PostgreSQL

pgbench -i creates four tables pgbench_accounts, pgbench_branches, pgbench_history, and pgbench_tellers, destroying any existing tables of these names. Be very careful to use another database if you have tables having these names!

At the default “[.quote]#scale factor”# of 1, the tables initially contain this many rows:

You can (and, for most purposes, probably should) increase the number of rows by using the -s (scale factor) option. The -F (fillfactor) option might also be used at this point.

Once you have done the necessary setup, you can run your benchmark with a command that doesn’t include -i, that is

In nearly all cases, you’ll need some options to make a useful test. The most important options are -c (number of clients), -t (number of transactions), -T (time limit), and -f (specify a custom script file). See below for a full list.

pgbench accepts the following command-line initialization arguments:

pgbench accepts the following command-line benchmarking arguments:

-b `scriptname[@weight]
#[.term]--builtin=scriptname[@weight]:: Add the specified built-in script to the list of scripts to be executed. Available built-in scripts are: `tpcb-like, simple-update and select-only. Unambiguous prefixes of built-in names are accepted. With the special name list, show the list of built-in scripts and exit immediately.
Optionally, write an integer weight after @ to adjust the probability of selecting this script versus other ones. The default weight is 1. See below for details. [.term]-c `clients
[.term]--client=clients:: Number of clients simulated, that is, number of concurrent database sessions. Default is 1. [.term]-C`
[.term]--connect:: Establish a new connection for each transaction, rather than doing it just once per client session. This is useful to measure the connection overhead. [.term]-d
[.term]--debug:: Print debugging output. [.term]-D `varname=value
[.term]--define=varname=value:: Define a variable for use by a custom script (see below). Multiple `-D options are allowed. [.term]-f `filename[@weight]
[.term]--file=filename[@weight]:: Add a transaction script read from filename to the list of scripts to be executed.
Optionally, write an integer weight after `@ to adjust the probability of selecting this script versus other ones. The default weight is 1. (To use a script file name that includes an @ character, append a weight so that there is no ambiguity, for example filen@me@1.) See below for details. [.term]-j `threads
[.term]--jobs=threads:: Number of worker threads within pgbench. Using more than one thread can be helpful on multi-CPU machines. Clients are distributed as evenly as possible among available threads. Default is 1. [.term]-l`
[.term]--log:: Write information about each transaction to a log file. See below for details. [.term]-L `limit
[.term]--latency-limit=limit:: Transactions that last more than limit milliseconds are counted and reported separately, as late.
When throttling is used (--rate=...), transactions that lag behind schedule by more than limit ms, and thus have no hope of meeting the latency limit, are not sent to the server at all. They are counted and reported separately as skipped. [.term]-M` `querymode
[.term]--protocol=querymode:: Protocol to use for submitting queries to the server:
* `simple: use simple query protocol. * extended: use extended query protocol. * prepared: use extended query protocol with prepared statements.
In the prepared mode, pgbench reuses the parse analysis result starting from the second query iteration, so pgbench runs faster than in other modes.
The default is simple query protocol. (See Chapter 52 for more information.) [.term]-n
[.term]--no-vacuum:: Perform no vacuuming before running the test. This option is [.emphasis]necessary# if you are running a custom test scenario that does not include the standard tables pgbench_accounts, pgbench_branches, pgbench_history, and pgbench_tellers. -N
#[.term]--skip-some-updates:: Run built-in simple-update script. Shorthand for -b simple-update. [.term]-P `sec
[.term]--progress=sec:: Show progress report every sec seconds. The report includes the time since the beginning of the run, the TPS since the last report, and the transaction latency average and standard deviation since the last report. Under throttling (-R), the latency is computed with respect to the transaction scheduled start time, not the actual transaction beginning time, thus it also includes the average schedule lag time. [.term]-r`
[.term]--report-latencies:: Report the average per-statement latency (execution time from the perspective of the client) of each command after the benchmark finishes. See below for details. [.term]-R `rate
[.term]--rate=rate:: Execute transactions targeting the specified rate instead of running as fast as possible (the default). The rate is given in transactions per second. If the targeted rate is above the maximum possible rate, the rate limit won’t impact the results.
The rate is targeted by starting transactions along a Poisson-distributed schedule time line. The expected start time schedule moves forward based on when the client first started, not when the previous transaction ended. That approach means that when transactions go past their original scheduled end time, it is possible for later ones to catch up again.
When throttling is active, the transaction latency reported at the end of the run is calculated from the scheduled start times, so it includes the time each transaction had to wait for the previous transaction to finish. The wait time is called the schedule lag time, and its average and maximum are also reported separately. The transaction latency with respect to the actual transaction start time, i.e., the time spent executing the transaction in the database, can be computed by subtracting the schedule lag time from the reported latency.
If `--latency-limit is used together with --rate, a transaction can lag behind so much that it is already over the latency limit when the previous transaction ends, because the latency is calculated from the scheduled start time. Such transactions are not sent to the server, but are skipped altogether and counted separately.
A high schedule lag time is an indication that the system cannot process transactions at the specified rate, with the chosen number of clients and threads. When the average transaction execution time is longer than the scheduled interval between each transaction, each successive transaction will fall further behind, and the schedule lag time will keep increasing the longer the test run is. When that happens, you will have to reduce the specified transaction rate. [.term]-s `scale_factor
[.term]--scale=scale_factor:: Report the specified scale factor in pgbench’s output. With the built-in tests, this is not necessary; the correct scale factor will be detected by counting the number of rows in the `pgbench_branches table. However, when testing only custom benchmarks (-f option), the scale factor will be reported as 1 unless this option is used. [.term]-S
[.term]--select-only:: Run built-in select-only script. Shorthand for -b select-only. [.term]-t `transactions
[.term]--transactions=transactions:: Number of transactions each client runs. Default is 10. [.term]-T` `seconds
[.term]--time=seconds:: Run the test for this many seconds, rather than a fixed number of transactions per client. `-t and -T are mutually exclusive. [.term]-v
[.term]--vacuum-all:: Vacuum all four standard tables before running the test. With neither -n nor -v, pgbench will vacuum the pgbench_tellers and pgbench_branches tables, and will truncate pgbench_history. [.term]--aggregate-interval=`seconds:: Length of aggregation interval (in seconds). May be used only with `-l option. With this option, the log contains per-interval summary data, as described below. [.term]--log-prefix=`prefix:: Set the filename prefix for the log files created by `--log. The default is pgbench_log. [.term]--progress-timestamp:: When showing progress (option -P), use a timestamp (Unix epoch) instead of the number of seconds since the beginning of the run. The unit is in seconds, with millisecond precision after the dot. This helps compare logs generated by various tools. [.term]--random-seed=`seed:: Set random generator seed. Seeds the system random number generator, which then produces a sequence of initial generator states, one for each thread. Values for seed may be: `time (the default, the seed is based on the current time), rand (use a strong random source, failing if none is available), or an unsigned decimal integer value. The random generator is invoked explicitly from a pgbench script (random... functions) or implicitly (for instance option --rate uses it to schedule transactions). When explicitly set, the value used for seeding is shown on the terminal. Any value allowed for `seed may also be provided through the environment variable `PGBENCH_RANDOM_SEED. To ensure that the provided seed impacts all possible uses, put this option first or use the environment variable.
Setting the seed explicitly allows to reproduce a pgbench run exactly, as far as random numbers are concerned. As the random state is managed per thread, this means the exact same pgbench run for an identical invocation if there is one client per thread and there are no external or data dependencies. From a statistical viewpoint reproducing runs exactly is a bad idea because it can hide the performance variability or improve performance unduly, e.g., by hitting the same pages as a previous run. However, it may also be of great help for debugging, for instance re-running a tricky case which leads to an error. Use wisely. [.term]--sampling-rate=`rate:: Sampling rate, used when writing data into the log, to reduce the amount of log generated. If this option is given, only the specified fraction of transactions are logged. 1.0 means all transactions will be logged, 0.05 means only 5% of the transactions will be logged.
Remember to take the sampling rate into account when processing the log file. For example, when computing TPS values, you need to multiply the numbers accordingly (e.g., with 0.01 sample rate, you’ll only get 1/100 of the actual TPS). [.term]--show-script=scriptname#:: Show the actual code of builtin script scriptname` on stderr, and exit immediately.

pgbench also accepts the following common command-line arguments for connection parameters:

-h `hostname
#[.term]--host=hostname:: The database server’s host name [.term]-p` `port
[.term]--port=port:: The database server’s port number [.term]-U` `login
[.term]--username=login:: The user name to connect as [.term]-V`
[.term]--version:: Print the pgbench version and exit. [.term]-?
[.term]--help#:: Show help about pgbench command line arguments, and exit.

pgbench executes test scripts chosen randomly from a specified list. The scripts may include built-in scripts specified with -b and user-provided scripts specified with -f. Each script may be given a relative weight specified after an @ so as to change its selection probability. The default weight is 1. Scripts with a weight of 0 are ignored.

The default built-in transaction script (also invoked with -b tpcb-like) issues seven commands per transaction over randomly chosen aid, tid, bid and delta. The scenario is inspired by the TPC-B benchmark, but is not actually TPC-B, hence the name.

If you select the simple-update built-in (also -N), steps 4 and 5 aren’t included in the transaction. This will avoid update contention on these tables, but it makes the test case even less like TPC-B.

If you select the select-only built-in (also -S), only the SELECT is issued.

pgbench has support for running custom benchmark scenarios by replacing the default transaction script (described above) with a transaction script read from a file (-f option). In this case a “[.quote]#transaction”# counts as one execution of a script file.

A script file contains one or more SQL commands terminated by semicolons. Empty lines and lines beginning with -- are ignored. Script files can also contain “[.quote]#meta commands”#, which are interpreted by pgbench itself, as described below.

Before PostgreSQL 9.6, SQL commands in script files were terminated by newlines, and so they could not be continued across lines. Now a semicolon is required to separate consecutive SQL commands (though a SQL command does not need one if it is followed by a meta command). If you need to create a script file that works with both old and new versions of pgbench, be sure to write each SQL command on a single line ending with a semicolon.

There is a simple variable-substitution facility for script files. Variable names must consist of letters (including non-Latin letters), digits, and underscores, with the first character not being a digit. Variables can be set by the command-line -D option, explained above, or by the meta commands explained below. In addition to any variables preset by -D command-line options, there are a few variables that are preset automatically, listed in Table 273. A value specified for these variables using -D takes precedence over the automatic presets. Once set, a variable’s value can be inserted into a SQL command by writing :`variablename`. When running more than one client session, each session has its own set of variables. pgbench supports up to 255 variable uses in one statement.

Table 273. pgbench Automatic Variables

Script file meta commands begin with a backslash (\) and normally extend to the end of the line, although they can be continued to additional lines by writing backslash-return. Arguments to a meta command are separated by white space. These meta commands are supported:

\if `expression
#[.term]\elif` `expression
[.term]\else`
[.term]\endif:: This group of commands implements nestable conditional blocks, similarly to psql’s \if` `expression. Conditional expressions are identical to those with `\set, with non-zero values interpreted as true. [.term]\set +`_`+varname`+ `_`+expression#:: Sets variable varname to a value calculated from expression. The expression may contain the `NULL constant, Boolean constants TRUE and FALSE, integer constants such as 5432, double constants such as 3.14159, references to variables :`variablename, operators with their usual SQL precedence and associativity, function calls, SQL CASE` generic conditional expressions and parentheses.
Functions and most operators return NULL on NULL input.
For conditional purposes, non zero numerical values are TRUE, zero numerical values and NULL are FALSE.
Too large or small integer and double constants, as well as integer arithmetic operators (+, -, * and /) raise errors on overflows.
When no final ELSE clause is provided to a CASE, the default value is NULL.
Examples:

The arithmetic, bitwise, comparison and logical operators listed in Table 274 are built into pgbench and may be used in expressions appearing in \set. The operators are listed in increasing precedence order. Except as noted, operators taking two numeric inputs will produce a double value if either input is double, otherwise they produce an integer result.

Table 274. pgbench Operators

The functions listed in Table 275 are built into pgbench and may be used in expressions appearing in \set.

Table 275. pgbench Functions

The random function generates values using a uniform distribution, that is all the values are drawn within the specified range with equal probability. The random_exponential, random_gaussian and random_zipfian functions require an additional double parameter which determines the precise shape of the distribution.

Hash functions hash, hash_murmur2 and hash_fnv1a accept an input value and an optional seed parameter. In case the seed isn’t provided the value of :default_seed is used, which is initialized randomly unless set by the command-line -D option. Hash functions can be used to scatter the distribution of random functions such as random_zipfian or random_exponential. For instance, the following pgbench script simulates possible real world workload typical for social media and blogging platforms where few accounts generate excessive load:

In some cases several distinct distributions are needed which don’t correlate with each other and this is when implicit seed parameter comes in handy:

As an example, the full definition of the built-in TPC-B-like transaction is:

This script allows each iteration of the transaction to reference different, randomly-chosen rows. (This example also shows why it’s important for each client session to have its own variables — otherwise they’d not be independently touching different rows.)

With the -l option (but without the --aggregate-interval option), pgbench writes information about each transaction to a log file. The log file will be named `prefix.nnn, where prefix defaults to `pgbench_log, and `nnn is the PID of the pgbench process. The prefix can be changed by using the `--log-prefix option. If the -j option is 2 or higher, so that there are multiple worker threads, each will have its own log file. The first worker will use the same name for its log file as in the standard single worker case. The additional log files for the other workers will be named `prefix.nnn.mmm, where mmm` is a sequential number for each worker starting with 1.

The format of the log is:

where `client_id indicates which client session ran the transaction, transaction_no counts how many transactions have been run by that session, time is the total elapsed transaction time in microseconds, script_no identifies which script file was used (useful when multiple scripts were specified with `-f or -b), and `time_epoch/time_us are a Unix-epoch time stamp and an offset in microseconds (suitable for creating an ISO 8601 time stamp with fractional seconds) showing when the transaction completed. The schedule_lag field is the difference between the transaction’s scheduled start time, and the time it actually started, in microseconds. It is only present when the `--rate option is used. When both --rate and --latency-limit are used, the `time for a skipped transaction will be reported as `skipped.

Here is a snippet of a log file generated in a single-client run:

Another example with --rate=100 and --latency-limit=5 (note the additional `schedule_lag` column):

In this example, transaction 82 was late, because its latency (6.173 ms) was over the 5 ms limit. The next two transactions were skipped, because they were already late before they were even started.

When running a long test on hardware that can handle a lot of transactions, the log files can become very large. The --sampling-rate option can be used to log only a random sample of transactions.

With the --aggregate-interval option, a different format is used for the log files:

where `interval_start is the start of the interval (as a Unix epoch time stamp), num_transactions is the number of transactions within the interval, sum_latency is the sum of the transaction latencies within the interval, sum_latency_2 is the sum of squares of the transaction latencies within the interval, min_latency is the minimum latency within the interval, and max_latency is the maximum latency within the interval. The next fields, sum_lag, sum_lag_2, min_lag, and max_lag, are only present if the `--rate option is used. They provide statistics about the time each transaction had to wait for the previous one to finish, i.e., the difference between each transaction’s scheduled start time and the time it actually started. The very last field, `skipped, is only present if the `--latency-limit option is used, too. It counts the number of transactions skipped because they would have started too late. Each transaction is counted in the interval when it was committed.

Here is some example output:

Notice that while the plain (unaggregated) log file shows which script was used for each transaction, the aggregated log does not. Therefore if you need per-script data, you need to aggregate the data on your own.

With the -r option, pgbench collects the elapsed transaction time of each statement executed by every client. It then reports an average of those values, referred to as the latency for each statement, after the benchmark has finished.

For the default script, the output will look similar to this:

If multiple script files are specified, the averages are reported separately for each script file.

Note that collecting the additional timing information needed for per-statement latency computation adds some overhead. This will slow average execution speed and lower the computed TPS. The amount of slowdown varies significantly depending on platform and hardware. Comparing average TPS values with and without latency reporting enabled is a good way to measure if the timing overhead is significant.

It is very easy to use pgbench to produce completely meaningless numbers. Here are some guidelines to help you get useful results.

In the first place, never believe any test that runs for only a few seconds. Use the -t or -T option to make the run last at least a few minutes, so as to average out noise. In some cases you could need hours to get numbers that are reproducible. It’s a good idea to try the test run a few times, to find out if your numbers are reproducible or not.

For the default TPC-B-like test scenario, the initialization scale factor (-s) should be at least as large as the largest number of clients you intend to test (-c); else you’ll mostly be measuring update contention. There are only -s rows in the pgbench_branches table, and every transaction wants to update one of them, so -c values in excess of -s will undoubtedly result in lots of transactions blocked waiting for other transactions.

The default test scenario is also quite sensitive to how long it’s been since the tables were initialized: accumulation of dead rows and dead space in the tables changes the results. To understand the results you must keep track of the total number of updates and when vacuuming happens. If autovacuum is enabled it can result in unpredictable changes in measured performance.

A limitation of pgbench is that it can itself become the bottleneck when trying to test a large number of client sessions. This can be alleviated by running pgbench on a different machine from the database server, although low network latency will be essential. It might even be useful to run several pgbench instances concurrently, on several client machines, against the same database server.

If untrusted users have access to a database that has not adopted a secure schema usage pattern, do not run pgbench in that database. pgbench uses unqualified names and does not manipulate the search path.