postgres

postgres

1

Application

postgres

PostgreSQL

database server

postgres

option

Description
===========

`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` instance. The `postgres` instance then starts a
separate server process to handle the connection.

One `postgres` instance always manages the data of 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` instance 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 [???](#app-initdb). Other possible
file layouts are discussed in [???](#runtime-config-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 [???](#app-initdb).
Sometimes it is used for debugging or disaster recovery; 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.

Options {#app-postgres-options}
=======

`postgres` accepts the following command-line arguments. For a detailed
discussion of the options consult [???](#runtime-config). 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.

General Purpose
---------------

`-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. Specifying this option is equivalent to setting the
    [???](#guc-shared-buffers) configuration parameter.

`-c name=value`

:   Sets a named run-time parameter. The configuration parameters
    supported by PostgreSQL are described in [???](#runtime-config).
    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.

`-C name`

:   Prints the value of the named run-time parameter, and exits. (See
    the `-c` option above for details.) This returns values from
    `postgresql.conf`, modified by any parameters supplied in this
    invocation. It does not reflect parameters supplied when the cluster
    was started.

    This can be used on a running server for most parameters. However,
    the server must be shut down for some runtime-computed parameters
    (e.g., [???](#guc-shared-memory-size),
    [???](#guc-shared-memory-size-in-huge-pages), and
    [???](#guc-wal-segment-size)).

    This option is meant for other programs that interact with a server
    instance, such as [???](#app-pg-ctl), to query configuration
    parameter values. User-facing applications should instead use
    [`SHOW`](#sql-show) or the pg\_settings view.

`-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 database configuration
    files. See [???](#runtime-config-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 [???](#datatype-datetime)
    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 [???](#guc-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 [???](#guc-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 [???](#guc-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 value can also be a comma-separated list of directories. An
    empty value specifies not listening on any Unix-domain sockets, in
    which case only TCP/IP sockets can be used to connect to the server.
    The default value is normally `/tmp`, but that can be changed at
    build time. Specifying this option is equivalent to setting the
    [???](#guc-unix-socket-directories) configuration parameter.

`-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 [???](#ssl-tcp).

`-N max-connections`

:   Sets the maximum number of client connections that this server will
    accept. The default value of this parameter is chosen automatically
    by initdb. Specifying this option is equivalent to setting the
    [???](#guc-max-connections) configuration parameter.

`-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 base amount of memory to be used by sorts and hash
    tables before resorting to temporary disk files. See the description
    of the `work_mem` configuration parameter in
    [???](#runtime-config-resource-memory).

`-V`; `--version`

:   Print the postgres version and exit.

`--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.

`-?`; `--help`

:   Show help about postgres command line arguments, and exit.

Semi-Internal Options
---------------------

The options described here are 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.
They are listed here only for use by PostgreSQL system developers.
Furthermore, these options might change or be removed in a future
release without notice.

`-f` `{ s | i | o | b | t | n | m | h }`

:   Forbids the use of particular scan and join methods: `s` and `i`
    disable sequential and index scans respectively, `o`, `b` and `t`
    disable index-only scans, bitmap index scans, and TID 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.

Options for Single-User Mode
----------------------------

single-user mode

The following options only apply to the single-user mode (see
[Single-User Mode](#app-postgres-single-user) below).

`--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. This must be the
    last argument on the command line. If it is omitted it defaults to
    the user name.

`-E`

:   Echo all commands to standard output before executing them.

`-j`

:   Use semicolon followed by two newlines, rather than just newline, as
    the command entry terminator.

`-r` filename

:   Send all server log output to filename. This option is only honored
    when supplied as a command-line option.

Environment
===========

PGCLIENTENCODING

:   Default character encoding used by clients. (The clients can
    override this individually.) This value can also be set in the
    configuration file.

PGDATA

:   Default data directory location

PGDATESTYLE

:   Default value of the [???](#guc-datestyle) run-time parameter. (The
    use of this environment variable is deprecated.)

PGPORT

:   Default port number (preferably set in the configuration file)

Diagnostics
===========

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 [???](#kernel-resources). You might
be able to postpone reconfiguring your kernel by decreasing
[???](#guc-shared-buffers) to reduce the shared memory consumption of
PostgreSQL, and/or by reducing [???](#guc-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 can remove the lock file mentioned in the message and
try again.

A failure message indicating inability to bind to a port might indicate
that that port is already in use by some non-PostgreSQL process. You
might 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 might 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.

Notes
=====

The utility command [???](#app-pg-ctl) can be used to start and shut
down the `postgres` server safely and comfortably.

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 might 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.

To cancel a running query, send the `SIGINT` signal to the process
running that command. To terminate a backend process cleanly, send
`SIGTERM` to that process. See also `pg_cancel_backend` and
`pg_terminate_backend` in [???](#functions-admin-signal) for the
SQL-callable equivalents of these two actions.

The `postgres` server uses `SIGQUIT` to tell subordinate server
processes to terminate without normal cleanup. This signal *should not*
be used by users. It is also unwise to send `SIGKILL` to a server
process MDASH 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.

Bugs {#app-postgres-bugs}
====

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.

Single-User Mode {#app-postgres-single-user}
================

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. The backslash
and adjacent newline are both dropped from the input command. Note that
this will happen even when within a string literal or comment.

But if you use the `-j` command line switch, a single newline does not
terminate command entry; instead, the sequence semicolon-newline-newline
does. That is, type a semicolon immediately followed by a completely
empty line. Backslash-newline is not treated specially in this mode.
Again, there is no intelligence about such a sequence appearing within a
string literal or comment.

In either input mode, if you type a semicolon that is not just before or
part of a command entry terminator, it is considered a command
separator. When you do type a command entry terminator, the multiple
statements you\'ve entered will be executed as a single transaction.

To quit the session, type EOF ([Control+D]{.keycombo}, usually). If
you\'ve entered any text since the last command entry terminator, then
EOF will be taken as a command entry terminator, and another EOF will be
needed to exit.

Note that the single-user mode server does not provide sophisticated
line-editing features (no command history, for example). Single-user
mode also does not do any background processing, such as automatic
checkpoints or replication.

Examples {#app-postgres-examples}
========

To start `postgres` in the background using default values, type:

    $ nohup postgres >logfile 2>&1 </dev/null &

To start `postgres` with a specific port, e.g., 1234:

    $ postgres -p 1234

To connect to this server using psql, specify this port with the -p
option:

    $ 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.

See Also
========

[???](#app-initdb), [???](#app-pg-ctl)
