>MacOS X</><secondary>IPC configuration</></>
      <listitem>
       <para>
        In OS X 10.2 and earlier, edit the file
        <filename>/System/Library/StartupItems/SystemTuning/SystemTuning</>
        and change the values in the following commands:
<programlisting>
sysctl -w kern.sysv.shmmax
sysctl -w kern.sysv.shmmin
sysctl -w kern.sysv.shmmni
sysctl -w kern.sysv.shmseg
sysctl -w kern.sysv.shmall
</programlisting>
        In OS X 10.3, these commands have been moved to <filename>/etc/rc</>
        and must be edited there.
       </para>
      </listitem>
     </varlistentry>


     <varlistentry>
      <term><systemitem class="osname">SCO OpenServer</></term>
      <indexterm><primary>SCO OpenServer</><secondary>IPC configuration</></>
      <listitem>
       <para>
        In the default configuration, only 512 kB of shared memory per
        segment is allowed, which is about enough for <option>-B 24 -N
        12</>. To increase the setting, first change to the directory
        <filename>/etc/conf/cf.d</>. To display the current value of
        <varname>SHMMAX</>, run
<programlisting>
./configure -y SHMMAX
</programlisting>
        To set a new value for <varname>SHMMAX</>, run
<programlisting>
./configure SHMMAX=<replaceable>value</>
</programlisting>
        where <replaceable>value</> is the new value you want to use
        (in bytes). After setting <varname>SHMMAX</>, rebuild the kernel:
<programlisting>
./link_unix
</programlisting>
        and reboot.
       </para>
      </listitem>
     </varlistentry>


     <varlistentry>
      <term><systemitem class="osname">Solaris</></term>
      <indexterm><primary>Solaris</><secondary>IPC configuration</></>
      <listitem>
       <para>
        At least in version 2.6, the default maximum size of a shared
        memory segments is too low for <productname>PostgreSQL</>. The
        relevant settings can be changed in <filename>/etc/system</>,
        for example:
<programlisting>
set shmsys:shminfo_shmmax=0x2000000
set shmsys:shminfo_shmmin=1
set shmsys:shminfo_shmmni=256
set shmsys:shminfo_shmseg=256

set semsys:seminfo_semmap=256
set semsys:seminfo_semmni=512
set semsys:seminfo_semmns=512
set semsys:seminfo_semmsl=32
</programlisting>
        You need to reboot for the changes to take effect.
       </para>

       <para>
        See also <ulink
        url="http://www.sunworld.com/swol-09-1997/swol-09-insidesolaris.html"></>
        for information on shared memory under
        <productname>Solaris</>.
       </para>
      </listitem>
     </varlistentry>


     <varlistentry>
      <term><systemitem class="osname">UnixWare</></term>
      <indexterm><primary>UnixWare</><secondary>IPC configuration</></>
      <listitem>
       <para>
        On <productname>UnixWare</> 7, the maximum size for shared
        memory segments is 512 kB in the default configuration. This
        is enough for about <option>-B 24 -N 12</>. To display the
        current value of <varname>SHMMAX</>, run
<programlisting>
/etc/conf/bin/idtune -g SHMMAX
</programlisting>
        which displays the current, default, minimum, and maximum
        values. To set a new value for <varname>SHMMAX</>,
        run
<programlisting>
/etc/conf/bin/idtune SHMMAX <replaceable>value</>
</programlisting>
        where <replaceable>value</> is the new value you want to use
        (in bytes). After setting <varname>SHMMAX</>, rebuild the
        kernel:
<programlisting>
/etc/conf/bin/idbuild -B
</programlisting>
        and reboot.
       </para>
      </listitem>
     </varlistentry>

    </variablelist>

  </sect2>


  <sect2>
   <title>Resource Limits</title>

   <para>
    Unix-like operating systems enforce various kinds of resource limits
    that might interfere with the operation of your
    <productname>PostgreSQL</productname> server. Of particular
    importance are limits on the number of processes per user, the
    number of open files per process, and the amount of memory available
    to each process. Each of these have a <quote>hard</quote> and a
    <quote>soft</quote> limit. The soft limit is what actually counts
    but it can be changed by the user up to the hard limit. The hard
    limit can only be changed by the root user. The system call
    <function>setrlimit</function> is responsible for setting these
    parameters. The shell's built-in command <command>ulimit</command>
    (Bourne shells) or <command>limit</command> (<application>csh</>) is
    used to control the resource limits from the command line. On
    BSD-derived systems the file <filename>/etc/login.conf</filename>
    controls the various resource limits set during login. See the
    operating system documentation for details. The relevant
    parameters are <varname>maxproc</varname>,
    <varname>openfiles</varname>, and <varname>datasize</varname>. For
    example:
<programlisting>
default:\
...
        :datasize-cur=256M:\
        :maxproc-cur=256:\
        :openfiles-cur=256:\
...
</programlisting>
    (<literal>-cur</literal> is the soft limit.  Append
    <literal>-max</literal> to set the hard limit.)
   </para>

   <para>
    Kernels can also have system-wide limits on some resources.
    <itemizedlist>
     <listitem>
      <para>
      On <productname>Linux</productname>
      <filename>/proc/sys/fs/file-max</filename> determines the
      maximum number of open files that the kernel will support.  It can
      be changed by writing a different number into the file or by
      adding an assignment in <filename>/etc/sysctl.conf</filename>.
      The maximum limit of files per process is fixed at the time the
      kernel is compiled; see
      <filename>/usr/src/linux/Documentation/proc.txt</filename> for
      more information.
      </para>
     </listitem>
    </itemizedlist>
   </para>

   <para>
    The <productname>PostgreSQL</productname> server uses one process
    per connection so you should provide for at least as many processes
    as allowed connections, in addition to what you need for the rest
    of your system.  This is usually not a problem but if you run
    several servers on one machine things might get tight.
   </para>

   <para>
    The factory default limit on open files is often set to
    <quote>socially friendly</quote> values that allow many users to
    coexist on a machine without using an inappropriate fraction of
    the system resources.  If you run many servers on a machine this
    is perhaps what you want, but on dedicated servers you may want to
    raise this limit.
   </para>

   <para>
    On the other side of the coin, some systems allow individual
    processes to open large numbers of files; if more than a few
    processes do so then the system-wide limit can easily be exceeded.
    If you find this happening, and you do not want to alter the system-wide
    limit, you can set <productname>PostgreSQL</productname>'s
    <varname>max_files_per_process</varname> configuration parameter to
    limit the consumption of open files.
   </para>
  </sect2>

  <sect2>
   <title>Linux Memory Overcommit</title>

   <para>
    In Linux 2.4 and later, the default virtual memory behavior is not
    optimal for <productname>PostgreSQL</productname>. Because of the
    way that the kernel implements memory overcommit, the kernel may
    terminate the <productname>PostgreSQL</productname> server (the
    <filename>postmaster</filename> process) if the memory demands of
    another process cause the system to run out of virtual memory.
   </para>

   <para>
    If this happens, you will see a kernel message that looks like
    this (consult your system documentation and configuration on where
    to look for such a message):
<programlisting>
Out of Memory: Killed process 12345 (postmaster). 
</programlisting>
    This indicates that the <filename>postmaster</filename> process
    has been terminated due to memory pressure.
    Although existing database connections will continue to function
    normally, no new connections will be accepted.  To recover,
    <productname>PostgreSQL</productname> will need to be restarted.
   </para>

   <para>
    One way to avoid this problem is to run
    <productname>PostgreSQL</productname>
    on a machine where you can be sure that other processes will not
    run the machine out of memory.
   </para>

   <para>
    On Linux 2.6 and later, a better solution is to modify the kernel's
    behavior so that it will not <quote>overcommit</> memory.  This is
    done by selecting strict overcommit mode via sysctl:
<programlisting>
sysctl -w vm.overcommit_memory=2
</programlisting>
    or placing an equivalent entry in <filename>/etc/sysctl.conf</>.
    You may also wish to modify the related setting 
    <literal>vm.overcommit_ratio</>.  For details see the kernel documentation
    file <filename>Documentation/vm/overcommit-accounting</>.
   </para>

   <para>
    Some vendors' Linux 2.4 kernels are reported to have early versions
    of the 2.6 overcommit sysctl.  However, setting
    <literal>vm.overcommit_memory</> to 2
    on a kernel that does not have the relevant code will make
    things worse not better.  It is recommended that you inspect
    the actual kernel source code (see the function
    <function>vm_enough_memory</> in the file <filename>mm/mmap.c</>)
    to verify what is supported in your copy before you try this in a 2.4
    installation.  The presence of the <filename>overcommit-accounting</>
    documentation file should <emphasis>not</> be taken as evidence that the
    feature is there.  If in any doubt, consult a kernel expert or your
    kernel vendor.
   </para>
  </sect2>
 </sect1>


 <sect1 id="postmaster-shutdown">
  <title>Shutting Down the Server</title>

  <indexterm zone="postmaster-shutdown">
   <primary>shutdown</>
  </indexterm>

  <para>
   There are several ways to shut down the database server. You control
   the type of shutdown by sending different signals to the
   <command>postmaster</command> process.

   <variablelist>
    <varlistentry>
     <term><systemitem>SIGTERM</systemitem><indexterm><primary>SIGTERM</></></term>
     <listitem>
      <para>
       After receiving <systemitem>SIGTERM</systemitem>, the server
       disallows new connections, but lets existing sessions end their
       work normally. It shuts down only after all of the sessions
       terminate normally. This is the <firstterm>Smart
       Shutdown</firstterm>.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><systemitem>SIGINT</systemitem><indexterm><primary>SIGINT</></></term>
     <listitem>
      <para>
       The server disallows new connections and sends all existing
       server processes <systemitem>SIGTERM</systemitem>, which will cause them
       to abort their current transactions and exit promptly. It then
       waits for the server processes to exit and finally shuts down. This is the
       <firstterm>Fast Shutdown</firstterm>.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><systemitem>SIGQUIT</systemitem><indexterm><primary>SIGQUIT</></></term>
     <listitem>
      <para>
      This is the <firstterm>Immediate Shutdown</firstterm>, which
      will cause the <command>postmaster</command> process to send a
      <systemitem>SIGQUIT</systemitem> to all child processes and exit
      immediately (without properly shutting itself down). The child processes
      likewise exit immediately upon receiving
      <systemitem>SIGQUIT</systemitem>. This will lead to recovery (by
      replaying the WAL log) upon next start-up. This is recommended
      only in emergencies.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>

   <important>
    <para>
     It is best not to use <systemitem>SIGKILL</systemitem> to shut down
     the server. This will prevent the server from releasing
     shared memory and semaphores, which may then have to be done by
     manually.
    </para>
   </important>

  <para>
   The <acronym>PID</> of the <command>postmaster</command> process can be found using the
   <command>ps</command> program, or from the file
   <filename>postmaster.pid</filename> in the data directory. So for
   example, to do a fast shutdown:
<screen>
$ <userinput>kill -INT `head -1 /usr/local/pgsql/data/postmaster.pid`</userinput>
</screen>
  </para>
  <para>
   The program <command>pg_ctl</command> is a shell script
   that provides a more convenient interface for shutting down the
   server.
  </para>
 </sect1>

 <sect1 id="ssl-tcp">
  <title>Secure TCP/IP Connections with SSL</title>

  <indexterm zone="ssl-tcp">
   <primary>SSL</primary>
  </indexterm>

  <para>
   <productname>PostgreSQL</> has native support for using
   <acronym>SSL</> connections to encrypt client/server communications
   for increased security. This requires that
   <productname>OpenSSL</productname> is installed on both client and
   server systems and that support in <productname>PostgreSQL</> is
   enabled at build time (see <xref linkend="installation">).
  </para>

  <para>
   With <acronym>SSL</> support compiled in, the
   <productname>PostgreSQL</> server can be started with
   <acronym>SSL</> enabled by setting the parameter
   <varname>ssl</varname> to on in <filename>postgresql.conf</>. When
   starting in <acronym>SSL</> mode, the server will look for the
   files <filename>server.key</> and <filename>server.crt</> in the
   data directory, which should contain the server private key
   and certificate, respectively. These files must be set up correctly
   before an <acronym>SSL</>-enabled server can start. If the private key is
   protected with a passphrase, the server will prompt for the
   passphrase and will not start until it has been entered.
  </para>

  <para>
   The server will listen for both standard and <acronym>SSL</>
   connections on the same TCP port, and will negotiate with any
   connecting client on whether to use <acronym>SSL</>. See <xref
   linkend="auth-pg-hba-conf"> about how to force the server to
   require use of <acronym>SSL</> for certain connections.
  </para>

  <para>
   For details on how to create your server private key and certificate,
   refer to the <productname>OpenSSL</> documentation. A simple
   self-signed certificate can be used to get started for testing, but a
   certificate signed by a certificate authority (<acronym>CA</>) (either one of the global
   <acronym>CAs</> or a local one) should be used in production so the
   client can verify the server's identity. To create a quick
   self-signed certificate, use the following
   <productname>OpenSSL</productname> command:
<programlisting>
openssl req -new -text -out server.req
</programlisting>
   Fill out the information that <command>openssl</> asks for. Make sure
   that you enter the local host name as <quote>Common Name</>; the challenge
   password can be left blank. The program will generate a key that is
   passphrase protected; it will not accept a passphrase that is less
   than four characters long. To remove the passphrase (as you must if
   you want automatic start-up of the server), run the commands
<programlisting>
openssl rsa -in privkey.pem -out server.key
rm privkey.pem
</programlisting>
   Enter the old passphrase to unlock the existing key. Now do
<programlisting>
openssl req -x509 -in server.req -text -key server.key -out server.crt
chmod og-rwx server.key
</programlisting>
   to turn the certificate into a self-signed certificate and to copy the
   key and certificate to where the server will look for them.
  </para>
 </sect1>

 <sect1 id="ssh-tunnels">
  <title>Secure TCP/IP Connections with <application>SSH</application> Tunnels</title>

  <indexterm zone="ssh-tunnels">
   <primary>ssh</primary>
  </indexterm>

  <para>
   One can use <application>SSH</application> to encrypt the network
   connection between clients and a
   <productname>PostgreSQL</productname> server. Done properly, this
   provides an adequately secure network connection.
  </para>

  <para>
   First make sure that an <application>SSH</application> server is
   running properly on the same machine as the
   <productname>PostgreSQL</productname> server and that you can log in using
   <command>ssh</command> as some user. Then you can establish a secure
   tunnel with a command like this from the client machine:
<programlisting>
ssh -L 3333:foo.com:5432 joe@foo.com
</programlisting>
   The first number in the <option>-L</option> argument, 3333, is the
   port number of your end of the tunnel; it can be chosen freely. The
   second number, 5432, is the remote end of the tunnel: the port
   number your server is using. The name or the address in between
   the port numbers is the host with the database server you are going
   to connect to. In order to connect to the database server using
   this tunnel, you connect to port 3333 on the local machine:
<programlisting>
psql -h localhost -p 3333 template1
</programlisting>
   To the database server it will then look as though you are really
   user <literal>joe@foo.com</literal> and it will use whatever
   authentication procedure was set up for this user. In order for the
   tunnel setup to succeed you must be allowed to connect via
   <command>ssh</command> as <literal>joe@foo.com</literal>, just
   as if you had attempted to use <command>ssh</command> to set up a
   terminal session.
  </para>

  <tip>
   <para>
    Several other applications exist that can provide secure tunnels using
    a procedure similar in concept to the one just described.
   </para>
  </tip>

 </sect1>

</Chapter>

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