postgresql/doc/src/sgml/runtime.sgml

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<Chapter Id="runtime">
 <Title>Server Runtime 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 Postgres user account</title>

  <para>
   As with any other server daemon that is connected to the world at
   large, it is advisable to run Postgres under a separate user
   account. This user account should only own the data itself that is
   being managed by the server, and should not be shared with other
   daemons. (Thus, using the user <quote>nobody</quote> is a bad
   idea.) It is not advisable to install the executables as owned by
   this user account because that runs the risk of user-defined
   functions gone astray or any other exploits compromising the
   executable programs.
  </para>

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

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

  <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> speaks of a catalog
   cluster instead.) A database cluster is a collection of databases
   that will be accessible through a single instance of a running
   database server. After initialization, a database cluster will
   contain one database named <literal>template1</literal>. As the
   name suggests, this will be used as a template for any subsequently
   created database; it should not be used for actual work.
  </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>,
   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>
&gt; <userinput>initdb -D /usr/local/pgsql/data</userinput>
</screen>
   Note that you must execute this command while being logged in to
   the Postgres 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>.
   </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 won't
   have the permission to do so (if you followed our advice and
   created an unprivileged account). In that case you can create the
   directory yourself (as root) and transfer ownership of it or grant
   write access to it. Here is how this might work:
<screen>
root# <userinput>mkdir /usr/local/pgsql/data</userinput>
root# <userinput>chown postgres /usr/local/pgsql/data</userinput>
root# <userinput>su postgres</userinput>
postgres&gt; <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 belongs to an already initialized installation.
  </para>

  <para>
   Because the data directory contains all the data stored in the
   database it is essential that it be well secured from unauthorized
   access. <command>initdb</command> therefore revokes access
   permissions from everyone but the Postgres user account.
  </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 is called
   <firstterm>postmaster</firstterm>.
   The postmaster must know where to find the data it is supposed
   to work on. This is done with the <option>-D</option> option. Thus,
   the simplest way to start the server is, for example,
<screen>
&gt; <userinput>postmaster -D /usr/local/pgsql/data</userinput>
</screen>
   which will leave the server running in the foreground. This must
   again be done while logged in to the Postgres user account. Without
   a <option>-D</option>, the server will try to use the data
   directory in the environment variable <envar>PGDATA</envar>; if
   neither of these works it will fail.
  </para>

  <para>
   To start the <application>postmaster</application> in the
   background, use the usual shell syntax:
<screen>
&gt; <userinput>postmaster -D /usr/local/pgsql/data &gt; logfile 1&gt;&amp;2 &amp;</userinput>
</screen>
   It is an extremely good idea to keep the server output around
   somewhere, as indicated here. It will help both for auditing
   purposes and to diagnose problems.
  </para>

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

  <para>
   Normally, you will want to start the database server when the
   computer boots up. This is not required; the
   <productname>PostgreSQL</productname> server can be run
   successfully from non-privileged accounts without root
   intervention.
  </para>

  <para>
   Different systems have different conventions for starting up
   daemons at boot time, so you are advised to familiarize yourself
   with them. Many systems have a file
   <filename>/etc/rc.local</filename> or
   <filename>/etc/rc.d/rc.local</filename> which is almost certainly
   no bad place to put such a command. Whatever you do, postmaster
   must be run by the <productname>Postgres</productname> user account
   <emphasis>and not by root</emphasis> or any other user. Therefore
   you probably always want to form your command lines along the lines
   of <literal>su -c '...' postgres</literal>, for example:
<programlisting>
nohup su -c 'postmaster -D /usr/local/pgsql/data > server.log 2>&1' postgres &
</programlisting>
   (using the program <application>nohup</application> to prevent the
   server from dying when you log out).
  </para>

  <para>
   Here are a few more operating system specific suggestions.

   <itemizedlist>
    <listitem>
     <para>
      Edit the file <filename>rc.local</filename> on
      <productname>NetBSD</productname> or file
      <filename>rc2.d</filename> on <productname>Solaris</productname> to contain the
      following single line:
<programlisting>
su postgres -c "/usr/local/pgsql/bin/postmaster -D /usr/local/pgsql/data"
</programlisting>
     </para>
    </listitem>

    <listitem>
     <para>
      On <productname>FreeBSD</productname> edit
      <filename>/usr/local/etc/rc.d/pgsql.sh</filename> to contain the
      following lines and make it <literal>chmod 755</literal> and
      <literal>chown root:bin</literal>.
<programlisting>
#!/bin/sh
[ -x /usr/local/pgsql/bin/postmaster ] && {
    su -l pgsql -c 'exec /usr/local/pgsql/bin/postmaster
        -D/usr/local/pgsql/data
        -S -o -F > /usr/local/pgsql/errlog' &
    echo -n ' pgsql'
}
</programlisting>
      You may put the line breaks as shown above. The shell is smart
      enough to keep parsing beyond end-of-line if there is an
      expression unfinished. The exec saves one layer of shell under
      the postmaster process so the parent is init.
     </para>
    </listitem>

    <listitem>
     <para>
      On <productname>RedHat Linux</productname> add a file
      <filename>/etc/rc.d/init.d/postgres.init</filename> 
       which is based on the example in <filename>contrib/linux/</filename>.
       Then make a softlink to this file from
       <filename>/etc/rc.d/rc5.d/S98postgres.init</filename>.
      </para>
     </listitem>
    </itemizedlist>
   </para>

   <para>
    While the <application>postmaster</application> is running, it's
    PID is in the file <filename>postmaster.pid</filename> in the data
    directory. This is used as in interlock against multiple running
    postmaster on the same data directory and can also be used for
    shutting down the postmaster.
   </para>

   <para>
    The shell script wrapper <application>pg_ctl</application> that
    comes with <productname>Postgres</productname> can also be used to
    control starting (and stopping!) of the database server in
    intelligent fashion.
   </para>

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

    <para>
     There are several common reasons for the postmaster to fail to
     start up. Check the postmaster's log file, or start it by hand
     (without redirecting standard output or standard error) to see
     what complaint messages appear. Some of the possible error
     messages are reasonably self-explanatory, but here are some that
     are not.
    </para>

    <para>
<screen>
FATAL: StreamServerPort: bind() failed: Address already in use
        Is another postmaster already running on that port?
</screen>
     This usually means just what it suggests: you accidentally
     started a second postmaster 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 wording, there may be a different problem. For
     example, trying to start a postmaster on a reserved port number
     may draw something like
<screen>
&gt; <userinput>postmaster -i -p 666</userinput>
FATAL: StreamServerPort: bind() failed: Permission denied
        Is another postmaster already running on that port?
</screen>
    </para>

    <para>
     A message like
<screen>
IpcMemoryCreate: shmget(5440001, 83918612, 01600) failed: Invalid argument
FATAL 1:  ShmemCreate: cannot create region
</screen>
     probably means that your kernel's limit on the size of shared
     memory areas is smaller than the buffer area that Postgres is
     trying to create (83918612 bytes in this example). Or it could
     mean that you don't have SysV-style shared memory support
     configured into your kernel at all. As a temporary workaround,
     you can try starting the postmaster 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, however. You may see this message
     when trying to start multiple postmasters on the same machine, if
     their total space requests exceed the kernel limit.
    </para>

    <para>
     An error like
<screen>
IpcSemaphoreCreate: semget(5440026, 16, 0600) failed: No space left on device
</screen>
     does <emphasis>not</emphasis> mean that you've run out of disk
     space; it means that your kernel's limit on the number of SysV
     semaphores is smaller than the number
     <productname>Postgres</productname> wants to create. As above,
     you may be able to work around the problem by starting the
     postmaster with a reduced number of backend processes
     (<option>-N</option> switch), but you'll eventually want to
     increase the kernel limit.
    </para>
   </sect2>

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

    <para>
     Although the possible error conditions on the client side are
     both virtually infinite 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>
connectDB() -- connect() failed: Connection refused
Is the postmaster running (with -i) at 'server.joe.com' and accepting connections on TCP/IP 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 the
     <option>-i</option> to the postmaster to allow TCP/IP
     connections.
    </para>

    <para>
     Alternatively, you'll get this when attempting
     Unix-socket communication to a local postmaster:
<screen>
connectDB() -- connect() failed: No such file or directory
Is the postmaster running at 'localhost' and accepting connections on Unix socket '5432'?
</screen>
    </para>

    <para>
     The last line is useful in verifying that the client is trying to
     connect where it is supposed to. If there is in fact no
     postmaster 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 particularly important to realize that
     <computeroutput>Connection refused</computeroutput> in this
     context does <emphasis>not</emphasis> mean that the postmaster
     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>

   <para>
    There are a lot of configuration parameters that affect the
    behavior of the database system in some way or other. Here we
    describe how to set them and the following subsections will
    discuss each of them.
   </para>

   <para>
    All parameter names are case-insensitive. Every parameter takes a
    value of one of the four types boolean, integer, floating point,
    string as described below. 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 options is to create a file
    <filename>postgresql.conf</filename> in the data directory (e.g.,
    <filename>/usr/local/pgsql/data</filename>). An example of how
    this file could look like is this:
<programlisting>
# This is a comment
log_connections = yes
syslog = 2
</programlisting>
    As you see, options are one per line. The equal sign between name
    and value is optional. White space is insignificant, blank lines
    are ignored. Hash marks (<quote>#</quote>) introduce comments
    anywhere.
   </para>

   <para>
    The configuration file is reread whenever the postmaster receives
    a SIGHUP signal. This signal is also propagated to all running
    backend processes, so that running sessions get the new default.
    Alternatively, you can send the signal to only one backend process
    directly.
   </para>

   <para>
    A second way to set these configuration parameters is to give them
    as a command line option to the postmaster, such as
<programlisting>
postmaster --log-connections=yes --syslog=2
</programlisting>
    which would have the same effect as the previous example.
   </para>

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

   <para>
    Finally, some options can be changed in individual SQL sessions
    with the <command>SET</command> command, for example
<screen>
=&gt; <userinput>SET ENABLE_SEQSCAN TO OFF;</userinput>
</screen>
    See the SQL command language reference for details on the syntax.
   </para>

   <sect2 id="runtime-config-optimizer">
    <title>Planner and Optimizer Tuning</title>

   <para>
    <variablelist>
     <varlistentry>
      <term>CPU_INDEX_TUPLE_COST (<type>floating point</type>)</term>
      <listitem>
       <para>
        Sets the query optimizer's estimate of the cost of processing
	each index tuple during an index scan. This is measured as a
	fraction of the cost of a sequential page fetch.
       </para>
      </listitem>
     </varlistentry>
    
     <varlistentry>
      <term>CPU_OPERATOR_COST (<type>floating point</type>)</term>
      <listitem>
       <para>
        Sets the optimizer's estimate of the cost of processing each
	operator in a WHERE clause. This is measured as a fraction of
	the cost of a sequential page fetch.
       </para>
      </listitem>
     </varlistentry>
    
     <varlistentry>
      <term>CPU_TUPLE_COST (<type>floating point</type>)</term>
      <listitem>
       <para>
        Sets the query optimizer's estimate of the cost of processing
	each tuple during a query. This is measured as a fraction of
	the cost of a sequential page fetch.
       </para>
      </listitem>
     </varlistentry>
    
     <varlistentry>
      <term>EFFECTIVE_CACHE_SIZE (<type>floating point</type>)</term>
      <listitem>
       <para>
        Sets the optimizer'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>Postgres</productname> data files). This is
	measured in disk pages, which are normally 8kB apiece.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>ENABLE_HASHJOIN (<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 mostly useful to debug the
        query planner.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>ENABLE_INDEXSCAN (<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 mostly useful to debug the
        query planner.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>ENABLE_MERGEJOIN (<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 mostly useful to debug the
        query planner.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>ENABLE_NESTLOOP (<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 is any other method available.
	The default is on. This is mostly useful to debug the query
	planner.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>ENABLE_SEQSCAN (<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 is any other method available.
	The default is on. This is mostly useful to debug the query
	planner.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>ENABLE_SORT (<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 is any other method available. The default
	is on. This is mostly useful to debug the query planner.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>ENABLE_TIDSCAN (<type>boolean</type>)</term>
      <listitem>
       <para>
        Enables or disables the query planner's use of TID scan plan
        types. The default is on. This is mostly useful to debug the
        query planner.
       </para>
      </listitem>
     </varlistentry>

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

     <varlistentry>
      <term>GEQO_EFFORT (<type>integer</type>)</term>
      <term>GEQO_GENERATIONS (<type>integer</type>)</term>
      <term>GEQO_POOL_SIZE (<type>integer</type>)</term>
      <term>GEQO_RANDOM_SEED (<type>integer</type>)</term>
      <term>GEQO_SELECTION_BIAS (<type>floating point</type>)</term>
      <listitem>
       <para>
        Various tuning parameters for the genetic query optimization
        algorithm: The pool size is the number of individuals in one
        population. Valid values are between 128 and 1024. If it is
        set to 0 (the default) a pool size of 2^(QS+1), where QS
        is the number of relations in the query, is taken. The effort
        is used to calculate a default for generations. Valid values
        are between 1 and 80, 40 being the default. Generations
        specifies the number of iterations in the algorithm. The
        number must be a positive integer. If 0 is specified then
        Effort * Log2(PoolSize) is used. 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. The random seed can be set to get
        reproduceable results from the algorithm. If it is set to -1
        then the algorithm behaves non-deterministically.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>GEQO_RELS (<type>integer</type>)</term>
      <listitem>
       <para>
        Only use genetic query optimization for queries with at least
        this many relations involved. The default is 11. For less
        relations it is probably more efficient to use the
        deterministic, exhaustive planner.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>KSQO (<type>boolean</type>)</term>
      <listitem>
       <para>
        The <firstterm>Key Set Query Optimizer</firstterm>
        (<abbrev>KSQO</abbrev>) causes the query planner to convert
        queries whose WHERE clause contains many OR'ed AND clauses
        (such as <literal>WHERE (a=1 AND b=2) OR (a=2 AND b=3)
        ...</literal>) into a UNION query. This method can be faster
        than the default implementation, but it doesn't necessarily
        give exactly the same results, since UNION implicitly adds a
        SELECT DISTINCT clause to eliminate identical output rows.
        KSQO is commonly used when working with products like
        <productname>Microsoft Access</productname>, which tend to
        generate queries of this form.
       </para>

       <para>
        The KSQO algorithm used to be absolutely essential for queries
        with many OR'ed AND clauses, but in
        <productname>Postgres</productname> 7.0 and later the standard
        planner handles these queries fairly successfully. Hence the
        default is OFF.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>RANDOM_PAGE_COST (<type>floating point</type>)</term>
      <listitem>
       <para>
        Sets the query optimizer'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.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>

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

   </sect2>

   <sect2 id="logging">
    <title>Logging and Debugging</title>

   <para>
    <variablelist>
     <varlistentry>
      <term>DEBUG_ASSERTIONS (<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 <literal>USE_ASSERT_CHECKING</>
        must be defined when Postgres is built.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>DEBUG_LEVEL (<type>integer</type>)</term>
      <listitem>
       <para>
        The higher this value is set, the more
        <quote>debugging</quote> output of various sorts is generated
        in the server log during operation. This option is 0 by
        default, which means no debugging output. Values up to about 4
        currently make sense.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>DEBUG_PRINT_PARSE (<type>boolean</type>)</term>
      <term>DEBUG_PRINT_PLAN (<type>boolean</type>)</term>
      <term>DEBUG_PRINT_REWRITTEN (<type>boolean</type>)</term>
      <term>DEBUG_PRINT_QUERY (<type>boolean</type>)</term>
      <term>DEBUG_PRETTY_PRINT (<type>boolean</type>)</term>
      <listitem>
       <para>
        For any executed query, prints either the query, the parse
        tree, the execution plan, or the query rewriter output to the
        server log. <option>DEBUG_PRETTY_PRINT</option> selects are
        nicer but longer output format.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>HOSTLOOKUP (<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>LOG_CONNECTIONS (<type>boolean</type>)</term>
      <listitem>
       <para>
        Prints a line informing about each successful connection to
        the server log. This is off by default, although it is
        probably very useful. This option can only be set at server
        start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>LOG_PID (<type>boolean</type>)</term>
      <listitem>
       <para>
        Prefixes each server log message with the process id of the
        backend process. This is useful to sort out which messages
        pertain to which connection. The default is off.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>LOG_TIMESTAMP (<type>boolean</type>)</term>
      <listitem>
       <para>
        Prefixes each server log message with a timestamp. The default
        is off.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>SHOW_QUERY_STATS (<type>boolean</type>)</term>
      <term>SHOW_PARSER_STATS (<type>boolean</type>)</term>
      <term>SHOW_PLANNER_STATS (<type>boolean</type>)</term>
      <term>SHOW_EXECUTOR_STATS (<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.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>SHOWPORTNUMBER (<type>boolean</type>)</term>
      <listitem>
       <para>
        Shows the 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>

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

     <varlistentry>
      <term>TRACE_NOTIFY (<type>boolean</type>)</term>
      <listitem>
       <para>
        Generates a great amount of debugging output for the
        <command>LISTEN</command> and <command>NOTIFY</command>
        commands.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>
   </sect2>

   <sect2 id="runtime-config-general">
    <title>General operation</title>

   <para>
    <variablelist>
     <varlistentry>
      <term>DEADLOCK_TIMEOUT (<type>integer</type>)</term>
      <listitem>
       <para>
        <productname>Postgres</productname> assumes that if
        transactions are stuck for this many milliseconds then a
        deadlock has occurred. Although it is technically possible to
        detect deadlocks <quote>properly</quote>, the present
        optimistic approach is much more efficient in practice. If you get
        too many deadlock detected messages when you provably did not
        have one, you might want to try raising this value. The
        default is 1000 (i.e., one second). This option can only be
        set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>FSYNC (<type>boolean</type>)</term>
      <listitem>
       <para>
        When this is on (default), an <function>fsync()</function>
        call is done after each transaction. Turning this off
        increases performance but an operating system crash or power
        outage might cause data corruption. (Note that a crash of
        <productname>Postgres</productname> itself is not affected.)
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>MAX_CONNECTIONS (<type>integer</type>)</term>
      <listitem>
       <para>
        Determines how many concurrent connections the database server
        will allow. The default is 32. There is also a compiled-in
        hard upper limit on this option, which is currently 1024. This
        parameter can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

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

     <varlistentry>
      <term>TCPIP_SOCKET (<type>boolean</type>)</term>
      <listitem>
       <para>
        If this is true, then the server will accept TCP/IP
        connections. 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>PORT (<type>integer</type>)</term>
      <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>SHMEM_BUFFERS (<type>integer</type>)</term>
      <listitem>
       <para>
        Sets the number of shared memory buffers the database server
        will use. The default is 64. Each buffer is typically 8192
        bytes. This option can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>SORT_MEM (<type>integer</type>)</term>
      <listitem>
       <para>
	Specifies the amount of memory to be used by internal sorts
	and hashes before resorting to temporary disk files. The value
	is specified in kilobytes, and defaults to 512 kilobytes. Note
	that for a complex query, several sorts and/or hashes might be
	running in parallel, and each one will be allowed to use as
	much memory as this value specifies before it starts to put
	data into temporary files.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>SQL_INHERITANCE (<type>bool</type>)</term>
      <listitem>
       <para>
        This controls the inheritance semantics, in particular whether
        subtables are included into the consideration of various
        commands by default. This was not the case in versions prior
        to 7.1. If you need the old behaviour 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>
        keyword to exclude subtables. See the SQL language reference
        and the <citetitle>User's Guide</citetitle> for more
        information about inheritance.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>
   </sect2>

   <sect2 id="runtime-config-short">
    <title>Short options</title>
   <para>
    For convenience there are also single letter option switches
    available for many parameters. They are described in the following
    table.

    <table>
     <title>Short option key</title>
     <tgroup cols="3">
      <colspec colnum="3" align="center">
      <thead>
       <row>
        <entry>Short option</entry>
        <entry>Equivalent</entry>
        <entry>Remark</entry>
       </row>
      </thead>
      <tbody>
       <row>
        <entry>-B <replaceable>x</replaceable></entry>
        <entry>shmem_buffers = <replaceable>x</replaceable></entry>
        <entry></entry>
       </row>
       <row>
        <entry>-d <replaceable>x</replaceable></entry>
        <entry>debug_level = <replaceable>x</replaceable></entry>
        <entry></entry>
       </row>
       <row>
        <entry>-F</entry>
        <entry>fsync = off</entry>
        <entry></entry>
       </row>
       <row>
        <entry>-i</entry>
        <entry>tcpip_socket = on</entry>
        <entry></entry>
       </row>
       <row>
        <entry>-N <replaceable>x</replaceable></entry>
        <entry>max_connections = <replaceable>x</replaceable></entry>
        <entry></entry>
       </row>
       <row>
        <entry>-p <replaceable>x</replaceable></entry>
        <entry>port = <replaceable>x</replaceable></entry>
        <entry></entry>
       </row>

       <row>
        <entry>-fi, -fh, -fm, -fn, -fs, -ft</entry>
        <entry>enable_indexscan=off, enable_hashjoin=off,
        enable_mergejoin=off, enable_nestloop=off, enable_seqscan=off,
        enable_tidscan=off</entry>
        <entry>*</entry>
       </row>
       <row>
        <entry>-S <replaceable>x</replaceable></entry>
        <entry>sort_mem = <replaceable>x</replaceable></entry>
        <entry>*</entry>
       </row>
       <row>
        <entry>-s</entry>
        <entry>show_query_stats = on</entry>
        <entry>*</entry>
       </row>
       <row>
        <entry>-tpa, -tpl, -te</entry>
        <entry>show_parser_stats=on, show_planner_stats=on, show_executor_stats=on</entry>
        <entry>*</entry>
       </row>
      </tbody>
     </tgroup>
    </table>
    For historical reasons, options marked <quote>*</quote> must be
    passed to the individual backend process via the
    <option>-o</option> postmaster option, for example,
<screen>
&gt; <userinput>postmaster -o '-S 1024 -s'</userinput>
</screen>
    or via <envar>PGOPTIONS</envar> from the client side, as explained
    above.
   </para>

   </sect2>
 </sect1>

 <sect1 id="postmaster-shutdown">
  <title>Shutting down the server</title>

  <para>
   Depending on your needs, there are several ways to shut down the
   database server when your work is done. The differentiation is
   done by what signal you send to the server process.
   <variablelist>
    <varlistentry>
     <term>SIGTERM</term>
     <listitem>
      <para>
       After receiving SIGTERM, the postmaster disallows new
       connections but lets active backend end their work and shuts
       down only after all of them terminated (by client request).
       This is the <firstterm>Smart Shutdown</firstterm>.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>SIGINT</term>
     <listitem>
      <para>
       The postmaster disallows new connections, sends all active
       backends SIGTERM (which will cause them to abort immediately),
       waits for children to exit and shuts down the data base. This
       is the <firstterm>Fast Shutdown</firstterm>.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>SIGQUIT</term>
     <listitem>
      <para>
       This is the <firstterm>Immediate Shutdown</firstterm> which
       will cause the postmaster to send a SIGUSR1 to all backends and
       exit immediately (without properly shutting down the database
       system). When WAL is implemented, this will lead to recovery on
       startup. Right now it's not recommendable to use this option.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>

   <caution>
    <para>
     If at all possible, do not use SIGKILL to shut down the
     postmaster. This can cause data corruption and will prevent the
     cleaning up of shared memory resources, which you will have to
     do yourself in that case.
    </para>
   </caution>

   The PID of the postmaster process can be found using the
   <application>ps</application> program, or from the file
   <filename>postmaster.pid</filename> in the data directory. So for
   example, to do a fast shutdown:
<screen>
&gt; <userinput>kill -INT `cat /usr/local/pgsql/data/postmaster.pid`</userinput>
</screen>
  </para>
  <para>
   The program <application>pg_ctl</application> is a shell script
   wrapper that provides a convenient interface to these functions.
  </para>
 </sect1>

 <sect1>
  <title>Secure TCP/IP Connection with SSH</title>

  <note>
   <title>Acknowledgement</title>
   <para>
    Idea taken from an email by Gene Selkov, Jr.
    (<email>selkovjr@mcs.anl.gov</>) written on 1999-09-08 in response
    to a question from Eric Marsden.
   </para>
  </note>

  <para>
   One can use <productname>ssh</productname> to encrypt the network
   connection between clients and a
   <productname>Postgres</productname> server. Done properly, this
   should lead to an adequately secure network connection.
  </para>

  <para>
   First make sure that an <productname>ssh</productname> server is
   running properly on the same machine as
   <productname>Postgres</productname> and that you can log in using
   ssh as some user. Then you can establish a secure tunnel with a
   command like this from the client machine:
<programlisting>
&gt; <userinput>ssh -L 3333:foo.com:5432 joe@foo.com</userinput>
</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 backend 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 ssh as
   joe@foo.com, just as if you had attempted to use ssh to set up a
   terminal session.
  </para>

 </sect1>

</Chapter>

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