本部分包含 PostgreSQL 伺服器應用程式和支援的實用工具參考訊息。這些指令只能在資料庫伺服器所在的主機上有效運行。PostgreSQL 用戶端工具中列出了其他實用工具。
pg_test_timing — measure timing overhead
pg_test_timing [option...]
pg_test_timing is a tool to measure the timing overhead on your system and confirm that the system time never moves backwards. Systems that are slow to collect timing data can give less accurate EXPLAIN ANALYZE results.
pg_test_timing accepts the following command-line options:-d duration
--duration=duration
Specifies the test duration, in seconds. Longer durations give slightly better accuracy, and are more likely to discover problems with the system clock moving backwards. The default test duration is 3 seconds.-V
--version
Print the pg_test_timing version and exit.-?
--help
Show help about pg_test_timing command line arguments, and exit.
Good results will show most (>90%) individual timing calls take less than one microsecond. Average per loop overhead will be even lower, below 100 nanoseconds. This example from an Intel i7-860 system using a TSC clock source shows excellent performance:
Note that different units are used for the per loop time than the histogram. The loop can have resolution within a few nanoseconds (ns), while the individual timing calls can only resolve down to one microsecond (us).
When the query executor is running a statement using EXPLAIN ANALYZE, individual operations are timed as well as showing a summary. The overhead of your system can be checked by counting rows with the psql program:
The i7-860 system measured runs the count query in 9.8 ms while the EXPLAIN ANALYZE version takes 16.6 ms, each processing just over 100,000 rows. That 6.8 ms difference means the timing overhead per row is 68 ns, about twice what pg_test_timing estimated it would be. Even that relatively small amount of overhead is making the fully timed count statement take almost 70% longer. On more substantial queries, the timing overhead would be less problematic.
On some newer Linux systems, it's possible to change the clock source used to collect timing data at any time. A second example shows the slowdown possible from switching to the slower acpi_pm time source, on the same system used for the fast results above:
In this configuration, the sample EXPLAIN ANALYZE above takes 115.9 ms. That's 1061 ns of timing overhead, again a small multiple of what's measured directly by this utility. That much timing overhead means the actual query itself is only taking a tiny fraction of the accounted for time, most of it is being consumed in overhead instead. In this configuration, any EXPLAIN ANALYZE totals involving many timed operations would be inflated significantly by timing overhead.
FreeBSD also allows changing the time source on the fly, and it logs information about the timer selected during boot:
Other systems may only allow setting the time source on boot. On older Linux systems the "clock" kernel setting is the only way to make this sort of change. And even on some more recent ones, the only option you'll see for a clock source is "jiffies". Jiffies are the older Linux software clock implementation, which can have good resolution when it's backed by fast enough timing hardware, as in this example:
Collecting accurate timing information is normally done on computers using hardware clocks with various levels of accuracy. With some hardware the operating systems can pass the system clock time almost directly to programs. A system clock can also be derived from a chip that simply provides timing interrupts, periodic ticks at some known time interval. In either case, operating system kernels provide a clock source that hides these details. But the accuracy of that clock source and how quickly it can return results varies based on the underlying hardware.
Inaccurate time keeping can result in system instability. Test any change to the clock source very carefully. Operating system defaults are sometimes made to favor reliability over best accuracy. And if you are using a virtual machine, look into the recommended time sources compatible with it. Virtual hardware faces additional difficulties when emulating timers, and there are often per operating system settings suggested by vendors.
The Time Stamp Counter (TSC) clock source is the most accurate one available on current generation CPUs. It's the preferred way to track the system time when it's supported by the operating system and the TSC clock is reliable. There are several ways that TSC can fail to provide an accurate timing source, making it unreliable. Older systems can have a TSC clock that varies based on the CPU temperature, making it unusable for timing. Trying to use TSC on some older multicore CPUs can give a reported time that's inconsistent among multiple cores. This can result in the time going backwards, a problem this program checks for. And even the newest systems can fail to provide accurate TSC timing with very aggressive power saving configurations.
Newer operating systems may check for the known TSC problems and switch to a slower, more stable clock source when they are seen. If your system supports TSC time but doesn't default to that, it may be disabled for a good reason. And some operating systems may not detect all the possible problems correctly, or will allow using TSC even in situations where it's known to be inaccurate.
The High Precision Event Timer (HPET) is the preferred timer on systems where it's available and TSC is not accurate. The timer chip itself is programmable to allow up to 100 nanosecond resolution, but you may not see that much accuracy in your system clock.
Advanced Configuration and Power Interface (ACPI) provides a Power Management (PM) Timer, which Linux refers to as the acpi_pm. The clock derived from acpi_pm will at best provide 300 nanosecond resolution.
Timers used on older PC hardware include the 8254 Programmable Interval Timer (PIT), the real-time clock (RTC), the Advanced Programmable Interrupt Controller (APIC) timer, and the Cyclone timer. These timers aim for millisecond resolution.
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 instance. The postgresinstance 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 postgresstarts 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 PGDATApoints directly to the data area directory created by initdb. Other possible file layouts are discussed in Section 19.2.
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; 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 19. 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.
-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 shared_buffers configuration parameter.
-c name=value
Sets a named run-time parameter. The configuration parameters supported by PostgreSQL are described in Chapter 19. 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 can be used on a running server, and returns values frompostgresql.conf, modified by any parameters supplied in this invocation. It does not reflect parameters supplied when the cluster was started.
This option is meant for other programs that interact with a server instance, such as pg_ctl, to query configuration parameter values. User-facing applications should instead useSHOW 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 Section 19.2 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 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 fsyncconfiguration 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 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 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 Section 18.9.
-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 max_connections configuration parameter.
-o extra-options
The command-line-style arguments specified in extra-options are passed to all server processes started by this postgres process.
Spaces within extra-options are considered to separate arguments, unless escaped with a backslash (\); write \\ to represent a literal backslash. Multiple arguments can also be specified via multiple uses of -o.
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 PGPORTenvironment 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 19.4.1.
-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.
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.
The following options only apply to the single-user mode (see 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. 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.
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 DateStyle run-time parameter. (The use of this environment variable is deprecated.)
PGPORT
Default port number (preferably set in the configuration file)
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 18.4. You might 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
or
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.
The utility command 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 Section 9.26.2 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 SIGKILLto 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
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, 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.
To start postgres in the background using default values, type:
To start postgres with a specific port, e.g. 1234:
To connect to this server using psql, specify this port with the -p option:
or set the environment variable PGPORT:
Named run-time parameters can be set in either of these styles:
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.