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<!--
$Header: /cvsroot/pgsql/doc/src/sgml/runtime.sgml,v 1.17 2000/07/22 14:49:00 petere Exp $
-->
<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>
> <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> <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>
> <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>
> <userinput>postmaster -D /usr/local/pgsql/data > logfile 1>&2 &</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>
> <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(key=5440001, size=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 System-V-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(key=5440026, num=16, 01600) 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 System
V 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>
<para>
Details about configuring System V IPC facilities are given in
<xref linkend="sysvipc">.
</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 what
this file could look like is:
<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>
=> <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>
This is the amount of time, in milliseconds, to wait on a lock
before checking to see if there is a deadlock condition or not.
The check for deadlock is relatively slow, so we don't want to
run it every time we wait for a lock. We (optimistically?)
assume that deadlocks are not common in production applications,
and just wait on the lock for awhile before starting to ask
questions about whether it can ever get unlocked.
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 the lock will be released before the waiter decides to
check for deadlock.
This option can only be set at server start.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>FSYNC (<type>boolean</type>)</term>
<listitem>
<para>
If this is option is on, the <productname>Postgres</> backend
will use the <function>fsync()</> system call in several
places to make sure that updates are physically written to
disk and will not hang around in the write caches. This
increases the chance that a database installation will still
be usable after a operating system or hardware crashes by a
large amount. (Crashes of the database server itself do
<emphasis>not</> affect this consideration.)
</para>
<para>
However, this operation severely slows down
<productname>Postgres</>, because at all those points it has
to block and wait for the operating system to flush the
buffers. Without <function>fsync</>, the operating system is
allowed to do its best in buffering, sorting, and delaying
writes, so this can be a <emphasis>very</> big perfomance
increase. However, if the system crashes, parts of the data of
a transaction that has already been committed -- according to
the information on disk -- will still hang around in memory.
Inconsistent data (i.e., data corruption) is therefore likely
to occur.
</para>
<para>
This option is the subject of an eternal debate in the
<productname>Postgres</> user and developer communities. Some
always leave it off, some turn it off only for bulk loads,
where there is a clear restart point if something goes wrong,
some leave it on just to be on the safe side. Because it is
the safe side, on is also the default. If you trust your
operating system, your utility company, and your hardware, you
might want to disable it.
</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 value, which is typically 1024
(both numbers can be altered when compiling the server). 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>
> <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="kernel-resources">
<title>Managing Kernel Resources</title>
<para>
A large <productname>Postgres</> installation can quickly hit
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 then keep reading.
</para>
<sect2 id="sysvipc">
<title>Shared Memory and Semaphores</title>
<para>
Shared memory and semaphores are collectively referred to as
<quote>System V IPC</> (together with message queues, which are
not relevant for <productname>Postgres</>). 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 QNX port,
<productname>Postgres</> 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 postmaster start. In
that case there's nothing left to do but to reconfigure your
kernel -- <productname>Postgres</> won't work without them.
</para>
<para>
When <productname>Postgres</> exceeds one of the various hard
limits of the IPC resources then the postmaster will refuse to
start up and should leave a marginally instructive error message
about which problem was encountered and what needs to be done
about it. The relevant kernel parameters have a relatively
consistent nomenclature across systems; <xref
linkend="sysvipc-parameters"> gives an overview. The methods to
set them, however, vary; suggestions for some platforms are given
below. Be aware, however, that you will have to reboot your
machine at best, or possibly recompile the kernel, to change these
settings.
</para>
<table id="sysvipc-parameters">
<title>System V 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>512 kB + 8192 * buffers + extra ... infinity</entry>
</row>
<row>
<entry><varname>SHMMIN</></>
<entry>Minimum size of shared memory segment (bytes)</>
<entry>1 (at most 144)</>
</row>
<row>
<entry><varname>SHMSEG</></>
<entry>Maximum number of shared memory segments per process</>
<entry>Must be at least 3, but the default is much higher.</>
</row>
<row>
<entry><varname>SHMMNI</></>
<entry>Maximum number of shared memory segments system-wide</>
<entry>like <varname>SHMSEG</> + room for other applications</>
</row>
<row>
<entry><varname>SEMMNI</></>
<entry>Maximum number of semaphore identifiers (i.e., sets)</>
<entry>>= ceil(max_connections % 16)</>
</row>
<row>
<entry><varname>SEMMNS</></>
<entry>Maximum number of semaphores system-wide</>
<entry>number of allowed connections, rounded up to multiple of 16</>
</row>
<row>
<entry><varname>SEMMSL</></>
<entry>Maximum number of semaphores per set</>
<entry>>= 16</>
</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>>= 255 (The default is often 32767, don't change unless asked to.)</>
</row>
</tbody>
</tgroup>
</table>
<para>
The most important shared memory parameter is <varname>SHMMAX</>,
the maximum size, in bytes, that a shared memory segment can have.
If you get an error message from <function>shmget</> along the
lines of <errorname>Invalid argument</> then it is possible that
this limit has been exceeded. The size of the required shared
memory segments 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 dominant item.
(You can therefore, as a temporary solution, lower these settings
to get rid of the failures.) As a rough approximation you can
estimate the required segment size as the number of buffers times
the block size (8192 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. (<productname>Postgres</> will
actually use three shared memory segments, but the size of the
other two is negligible for this consideration.)
</para>
<para>
Less likely to cause problems is the minimum size for shared
memory segments (<varname>SHMMIN</>), which must be at least 144
for <productname>Postgres</> (it's usually just 1), and the
maximum number of segments system-wide (<varname>SHMMNI</>, as
mentioned, 3 are needed) or per-process (<varname>SHMSEG</>,
ditto). 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>Postgres</> uses one semaphore per allowed connection
(<option>-N</> option), in sets of 16. The maximum number of
semaphores in the system is set by <varname>SEMMNS</>, which
consequently must be at least as high as the connection setting.
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 <quote><errorname>No space left on
device</></>, from the function <function>semget()</>.
</para>
<para>
In some cases it might also turn out to 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 gets lost (until reboot). Fragmentation
of the semaphore space could therefore over time lead to less
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 16 for
<productname>Postgres</>.
</para>
<para>
Various settings related to <quote>semaphore undo</>, such as
<varname>SEMMNU</> and <varname>SEMUME</>, are not of relevance
with <productname>Postgres</>.
</para>
<para>
<variablelist>
<varlistentry>
<term>BSD/OS</>
<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 number of buffers supported by the
postmaster, increase <varname>SHMMAXPGS</> by 1024 for every
additional 4 MB of shared memory:
<programlisting>
/sys/sys/shm.h:69:#define SHMMAXPGS 1024 /* max hardware pages... */
</programlisting>
The default setting of 1024 is for a maximum of 4 MB of shared
memory.
</para>
</formalpara>
<para>
For those running 4.1 or later, just recompile the kernel and
reboot. For those running earlier releases, use
<application>bpatch</> to find the <varname>sysptsize</> value
for the current kernel. This is computed dynamically at
bootup.
<screen>
$ <userinput>bpatch -r sysptsize</>
<computeroutput>0x9 = 9</>
</screen>
Next, change <varname>SYSPTSIZE</> to a hard-coded value. Use
the bpatch value, plus add 1 for every additional 4 MB of
shared memory you desire.
<programlisting>
/sys/i386/i386/i386_param.c:28:#define SYSPTSIZE 0 /* dynamically... */
</programlisting>
<varname>sysptsize</> can not be changed by sysctl on the fly.
</para>
<formalpara>
<title>Semaphores</>
<para>
You may need to increase the number of semaphores. By
default, <productname>Postgres</> allocates 32 semaphores,
one for each backend connection. This is just over half the
default system total of 60.
</para>
</formalpara>
<para>
The defaults are in <filename>/sys/sys/sem.h</>:
<programlisting>
/* Configuration parameters */
#ifndef SEMMNI
#define SEMMNI 10 /* # of semaphore identifiers */
#endif
#ifndef SEMMNS
#define SEMMNS 60 /* # of semaphores in system */
#endif
#ifndef SEMUME
#define SEMUME 10 /* max # of undo entries per process */
#endif
#ifndef SEMMNU
#define SEMMNU 30 /* # of undo structures in system */
#endif
</programlisting>
Set the values you want in your kernel config file, e.g.:
<programlisting>
options "SEMMNI=40"
options "SEMMNS=240"
options "SEMUME=40"
options "SEMMNU=120"
</programlisting>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>FreeBSD</>
<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 on 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>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>HPUX</>
<listitem>
<para>
The default settings tend to suffice for normal installations.
On <productname>HPUX</> 10, the factory default for
<varname>SEMMNS</> is 128, which might be too low for larger
database sites.
</para>
<para>
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>Linux</>
<listitem>
<para>
System V IPC is enabled by default and sufficiently sized for
most uses. The relevant parameters are in
<filename>/usr/src/linux/include/asm-<replaceable>xxx</>/shmparam.h</>
and <filename>/usr/src/linux/include/linux/sem.h</>. Be sure
to do <command>make dep</> before rebuilding the kernel.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>SCO OpenServer</>
<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 the directory to
<filename>/etc/conf/cf.d</>. To display the current value of
<varname>SHMMAX</>, in bytes, 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>Solaris</>
<listitem>
<para>
At least in version 2.6, the maximum size of a shared memory
segment is set too low for <productname>Postgres</>. 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 to make the changes effective.
</para>
<para>
See also <ulink
url="http://www.sunworld.com/swol-09-1997/swol-09-insidesolaris.html">http://www.sunworld.com/swol-09-1997/swol-09-insidesolaris.html</>
for information on shared memory under
<productname>Solaris</>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>UnixWare</>
<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, in bytes. 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>
<note>
<para>
If your platform is not listed here, please consider
contributing some information.
</para>
</note>
</para>
</sect2>
<!--
Other fun things to write about one day:
* number of processes per user and system-wide (soft/hard limit)
* open files/inodes per user and system-wide (soft/hard limit)
(Think about this both ways: Increasing it to allow Postgres to
open more files, and decreasing it to prevent Postgres from taking
up all file descriptors.)
* stack and data segment size, plain-old memory limit
-->
</sect1>
<sect1 id="locale">
<title>Locale Support</title>
<note>
<title>Acknowledgement</title>
<para>
Written by Oleg Bartunov. See <ulink
url="http://www.sai.msu.su/~megera/postgres/">Oleg's web
page</ulink> for additional information on locale and Russian
language support.
</para>
</note>
<para>
While doing a project for a company in Moscow, Russia, I
encountered the problem that <productname>Postgres</> had no
support of national alphabets. After looking for possible
workarounds I decided to develop support of locale myself. I'm not
a C programmer but already had some experience with locale
programming when I work with <productname>Perl</> (debugging) and
<productname>Glimpse</>. After several days of digging through the
<productname>Postgres</> source tree I made very minor corections
to <filename>src/backend/utils/adt/varlena.c</> and
<filename>src/backend/main/main.c</> and got what I needed! I did
support only for <envar>LC_CTYPE</envar> and
<envar>LC_COLLATE</envar>, but later <envar>LC_MONETARY</envar> was
added by others. I got many messages from people about this patch
so I decided to send it to developers and (to my surprise) it was
incorporated into the <productname>Postgres</> distribution.
</para>
<para>
People often complain that locale doesn't work for them. There are
several common mistakes:
<itemizedlist>
<listitem>
<para>
Didn't properly configure <productname>Postgres</> before
compilation. You must run <filename>configure</> with the
<option>--enable-locale</> option to enable locale support.
</para>
</listitem>
<listitem>
<para>
Didn't setup environment correctly when starting postmaster. You
must define environment variables <envar>LC_CTYPE</envar> and
<envar>LC_COLLATE</envar> before running postmaster because
backend gets information about locale from environment. I use
following shell script:
<programlisting>
#!/bin/sh
export LC_CTYPE=koi8-r
export LC_COLLATE=koi8-r
postmaster -B 1024 -S -D/usr/local/pgsql/data/ -o '-Fe'
</programlisting>
</para>
</listitem>
<listitem>
<para>
Broken locale support in the operating system (for example,
locale support in libc under Linux several times has changed and
this caused a lot of problems). Perl has also support of locale
and if locale is broken <command>perl -v</> will complain
something like:
<screen>
<prompt>$</> <userinput>export LC_CTYPE='not_exist'</>
<prompt>$</> <userinput>perl -v</>
<computeroutput>
perl: warning: Setting locale failed.
perl: warning: Please check that your locale settings:
LC_ALL = (unset),
LC_CTYPE = "not_exist",
LANG = (unset)
are supported and installed on your system.
perl: warning: Falling back to the standard locale ("C").
</computeroutput>
</screen>
</para>
</listitem>
<listitem>
<para>
Wrong location of locale files. Possible locations include:
<filename>/usr/lib/locale</filename> (Linux, Solaris),
<filename>/usr/share/locale</filename> (Linux),
<filename>/usr/lib/nls/loc</filename> (DUX 4.0).
Check <command>man locale</command> to find the correct
location. Under Linux I made a symbolic link between
<filename>/usr/lib/locale</filename> and
<filename>/usr/share/locale</filename> to be sure that the next
libc will not break my locale.
</para>
</listitem>
</itemizedlist>
</para>
<formalpara>
<title>What are the Benefits?</title>
<para>
You can use ~* and order by operators for strings contain
characters from national alphabets. Non-english users definitely
need that.
</para>
</formalpara>
<formalpara>
<title>What are the Drawbacks?</title>
<para>
There is one evident drawback of using locale - its speed! So, use
locale only if you really need it.
</para>
</formalpara>
</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>
> <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>
> <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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