postgres — PostgreSQL database server
postgres
[option
...]
postgres
is the
PostgreSQL database server. In order
for a client application to access a database it connects (over a
network or locally) to a running postgres
process.
The postgres
instance then starts a separate server
process to handle the connection.
One postgres
instance always manages the data from
exactly one database cluster. A database cluster is a collection
of databases that is stored at a common file system location (the
“data area”). More than one
postgres
process can run on a system at one
time, so long as they use different data areas and different
communication ports (see below). When
postgres
starts it needs to know the location
of the data area. The location must be specified by the
-D
option or the PGDATA
environment
variable; there is no default. Typically, -D
or
PGDATA
points directly to the data area directory
created by initdb. Other possible file layouts are
discussed in Section 17.2, “File Locations”.
By default postgres
starts in the
foreground and prints log messages to the standard error stream. In
practical applications postgres
should be started as a background process, perhaps at boot time.
The postgres
command can also be called in
single-user mode. The primary use for this mode is during
bootstrapping by initdb. Sometimes it is used
for debugging or disaster recovery (but note that running a single-user
server is not truly suitable for debugging the server, since no
realistic interprocess communication and locking will happen).
When invoked in single-user
mode from the shell, the user can enter queries and the results
will be printed to the screen, but in a form that is more useful
for developers than end users. In the single-user mode,
the session user will be set to the user with ID 1, and implicit
superuser powers are granted to this user.
This user does not actually have to exist, so the single-user mode
can be used to manually recover from certain
kinds of accidental damage to the system catalogs.
postgres
accepts the following command-line
arguments. For a detailed discussion of the options consult Chapter 17, Server Configuration. You can save typing most of these
options by setting up a configuration file. Some (safe) options
can also be set from the connecting client in an
application-dependent way to apply only for that session. For
example, if the environment variable PGOPTIONS
is
set, then libpq-based clients will pass that
string to the server, which will interpret it as
postgres
command-line options.
-A 0|1
Enables run-time assertion checks, which is a debugging aid to detect programming mistakes. This option is only available if assertions were enabled when PostgreSQL was compiled. If so, the default is on.
-B nbuffers
Sets the number of shared buffers for use by the server processes. The default value of this parameter is chosen automatically by initdb; refer to Section 17.4.1, “Memory” for more information.
-c name
=value
Sets a named run-time parameter. The configuration parameters
supported by PostgreSQL are
described in Chapter 17, Server Configuration. Most of the
other command line options are in fact short forms of such a
parameter assignment. -c
can appear multiple times
to set multiple parameters.
-d debug-level
Sets the debug level. The higher this value is set, the more
debugging output is written to the server log. Values are
from 1 to 5. It is also possible to pass -d
0
for a specific session, which will prevent the
server log level of the parent postgres
process from being
propagated to this session.
-D datadir
Specifies the file system location of the data directory or configuration file(s). See Section 17.2, “File Locations” for details.
-e
Sets the default date style to “European”, that is
DMY
ordering of input date fields. This also causes
the day to be printed before the month in certain date output formats.
See Section 8.5, “Date/Time Types” for more information.
-F
Disables fsync
calls for improved
performance, at the risk of data corruption in the event of a
system crash. Specifying this option is equivalent to
disabling the fsync configuration
parameter. Read the detailed documentation before using this!
-h hostname
Specifies the IP host name or address on which
postgres
is to listen for TCP/IP
connections from client applications. The value can also be a
comma-separated list of addresses, or *
to specify
listening on all available interfaces. An empty value
specifies not listening on any IP addresses, in which case
only Unix-domain sockets can be used to connect to the
server. Defaults to listening only on
localhost.
Specifying this option is equivalent to setting the listen_addresses configuration parameter.
-i
Allows remote clients to connect via TCP/IP (Internet domain)
connections. Without this option, only local connections are
accepted. This option is equivalent to setting
listen_addresses
to *
in
postgresql.conf
or via -h
.
This option is deprecated since it does not allow access to the
full functionality of listen_addresses.
It's usually better to set listen_addresses
directly.
-k directory
Specifies the directory of the Unix-domain socket on which
postgres
is to listen for
connections from client applications. The default is normally
/tmp
, but can be changed at build time.
-l
Enables secure connections using SSL. PostgreSQL must have been compiled with support for SSL for this option to be available. For more information on using SSL, refer to Section 16.7, “Secure TCP/IP Connections with SSL”.
-N max-connections
Sets the maximum number of client connections that this
server will accept. By
default, this value is 32, but it can be set as high as your
system will support. (Note that
-B
is required to be at least twice
-N
. See Section 16.4, “Managing Kernel Resources” for a discussion of
system resource requirements for large numbers of client
connections.) Specifying this option is equivalent to setting the
max_connections configuration parameter.
-o extra-options
The command line-style options specified in extra-options
are passed to
all server processes started by this
postgres
process. If the option string contains
any spaces, the entire string must be quoted.
The use of this option is obsolete; all command-line options
for server processes can be specified directly on the
postgres
command line.
-p port
Specifies the TCP/IP port or local Unix domain socket file
extension on which postgres
is to listen for connections from client applications.
Defaults to the value of the PGPORT
environment
variable, or if PGPORT
is not set, then
defaults to the value established during compilation (normally
5432). If you specify a port other than the default port,
then all client applications must specify the same port using
either command-line options or PGPORT
.
-s
Print time information and other statistics at the end of each command. This is useful for benchmarking or for use in tuning the number of buffers.
-S
work-mem
Specifies the amount of memory to be used by internal sorts and hashes
before resorting to temporary disk files. See the description of the
work_mem
configuration parameter in Section 17.4.1, “Memory”.
--name
=value
Sets a named run-time parameter; a shorter form of
-c
.
--describe-config
This option dumps out the server's internal configuration variables,
descriptions, and defaults in tab-delimited COPY
format.
It is designed primarily for use by administration tools.
There are several other options that may be specified, used mainly for debugging purposes and in some cases to assist with recovery of severely damaged databases. There should be no reason to use them in a production database setup. These are listed here only for the use by PostgreSQL system developers. Furthermore, any of these options may disappear or change in a future release without notice.
-f
{ s | i | m | n | h }
Forbids the use of particular scan and join methods:
s
and i
disable sequential and index scans respectively, while
n
, m
, and h
disable nested-loop, merge and hash joins respectively.
Neither sequential scans nor nested-loop joins can be disabled
completely; the -fs
and
-fn
options simply discourage the optimizer
from using those plan types if it has any other alternative.
-n
This option is for debugging problems that cause a server
process to die abnormally. The ordinary strategy in this
situation is to notify all other server processes that they
must terminate and then reinitialize the shared memory and
semaphores. This is because an errant server process could
have corrupted some shared state before terminating. This
option specifies that postgres
will
not reinitialize shared data structures. A knowledgeable
system programmer can then use a debugger to examine shared
memory and semaphore state.
-O
Allows the structure of system tables to be modified. This is
used by initdb
.
-P
Ignore system indexes when reading system tables (but still update the indexes when modifying the tables). This is useful when recovering from damaged system indexes.
-t
pa[rser] | pl[anner] | e[xecutor]
Print timing statistics for each query relating to each of the
major system modules. This option cannot be used together
with the -s
option.
-T
This option is for debugging problems that cause a server
process to die abnormally. The ordinary strategy in this
situation is to notify all other server processes that they
must terminate and then reinitialize the shared memory and
semaphores. This is because an errant server process could
have corrupted some shared state before terminating. This
option specifies that postgres
will
stop all other server processes by sending the signal
SIGSTOP
, but will not cause them to
terminate. This permits system programmers to collect core
dumps from all server processes by hand.
-v
protocol
Specifies the version number of the frontend/backend protocol to be used for a particular session. This option is for internal use only.
-W
seconds
A delay of this many seconds occurs when a new server process is started, after it conducts the authentication procedure. This is intended to give an opportunity to attach to the server process with a debugger.
-y
database
Indicates that this is a subprocess started by a parent
postgres
process, and specifies the database to
use. This option is for internal use only.
The following options only apply to the single-user mode.
--single
Selects the single-user mode. This must be the first argument on the command line.
database
Specifies the name of the database to be accessed. If it is omitted it defaults to the user name.
-E
Echo all commands.
-j
Disables use of newline as a statement delimiter.
-r
filename
Send all server log output to filename
. In normal multiuser
mode, this option is ignored, and stderr is
used by all processes.
PGCLIENTENCODING
Default character encoding used by clients. (The clients may override this individually.) This value can also be set in the configuration file.
PGDATA
Default data directory location
PGDATESTYLE
Default value of the DateStyle run-time parameter. (The use of this environment variable is deprecated.)
PGPORT
Default port (preferably set in the configuration file)
TZ
Server time zone
A failure message mentioning semget
or
shmget
probably indicates you need to configure your
kernel to provide adequate shared memory and semaphores. For more
discussion see Section 16.4, “Managing Kernel Resources”. You may be able
to postpone reconfiguring your kernel by decreasing shared_buffers to reduce the shared memory
consumption of PostgreSQL, and/or by reducing
max_connections to reduce the semaphore
consumption.
A failure message suggesting that another server is already running should be checked carefully, for example by using the command
$
ps ax | grep postgres
or
$
ps -ef | grep postgres
depending on your system. If you are certain that no conflicting server is running, you may remove the lock file mentioned in the message and try again.
A failure message indicating inability to bind to a port may
indicate that that port is already in use by some
non-PostgreSQL process. You may also
get this error if you terminate postgres
and immediately restart it using the same port; in this case, you
must simply wait a few seconds until the operating system closes
the port before trying again. Finally, you may get this error if
you specify a port number that your operating system considers to
be reserved. For example, many versions of Unix consider port
numbers under 1024 to be “trusted” and only permit
the Unix superuser to access them.
If at all possible, do not use
SIGKILL
to kill the main
postgres
server. Doing so will prevent
postgres
from freeing the system
resources (e.g., shared memory and semaphores) that it holds before
terminating. This may cause problems for starting a fresh
postgres
run.
To terminate the postgres
server normally, the
signals SIGTERM
, SIGINT
, or
SIGQUIT
can be used. The first will wait for
all clients to terminate before quitting, the second will
forcefully disconnect all clients, and the third will quit
immediately without proper shutdown, resulting in a recovery run
during restart. The SIGHUP
signal will reload
the server configuration files. It is also possible to send
SIGHUP
to an individual server process, but that
is usually not sensible.
The utility command pg_ctl can be used to
start and shut down the postgres
server
safely and comfortably.
To cancel a running query, send the SIGINT
signal
to the process running that command.
The postgres
server uses SIGTERM
to tell subordinate server processes to quit normally and
SIGQUIT
to terminate without the normal cleanup.
These signals should not be used by users. It
is also unwise to send SIGKILL
to a server
process — the main postgres
process will
interpret this as a crash and will force all the sibling processes
to quit as part of its standard crash-recovery procedure.
The --
options will not work on FreeBSD or OpenBSD.
Use -c
instead. This is a bug in the affected operating
systems; a future release of PostgreSQL
will provide a workaround if this is not fixed.
To start a single-user mode server, use a command like
postgres --single -D /usr/local/pgsql/data other-options
my_database
Provide the correct path to the database directory with -D
, or
make sure that the environment variable PGDATA
is set.
Also specify the name of the particular database you want to work in.
Normally, the single-user mode server treats newline as the command entry terminator; there is no intelligence about semicolons, as there is in psql. To continue a command across multiple lines, you must type backslash just before each newline except the last one.
But if you use the -j
command line switch, then newline does
not terminate command entry. In this case, the server will read the standard input
until the end-of-file (EOF) marker, then
process the input as a single command string. Backslash-newline is not
treated specially in this case.
To quit the session, type EOF
(Control+D, usually).
If you've
used -j
, two consecutive EOFs are needed to exit.
Note that the single-user mode server does not provide sophisticated line-editing features (no command history, for example).
To start postgres
in the background
using default values, type:
$
nohup postgres >logfile 2>&1 </dev/null &
To start postgres
with a specific
port:
$
postgres -p 1234
This command will start up postgres
communicating through the port 1234. In order to connect to this
server using psql, you would need to run it as
$
psql -p 1234
or set the environment variable PGPORT
:
$
export PGPORT=1234
$
psql
Named run-time parameters can be set in either of these styles:
$
postgres -c work_mem=1234
$
postgres --work-mem=1234
Either form overrides whatever setting might exist for
work_mem
in postgresql.conf
. Notice that
underscores in parameter names can be written as either underscore
or dash on the command line. Except for short-term experiments,
it's probably better practice to edit the setting in
postgresql.conf
than to rely on a command-line switch
to set a parameter.