PostgreSQL User's Guide
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# postgres

## Name

postgres — Run a Postgres single-user backend

    postgres [ dbname ]
    postgres [ -B nBuffers ] [ -C ] [ -D DataDir ] [ -E ] [ -F ]
       [ -O ] [ -P ] [ -Q ] [ -S SortSize ] [ -d [ DebugLevel ] ] [ -e ]
       [ -o ] [ OutputFile ] [ -s ] [ -v protocol ] [ dbname ]

### Inputs

postgres accepts the following command line arguments:

`dbname`

  The optional argument `dbname` specifies the name of the database to be accessed. `dbname` defaults to the value of the `USER` environment variable.

-B `nBuffers`

  If the backend is running under the postmaster, `nBuffers` is the number of shared-memory buffers that the postmaster has allocated for the backend server processes that it starts. If the backend is running standalone, this specifies the number of buffers to allocate. This value defaults to 64 buffers, where each buffer is 8k bytes (or whatever BLCKSZ is set to in config.h).

-C

  Do not show the server version number.

-D `DataDir`

  Specifies the directory to use as the root of the tree of database directories. If -D is not given, the default data directory name is the value of the environment variable `PGDATA`. If `PGDATA` is not set, then the directory used is `$POSTGRESHOME/data`. If neither environment variable is set and this command-line option is not specified, the default directory that was set at compile-time is used.

-E

  Echo all queries.

-F

  Disable an automatic `fsync()` call after each transaction. This option improves performance, but an operating system crash while a transaction is in progress may cause the loss of the most recently entered data. Without the `fsync()` call the data is buffered by the operating system, and written to disk sometime later.

-O

  Override restrictions, so system table structures can be modified. These tables are typically those with a leading "`pg_`" in the table name.

-P

  Ignore system indexes to scan/update system tuples. REINDEX for system tables/indexes requires this option. System tables are typically those with a leading "`pg_`" in the table name.

-Q

  Specifies "quiet" mode.

-S `SortSize`

  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 as `SortSize` kilobytes before it starts to put data into temporary files.

-d [ `DebugLevel` ]

  The optional argument `DebugLevel` determines the amount of debugging output the backend servers will produce. If `DebugLevel` is one, the postmaster will trace all connection traffic, and nothing else. For levels two and higher, debugging is turned on in the backend process and the postmaster displays more information, including the backend environment and process traffic. Note that if no file is specified for backend servers to send their debugging output then this output will appear on the controlling tty of their parent postmaster.

-e

  This option controls how dates are interpreted upon input to and output from the database. If the -e option is supplied, then dates passed to and from the frontend processes will be assumed to be in "European" format (`DD-MM-YYYY`), otherwise dates are assumed to be in "American" format (`MM-DD-YYYY`). Dates are accepted by the backend in a wide variety of formats, and for input dates this switch mostly affects the interpretation for ambiguous cases. See the PostgreSQL User's Guide for more information.

-o `OutputFile`

  Sends all debugging and error output to `OutputFile`. If the backend is running under the postmaster, error messages are still sent to the frontend process as well as to `OutputFile`, but debugging output is sent to the controlling tty of the postmaster (since only one file descriptor can be sent to an actual file).

-s

  Print time information and other statistics at the end of each query. This is useful for benchmarking or for use in tuning the number of buffers.

-v `protocol`

  Specifies the number of the frontend/backend protocol to be used for this particular session.

There are several other options that may be specified, used mainly for debugging purposes. These are listed here only for the use by Postgres system developers. Use of any of these options is highly discouraged. Furthermore, any of these options may disappear or change at any time.

These special-case options are:

-A n|r|b|Q\fIn\fP|X\fIn\fP

  This option generates a tremendous amount of output.

-L

  Turns off the locking system.

-N

  Disables use of newline as a query delimiter.

-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.

  > Note: 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.

-i

  Prevents query execution, but shows the plan tree.

-p `dbname`

  Indicates to the backend server that it has been started by a postmaster and make different assumptions about buffer pool management, file descriptors, etc. Switches following -p are restricted to those considered "secure".

-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 with -s.

### Outputs

Of the nigh-infinite number of error messages you may see when you execute the backend server directly, the most common will probably be:

`semget: No space left on device`

  If you see this message, you should run the ipcclean command. After doing this, try starting postmaster again. If this still doesn't work, you probably need to configure your kernel for shared memory and semaphores as described in the installation notes. If you have a kernel with particularly small shared memory and/or semaphore limits, you may have to reconfigure your kernel to increase its shared memory or semaphore parameters.

  > Tip: You may be able to postpone reconfiguring your kernel by decreasing -B to reduce Postgres' shared memory consumption.

## Description

The Postgres backend server can be executed directly from the user shell. This should be done only while debugging by the DBA, and should not be done while other Postgres backends are being managed by a postmaster on this set of databases.

Some of the switches explained here can be passed to the backend through the "database options" field of a connection request, and thus can be set for a particular backend without going to the trouble of restarting the postmaster. This is particularly handy for debugging-related switches.

The optional argument `dbname` specifies the name of the database to be accessed. `dbname` defaults to the value of the `USER` environment variable.

## Notes

Useful utilities for dealing with shared memory problems include ipcs(1), ipcrm(1), and ipcclean(1). See also postmaster.

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