Google Git
Sign in
apache/cloudberry/refs/heads/string_replace_fix/./doc/src/sgml/ref/pgbench.sgml
blob: 0c60077e1f9b8425f12b6355e805a7ab5ac9e783 [file]
<!--
doc/src/sgml/ref/pgbench.sgml
PostgreSQL documentation
-->
<refentry id="pgbench">
<indexterm zone="pgbench">
<primary>pgbench</primary>
</indexterm>
<refmeta>
<refentrytitle><application>pgbench</application></refentrytitle>
<manvolnum>1</manvolnum>
<refmiscinfo>Application</refmiscinfo>
</refmeta>
<refnamediv>
<refname>pgbench</refname>
<refpurpose>run a benchmark test on <productname>PostgreSQL</productname></refpurpose>
</refnamediv>
<refsynopsisdiv>
<cmdsynopsis>
<command>pgbench</command>
<arg choice="plain"><option>-i</option></arg>
<arg rep="repeat"><replaceable>option</replaceable></arg>
<arg choice="opt"><replaceable>dbname</replaceable></arg>
</cmdsynopsis>
<cmdsynopsis>
<command>pgbench</command>
<arg rep="repeat"><replaceable>option</replaceable></arg>
<arg choice="opt"><replaceable>dbname</replaceable></arg>
</cmdsynopsis>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para>
<application>pgbench</application> is a simple program for running benchmark
tests on <productname>PostgreSQL</productname>. It runs the same sequence of SQL
commands over and over, possibly in multiple concurrent database sessions,
and then calculates the average transaction rate (transactions per second).
By default, <application>pgbench</application> tests a scenario that is
loosely based on TPC-B, involving five <command>SELECT</command>,
<command>UPDATE</command>, and <command>INSERT</command> commands per transaction.
However, it is easy to test other cases by writing your own transaction
script files.
</para>
<para>
Typical output from <application>pgbench</application> looks like:
<screen>
transaction type: &lt;builtin: TPC-B (sort of)&gt;
scaling factor: 10
query mode: simple
number of clients: 10
number of threads: 1
number of transactions per client: 1000
number of transactions actually processed: 10000/10000
latency average = 11.013 ms
latency stddev = 7.351 ms
initial connection time = 45.758 ms
tps = 896.967014 (without initial connection time)
</screen>
The first six lines report some of the most important parameter
settings. The next line reports the number of transactions completed
and intended (the latter being just the product of number of clients
and number of transactions per client); these will be equal unless the run
failed before completion. (In <option>-T</option> mode, only the actual
number of transactions is printed.)
The last line reports the number of transactions per second.
</para>
<para>
The default TPC-B-like transaction test requires specific tables to be
set up beforehand. <application>pgbench</application> should be invoked with
the <option>-i</option> (initialize) option to create and populate these
tables. (When you are testing a custom script, you don't need this
step, but will instead need to do whatever setup your test needs.)
Initialization looks like:
<programlisting>
pgbench -i <optional> <replaceable>other-options</replaceable> </optional> <replaceable>dbname</replaceable>
</programlisting>
where <replaceable>dbname</replaceable> is the name of the already-created
database to test in. (You may also need <option>-h</option>,
<option>-p</option>, and/or <option>-U</option> options to specify how to
connect to the database server.)
</para>
<caution>
<para>
<literal>pgbench -i</literal> creates four tables <structname>pgbench_accounts</structname>,
<structname>pgbench_branches</structname>, <structname>pgbench_history</structname>, and
<structname>pgbench_tellers</structname>,
destroying any existing tables of these names.
Be very careful to use another database if you have tables having these
names!
</para>
</caution>
<para>
At the default <quote>scale factor</quote> of 1, the tables initially
contain this many rows:
<screen>
table # of rows
---------------------------------
pgbench_branches 1
pgbench_tellers 10
pgbench_accounts 100000
pgbench_history 0
</screen>
You can (and, for most purposes, probably should) increase the number
of rows by using the <option>-s</option> (scale factor) option. The
<option>-F</option> (fillfactor) option might also be used at this point.
</para>
<para>
Once you have done the necessary setup, you can run your benchmark
with a command that doesn't include <option>-i</option>, that is
<programlisting>
pgbench <optional> <replaceable>options</replaceable> </optional> <replaceable>dbname</replaceable>
</programlisting>
In nearly all cases, you'll need some options to make a useful test.
The most important options are <option>-c</option> (number of clients),
<option>-t</option> (number of transactions), <option>-T</option> (time limit),
and <option>-f</option> (specify a custom script file).
See below for a full list.
</para>
</refsect1>
<refsect1>
<title>Options</title>
<para>
The following is divided into three subsections. Different options are
used during database initialization and while running benchmarks, but some
options are useful in both cases.
</para>
<refsect2 id="pgbench-init-options">
<title>Initialization Options</title>
<para>
<application>pgbench</application> accepts the following command-line
initialization arguments:
<variablelist>
<varlistentry>
<term><replaceable class="parameter">dbname</replaceable></term>
<listitem>
<para>
Specifies the name of the database to test in. If this is
not specified, the environment variable
<envar>PGDATABASE</envar> is used. If that is not set, the
user name specified for the connection is used.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-i</option></term>
<term><option>--initialize</option></term>
<listitem>
<para>
Required to invoke initialization mode.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-I <replaceable>init_steps</replaceable></option></term>
<term><option>--init-steps=<replaceable>init_steps</replaceable></option></term>
<listitem>
<para>
Perform just a selected set of the normal initialization steps.
<replaceable>init_steps</replaceable> specifies the
initialization steps to be performed, using one character per step.
Each step is invoked in the specified order.
The default is <literal>dtgvp</literal>.
The available steps are:
<variablelist>
<varlistentry>
<term><literal>d</literal> (Drop)</term>
<listitem>
<para>
Drop any existing <application>pgbench</application> tables.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>t</literal> (create Tables)</term>
<listitem>
<para>
Create the tables used by the
standard <application>pgbench</application> scenario, namely
<structname>pgbench_accounts</structname>,
<structname>pgbench_branches</structname>,
<structname>pgbench_history</structname>, and
<structname>pgbench_tellers</structname>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>g</literal> or <literal>G</literal> (Generate data, client-side or server-side)</term>
<listitem>
<para>
Generate data and load it into the standard tables,
replacing any data already present.
</para>
<para>
With <literal>g</literal> (client-side data generation),
data is generated in <command>pgbench</command> client and then
sent to the server. This uses the client/server bandwidth
extensively through a <command>COPY</command>.
Using <literal>g</literal> causes logging to print one message
every 100,000 rows while generating data for the
<structname>pgbench_accounts</structname> table.
</para>
<para>
With <literal>G</literal> (server-side data generation),
only small queries are sent from the <command>pgbench</command>
client and then data is actually generated in the server.
No significant bandwidth is required for this variant, but
the server will do more work.
Using <literal>G</literal> causes logging not to print any progress
message while generating data.
</para>
<para>
The default initialization behavior uses client-side data
generation (equivalent to <literal>g</literal>).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>v</literal> (Vacuum)</term>
<listitem>
<para>
Invoke <command>VACUUM</command> on the standard tables.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>p</literal> (create Primary keys)</term>
<listitem>
<para>
Create primary key indexes on the standard tables.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>f</literal> (create Foreign keys)</term>
<listitem>
<para>
Create foreign key constraints between the standard tables.
(Note that this step is not performed by default.)
</para>
</listitem>
</varlistentry>
</variablelist></para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-F</option> <replaceable>fillfactor</replaceable></term>
<term><option>--fillfactor=</option><replaceable>fillfactor</replaceable></term>
<listitem>
<para>
Create the <structname>pgbench_accounts</structname>,
<structname>pgbench_tellers</structname> and
<structname>pgbench_branches</structname> tables with the given fillfactor.
Default is 100.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-n</option></term>
<term><option>--no-vacuum</option></term>
<listitem>
<para>
Perform no vacuuming during initialization.
(This option suppresses the <literal>v</literal> initialization step,
even if it was specified in <option>-I</option>.)
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-q</option></term>
<term><option>--quiet</option></term>
<listitem>
<para>
Switch logging to quiet mode, producing only one progress message per 5
seconds. The default logging prints one message each 100,000 rows, which
often outputs many lines per second (especially on good hardware).
</para>
<para>
This setting has no effect if <literal>G</literal> is specified
in <option>-I</option>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-s</option> <replaceable>scale_factor</replaceable></term>
<term><option>--scale=</option><replaceable>scale_factor</replaceable></term>
<listitem>
<para>
Multiply the number of rows generated by the scale factor.
For example, <literal>-s 100</literal> will create 10,000,000 rows
in the <structname>pgbench_accounts</structname> table. Default is 1.
When the scale is 20,000 or larger, the columns used to
hold account identifiers (<structfield>aid</structfield> columns)
will switch to using larger integers (<type>bigint</type>),
in order to be big enough to hold the range of account
identifiers.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--foreign-keys</option></term>
<listitem>
<para>
Create foreign key constraints between the standard tables.
(This option adds the <literal>f</literal> step to the initialization
step sequence, if it is not already present.)
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--index-tablespace=<replaceable>index_tablespace</replaceable></option></term>
<listitem>
<para>
Create indexes in the specified tablespace, rather than the default
tablespace.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--partition-method=<replaceable>NAME</replaceable></option></term>
<listitem>
<para>
Create a partitioned <literal>pgbench_accounts</literal> table with
<replaceable>NAME</replaceable> method.
Expected values are <literal>range</literal> or <literal>hash</literal>.
This option requires that <option>--partitions</option> is set to non-zero.
If unspecified, default is <literal>range</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--partitions=<replaceable>NUM</replaceable></option></term>
<listitem>
<para>
Create a partitioned <literal>pgbench_accounts</literal> table with
<replaceable>NUM</replaceable> partitions of nearly equal size for
the scaled number of accounts.
Default is <literal>0</literal>, meaning no partitioning.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--tablespace=<replaceable>tablespace</replaceable></option></term>
<listitem>
<para>
Create tables in the specified tablespace, rather than the default
tablespace.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--unlogged-tables</option></term>
<listitem>
<para>
Create all tables as unlogged tables, rather than permanent tables.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</refsect2>
<refsect2 id="pgbench-run-options">
<title>Benchmarking Options</title>
<para>
<application>pgbench</application> accepts the following command-line
benchmarking arguments:
<variablelist>
<varlistentry>
<term><option>-b</option> <replaceable>scriptname[@weight]</replaceable></term>
<term><option>--builtin</option>=<replaceable>scriptname[@weight]</replaceable></term>
<listitem>
<para>
Add the specified built-in script to the list of scripts to be executed.
Available built-in scripts are: <literal>tpcb-like</literal>,
<literal>simple-update</literal> and <literal>select-only</literal>.
Unambiguous prefixes of built-in names are accepted.
With the special name <literal>list</literal>, show the list of built-in scripts
and exit immediately.
</para>
<para>
Optionally, write an integer weight after <literal>@</literal> to
adjust the probability of selecting this script versus other ones.
The default weight is 1.
See below for details.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-c</option> <replaceable>clients</replaceable></term>
<term><option>--client=</option><replaceable>clients</replaceable></term>
<listitem>
<para>
Number of clients simulated, that is, number of concurrent database
sessions. Default is 1.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-C</option></term>
<term><option>--connect</option></term>
<listitem>
<para>
Establish a new connection for each transaction, rather than
doing it just once per client session.
This is useful to measure the connection overhead.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-d</option></term>
<term><option>--debug</option></term>
<listitem>
<para>
Print debugging output.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-D</option> <replaceable>varname</replaceable><literal>=</literal><replaceable>value</replaceable></term>
<term><option>--define=</option><replaceable>varname</replaceable><literal>=</literal><replaceable>value</replaceable></term>
<listitem>
<para>
Define a variable for use by a custom script (see below).
Multiple <option>-D</option> options are allowed.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-f</option> <replaceable>filename[@weight]</replaceable></term>
<term><option>--file=</option><replaceable>filename[@weight]</replaceable></term>
<listitem>
<para>
Add a transaction script read from <replaceable>filename</replaceable>
to the list of scripts to be executed.
</para>
<para>
Optionally, write an integer weight after <literal>@</literal> 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 <literal>@</literal>
character, append a weight so that there is no ambiguity, for
example <literal>filen@me@1</literal>.)
See below for details.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-j</option> <replaceable>threads</replaceable></term>
<term><option>--jobs=</option><replaceable>threads</replaceable></term>
<listitem>
<para>
Number of worker threads within <application>pgbench</application>.
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.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-l</option></term>
<term><option>--log</option></term>
<listitem>
<para>
Write information about each transaction to a log file.
See below for details.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-L</option> <replaceable>limit</replaceable></term>
<term><option>--latency-limit=</option><replaceable>limit</replaceable></term>
<listitem>
<para>
Transactions that last more than <replaceable>limit</replaceable> milliseconds
are counted and reported separately, as <firstterm>late</firstterm>.
</para>
<para>
When throttling is used (<option>--rate=...</option>), transactions that
lag behind schedule by more than <replaceable>limit</replaceable> 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
<firstterm>skipped</firstterm>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-M</option> <replaceable>querymode</replaceable></term>
<term><option>--protocol=</option><replaceable>querymode</replaceable></term>
<listitem>
<para>
Protocol to use for submitting queries to the server:
<itemizedlist>
<listitem>
<para><literal>simple</literal>: use simple query protocol.</para>
</listitem>
<listitem>
<para><literal>extended</literal>: use extended query protocol.</para>
</listitem>
<listitem>
<para><literal>prepared</literal>: use extended query protocol with prepared statements.</para>
</listitem>
</itemizedlist>
In the <literal>prepared</literal> mode, <application>pgbench</application>
reuses the parse analysis result starting from the second query
iteration, so <application>pgbench</application> runs faster
than in other modes.
</para>
<para>
The default is simple query protocol. (See <xref linkend="protocol"/>
for more information.)
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-n</option></term>
<term><option>--no-vacuum</option></term>
<listitem>
<para>
Perform no vacuuming before running the test.
This option is <emphasis>necessary</emphasis>
if you are running a custom test scenario that does not include
the standard tables <structname>pgbench_accounts</structname>,
<structname>pgbench_branches</structname>, <structname>pgbench_history</structname>, and
<structname>pgbench_tellers</structname>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-N</option></term>
<term><option>--skip-some-updates</option></term>
<listitem>
<para>
Run built-in simple-update script.
Shorthand for <option>-b simple-update</option>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-P</option> <replaceable>sec</replaceable></term>
<term><option>--progress=</option><replaceable>sec</replaceable></term>
<listitem>
<para>
Show progress report every <replaceable>sec</replaceable> 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 (<option>-R</option>),
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.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-r</option></term>
<term><option>--report-latencies</option></term>
<listitem>
<para>
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.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-R</option> <replaceable>rate</replaceable></term>
<term><option>--rate=</option><replaceable>rate</replaceable></term>
<listitem>
<para>
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.
</para>
<para>
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.
</para>
<para>
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.
</para>
<para>
If <option>--latency-limit</option> is used together with <option>--rate</option>,
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.
</para>
<para>
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.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-s</option> <replaceable>scale_factor</replaceable></term>
<term><option>--scale=</option><replaceable>scale_factor</replaceable></term>
<listitem>
<para>
Report the specified scale factor in <application>pgbench</application>'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 <structname>pgbench_branches</structname> table.
However, when testing only custom benchmarks (<option>-f</option> option),
the scale factor will be reported as 1 unless this option is used.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-S</option></term>
<term><option>--select-only</option></term>
<listitem>
<para>
Run built-in select-only script.
Shorthand for <option>-b select-only</option>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-t</option> <replaceable>transactions</replaceable></term>
<term><option>--transactions=</option><replaceable>transactions</replaceable></term>
<listitem>
<para>
Number of transactions each client runs. Default is 10.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-T</option> <replaceable>seconds</replaceable></term>
<term><option>--time=</option><replaceable>seconds</replaceable></term>
<listitem>
<para>
Run the test for this many seconds, rather than a fixed number of
transactions per client. <option>-t</option> and
<option>-T</option> are mutually exclusive.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-v</option></term>
<term><option>--vacuum-all</option></term>
<listitem>
<para>
Vacuum all four standard tables before running the test.
With neither <option>-n</option> nor <option>-v</option>, <application>pgbench</application> will vacuum the
<structname>pgbench_tellers</structname> and <structname>pgbench_branches</structname>
tables, and will truncate <structname>pgbench_history</structname>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--aggregate-interval=<replaceable>seconds</replaceable></option></term>
<listitem>
<para>
Length of aggregation interval (in seconds). May be used only
with <option>-l</option> option. With this option, the log contains
per-interval summary data, as described below.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--log-prefix=<replaceable>prefix</replaceable></option></term>
<listitem>
<para>
Set the filename prefix for the log files created by
<option>--log</option>. The default is <literal>pgbench_log</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--progress-timestamp</option></term>
<listitem>
<para>
When showing progress (option <option>-P</option>), 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.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--random-seed=</option><replaceable>seed</replaceable></term>
<listitem>
<para>
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 <replaceable>seed</replaceable> may be:
<literal>time</literal> (the default, the seed is based on the current time),
<literal>rand</literal> (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
(<literal>random...</literal> functions) or implicitly (for instance option
<option>--rate</option> uses it to schedule transactions).
When explicitly set, the value used for seeding is shown on the terminal.
Any value allowed for <replaceable>seed</replaceable> may also be
provided through the environment variable
<literal>PGBENCH_RANDOM_SEED</literal>.
To ensure that the provided seed impacts all possible uses, put this option
first or use the environment variable.
</para>
<para>
Setting the seed explicitly allows to reproduce a <command>pgbench</command>
run exactly, as far as random numbers are concerned.
As the random state is managed per thread, this means the exact same
<command>pgbench</command> 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.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--sampling-rate=<replaceable>rate</replaceable></option></term>
<listitem>
<para>
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.
</para>
<para>
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).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--show-script=</option><replaceable>scriptname</replaceable></term>
<listitem>
<para>
Show the actual code of builtin script <replaceable>scriptname</replaceable>
on stderr, and exit immediately.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</refsect2>
<refsect2 id="pgbench-common-options">
<title>Common Options</title>
<para>
<application>pgbench</application> also accepts the following common command-line
arguments for connection parameters:
<variablelist>
<varlistentry>
<term><option>-h</option> <replaceable>hostname</replaceable></term>
<term><option>--host=</option><replaceable>hostname</replaceable></term>
<listitem>
<para>
The database server's host name
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-p</option> <replaceable>port</replaceable></term>
<term><option>--port=</option><replaceable>port</replaceable></term>
<listitem>
<para>
The database server's port number
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-U</option> <replaceable>login</replaceable></term>
<term><option>--username=</option><replaceable>login</replaceable></term>
<listitem>
<para>
The user name to connect as
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-V</option></term>
<term><option>--version</option></term>
<listitem>
<para>
Print the <application>pgbench</application> version and exit.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-?</option></term>
<term><option>--help</option></term>
<listitem>
<para>
Show help about <application>pgbench</application> command line
arguments, and exit.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</refsect2>
</refsect1>
<refsect1>
<title>Exit Status</title>
<para>
A successful run will exit with status 0. Exit status 1 indicates static
problems such as invalid command-line options. Errors during the run such
as database errors or problems in the script will result in exit status 2.
In the latter case, <application>pgbench</application> will print partial
results.
</para>
</refsect1>
<refsect1>
<title>Environment</title>
<variablelist>
<varlistentry>
<term><envar>PGDATABASE</envar></term>
<term><envar>PGHOST</envar></term>
<term><envar>PGPORT</envar></term>
<term><envar>PGUSER</envar></term>
<listitem>
<para>
Default connection parameters.
</para>
</listitem>
</varlistentry>
</variablelist>
<para>
This utility, like most other <productname>PostgreSQL</productname> utilities,
uses the environment variables supported by <application>libpq</application>
(see <xref linkend="libpq-envars"/>).
</para>
<para>
The environment variable <envar>PG_COLOR</envar> specifies whether to use
color in diagnostic messages. Possible values are
<literal>always</literal>, <literal>auto</literal> and
<literal>never</literal>.
</para>
</refsect1>
<refsect1>
<title>Notes</title>
<refsect2>
<title>What Is the <quote>Transaction</quote> Actually Performed in <application>pgbench</application>?</title>
<para>
<application>pgbench</application> executes test scripts chosen randomly
from a specified list.
The scripts may include built-in scripts specified with <option>-b</option>
and user-provided scripts specified with <option>-f</option>.
Each script may be given a relative weight specified after an
<literal>@</literal> so as to change its selection probability.
The default weight is <literal>1</literal>.
Scripts with a weight of <literal>0</literal> are ignored.
</para>
<para>
The default built-in transaction script (also invoked with <option>-b tpcb-like</option>)
issues seven commands per transaction over randomly chosen <literal>aid</literal>,
<literal>tid</literal>, <literal>bid</literal> and <literal>delta</literal>.
The scenario is inspired by the TPC-B benchmark, but is not actually TPC-B,
hence the name.
</para>
<orderedlist>
<listitem><para><literal>BEGIN;</literal></para></listitem>
<listitem><para><literal>UPDATE pgbench_accounts SET abalance = abalance + :delta WHERE aid = :aid;</literal></para></listitem>
<listitem><para><literal>SELECT abalance FROM pgbench_accounts WHERE aid = :aid;</literal></para></listitem>
<listitem><para><literal>UPDATE pgbench_tellers SET tbalance = tbalance + :delta WHERE tid = :tid;</literal></para></listitem>
<listitem><para><literal>UPDATE pgbench_branches SET bbalance = bbalance + :delta WHERE bid = :bid;</literal></para></listitem>
<listitem><para><literal>INSERT INTO pgbench_history (tid, bid, aid, delta, mtime) VALUES (:tid, :bid, :aid, :delta, CURRENT_TIMESTAMP);</literal></para></listitem>
<listitem><para><literal>END;</literal></para></listitem>
</orderedlist>
<para>
If you select the <literal>simple-update</literal> built-in (also <option>-N</option>),
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.
</para>
<para>
If you select the <literal>select-only</literal> built-in (also <option>-S</option>),
only the <command>SELECT</command> is issued.
</para>
</refsect2>
<refsect2>
<title>Custom Scripts</title>
<para>
<application>pgbench</application> has support for running custom
benchmark scenarios by replacing the default transaction script
(described above) with a transaction script read from a file
(<option>-f</option> option). In this case a <quote>transaction</quote>
counts as one execution of a script file.
</para>
<para>
A script file contains one or more SQL commands terminated by
semicolons. Empty lines and lines beginning with
<literal>--</literal> are ignored. Script files can also contain
<quote>meta commands</quote>, which are interpreted by <application>pgbench</application>
itself, as described below.
</para>
<note>
<para>
Before <productname>PostgreSQL</productname> 9.6, SQL commands in script files
were terminated by newlines, and so they could not be continued across
lines. Now a semicolon is <emphasis>required</emphasis> to separate consecutive
SQL commands (though an 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 <application>pgbench</application>, be sure to write
each SQL command on a single line ending with a semicolon.
</para>
</note>
<para>
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 <option>-D</option> option,
explained above, or by the meta commands explained below.
In addition to any variables preset by <option>-D</option> command-line options,
there are a few variables that are preset automatically, listed in
<xref linkend="pgbench-automatic-variables"/>. A value specified for these
variables using <option>-D</option> takes precedence over the automatic presets.
Once set, a variable's
value can be inserted into an SQL command by writing
<literal>:</literal><replaceable>variablename</replaceable>. When running more than
one client session, each session has its own set of variables.
<application>pgbench</application> supports up to 255 variable uses in one
statement.
</para>
<table id="pgbench-automatic-variables">
<title>pgbench Automatic Variables</title>
<tgroup cols="2">
<colspec colname="col1" colwidth="1*"/>
<colspec colname="col2" colwidth="2*"/>
<thead>
<row>
<entry>Variable</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry> <literal>client_id</literal> </entry>
<entry>unique number identifying the client session (starts from zero)</entry>
</row>
<row>
<entry> <literal>default_seed</literal> </entry>
<entry>seed used in hash and pseudorandom permutation functions by default</entry>
</row>
<row>
<entry> <literal>random_seed</literal> </entry>
<entry>random generator seed (unless overwritten with <option>-D</option>)</entry>
</row>
<row>
<entry> <literal>scale</literal> </entry>
<entry>current scale factor</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
Script file meta commands begin with a backslash (<literal>\</literal>) 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:
</para>
<variablelist>
<varlistentry id='pgbench-metacommand-gset'>
<term>
<literal>\gset [<replaceable>prefix</replaceable>]</literal>
<literal>\aset [<replaceable>prefix</replaceable>]</literal>
</term>
<listitem>
<para>
These commands may be used to end SQL queries, taking the place of the
terminating semicolon (<literal>;</literal>).
</para>
<para>
When the <literal>\gset</literal> command is used, the preceding SQL query is
expected to return one row, the columns of which are stored into variables
named after column names, and prefixed with <replaceable>prefix</replaceable>
if provided.
</para>
<para>
When the <literal>\aset</literal> command is used, all combined SQL queries
(separated by <literal>\;</literal>) have their columns stored into variables
named after column names, and prefixed with <replaceable>prefix</replaceable>
if provided. If a query returns no row, no assignment is made and the variable
can be tested for existence to detect this. If a query returns more than one
row, the last value is kept.
</para>
<para>
<literal>\gset</literal> and <literal>\aset</literal> cannot be used in
pipeline mode, since the query results are not yet available by the time
the commands would need them.
</para>
<para>
The following example puts the final account balance from the first query
into variable <replaceable>abalance</replaceable>, and fills variables
<replaceable>p_two</replaceable> and <replaceable>p_three</replaceable>
with integers from the third query.
The result of the second query is discarded.
The result of the two last combined queries are stored in variables
<replaceable>four</replaceable> and <replaceable>five</replaceable>.
<programlisting>
UPDATE pgbench_accounts
SET abalance = abalance + :delta
WHERE aid = :aid
RETURNING abalance \gset
-- compound of two queries
SELECT 1 \;
SELECT 2 AS two, 3 AS three \gset p_
SELECT 4 AS four \; SELECT 5 AS five \aset
</programlisting></para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>\if</literal> <replaceable class="parameter">expression</replaceable></term>
<term><literal>\elif</literal> <replaceable class="parameter">expression</replaceable></term>
<term><literal>\else</literal></term>
<term><literal>\endif</literal></term>
<listitem>
<para>
This group of commands implements nestable conditional blocks,
similarly to <literal>psql</literal>'s <xref linkend="psql-metacommand-if"/>.
Conditional expressions are identical to those with <literal>\set</literal>,
with non-zero values interpreted as true.
</para>
</listitem>
</varlistentry>
<varlistentry id='pgbench-metacommand-set'>
<term>
<literal>\set <replaceable>varname</replaceable> <replaceable>expression</replaceable></literal>
</term>
<listitem>
<para>
Sets variable <replaceable>varname</replaceable> to a value calculated
from <replaceable>expression</replaceable>.
The expression may contain the <literal>NULL</literal> constant,
Boolean constants <literal>TRUE</literal> and <literal>FALSE</literal>,
integer constants such as <literal>5432</literal>,
double constants such as <literal>3.14159</literal>,
references to variables <literal>:</literal><replaceable>variablename</replaceable>,
<link linkend="pgbench-builtin-operators">operators</link>
with their usual SQL precedence and associativity,
<link linkend="pgbench-builtin-functions">function calls</link>,
SQL <link linkend="functions-case"><token>CASE</token> generic conditional
expressions</link> and parentheses.
</para>
<para>
Functions and most operators return <literal>NULL</literal> on
<literal>NULL</literal> input.
</para>
<para>
For conditional purposes, non zero numerical values are
<literal>TRUE</literal>, zero numerical values and <literal>NULL</literal>
are <literal>FALSE</literal>.
</para>
<para>
Too large or small integer and double constants, as well as
integer arithmetic operators (<literal>+</literal>,
<literal>-</literal>, <literal>*</literal> and <literal>/</literal>)
raise errors on overflows.
</para>
<para>
When no final <token>ELSE</token> clause is provided to a
<token>CASE</token>, the default value is <literal>NULL</literal>.
</para>
<para>
Examples:
<programlisting>
\set ntellers 10 * :scale
\set aid (1021 * random(1, 100000 * :scale)) % \
(100000 * :scale) + 1
\set divx CASE WHEN :x &lt;&gt; 0 THEN :y/:x ELSE NULL END
</programlisting></para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<literal>\sleep <replaceable>number</replaceable> [ us | ms | s ]</literal>
</term>
<listitem>
<para>
Causes script execution to sleep for the specified duration in
microseconds (<literal>us</literal>), milliseconds (<literal>ms</literal>) or seconds
(<literal>s</literal>). If the unit is omitted then seconds are the default.
<replaceable>number</replaceable> can be either an integer constant or a
<literal>:</literal><replaceable>variablename</replaceable> reference to a variable
having an integer value.
</para>
<para>
Example:
<programlisting>
\sleep 10 ms
</programlisting></para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<literal>\setshell <replaceable>varname</replaceable> <replaceable>command</replaceable> [ <replaceable>argument</replaceable> ... ]</literal>
</term>
<listitem>
<para>
Sets variable <replaceable>varname</replaceable> to the result of the shell command
<replaceable>command</replaceable> with the given <replaceable>argument</replaceable>(s).
The command must return an integer value through its standard output.
</para>
<para>
<replaceable>command</replaceable> and each <replaceable>argument</replaceable> can be either
a text constant or a <literal>:</literal><replaceable>variablename</replaceable> reference
to a variable. If you want to use an <replaceable>argument</replaceable> starting
with a colon, write an additional colon at the beginning of
<replaceable>argument</replaceable>.
</para>
<para>
Example:
<programlisting>
\setshell variable_to_be_assigned command literal_argument :variable ::literal_starting_with_colon
</programlisting></para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<literal>\shell <replaceable>command</replaceable> [ <replaceable>argument</replaceable> ... ]</literal>
</term>
<listitem>
<para>
Same as <literal>\setshell</literal>, but the result of the command
is discarded.
</para>
<para>
Example:
<programlisting>
\shell command literal_argument :variable ::literal_starting_with_colon
</programlisting></para>
</listitem>
</varlistentry>
<varlistentry id='pgbench-metacommand-pipeline'>
<term><literal>\startpipeline</literal></term>
<term><literal>\endpipeline</literal></term>
<listitem>
<para>
These commands delimit the start and end of a pipeline of SQL
statements. In pipeline mode, statements are sent to the server
without waiting for the results of previous statements. See
<xref linkend="libpq-pipeline-mode"/> for more details.
Pipeline mode requires the use of extended query protocol.
</para>
</listitem>
</varlistentry>
</variablelist>
</refsect2>
<refsect2 id="pgbench-builtin-operators">
<title>Built-in Operators</title>
<para>
The arithmetic, bitwise, comparison and logical operators listed in
<xref linkend="pgbench-operators"/> are built into <application>pgbench</application>
and may be used in expressions appearing in
<link linkend="pgbench-metacommand-set"><literal>\set</literal></link>.
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.
</para>
<table id="pgbench-operators">
<title>pgbench Operators</title>
<tgroup cols="1">
<thead>
<row>
<entry role="func_table_entry"><para role="func_signature">
Operator
</para>
<para>
Description
</para>
<para>
Example(s)
</para></entry>
</row>
</thead>
<tbody>
<row>
<entry role="func_table_entry"><para role="func_signature">
<replaceable>boolean</replaceable> <literal>OR</literal> <replaceable>boolean</replaceable>
<returnvalue><replaceable>boolean</replaceable></returnvalue>
</para>
<para>
Logical OR
</para>
<para>
<literal>5 or 0</literal>
<returnvalue>TRUE</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<replaceable>boolean</replaceable> <literal>AND</literal> <replaceable>boolean</replaceable>
<returnvalue><replaceable>boolean</replaceable></returnvalue>
</para>
<para>
Logical AND
</para>
<para>
<literal>3 and 0</literal>
<returnvalue>FALSE</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<literal>NOT</literal> <replaceable>boolean</replaceable>
<returnvalue><replaceable>boolean</replaceable></returnvalue>
</para>
<para>
Logical NOT
</para>
<para>
<literal>not false</literal>
<returnvalue>TRUE</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<replaceable>boolean</replaceable> <literal>IS [NOT] (NULL|TRUE|FALSE)</literal>
<returnvalue><replaceable>boolean</replaceable></returnvalue>
</para>
<para>
Boolean value tests
</para>
<para>
<literal>1 is null</literal>
<returnvalue>FALSE</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<replaceable>value</replaceable> <literal>ISNULL|NOTNULL</literal>
<returnvalue><replaceable>boolean</replaceable></returnvalue>
</para>
<para>
Nullness tests
</para>
<para>
<literal>1 notnull</literal>
<returnvalue>TRUE</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<replaceable>number</replaceable> <literal>=</literal> <replaceable>number</replaceable>
<returnvalue><replaceable>boolean</replaceable></returnvalue>
</para>
<para>
Equal
</para>
<para>
<literal>5 = 4</literal>
<returnvalue>FALSE</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<replaceable>number</replaceable> <literal>&lt;&gt;</literal> <replaceable>number</replaceable>
<returnvalue><replaceable>boolean</replaceable></returnvalue>
</para>
<para>
Not equal
</para>
<para>
<literal>5 &lt;&gt; 4</literal>
<returnvalue>TRUE</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<replaceable>number</replaceable> <literal>!=</literal> <replaceable>number</replaceable>
<returnvalue><replaceable>boolean</replaceable></returnvalue>
</para>
<para>
Not equal
</para>
<para>
<literal>5 != 5</literal>
<returnvalue>FALSE</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<replaceable>number</replaceable> <literal>&lt;</literal> <replaceable>number</replaceable>
<returnvalue><replaceable>boolean</replaceable></returnvalue>
</para>
<para>
Less than
</para>
<para>
<literal>5 &lt; 4</literal>
<returnvalue>FALSE</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<replaceable>number</replaceable> <literal>&lt;=</literal> <replaceable>number</replaceable>
<returnvalue><replaceable>boolean</replaceable></returnvalue>
</para>
<para>
Less than or equal to
</para>
<para>
<literal>5 &lt;= 4</literal>
<returnvalue>FALSE</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<replaceable>number</replaceable> <literal>&gt;</literal> <replaceable>number</replaceable>
<returnvalue><replaceable>boolean</replaceable></returnvalue>
</para>
<para>
Greater than
</para>
<para>
<literal>5 &gt; 4</literal>
<returnvalue>TRUE</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<replaceable>number</replaceable> <literal>&gt;=</literal> <replaceable>number</replaceable>
<returnvalue><replaceable>boolean</replaceable></returnvalue>
</para>
<para>
Greater than or equal to
</para>
<para>
<literal>5 &gt;= 4</literal>
<returnvalue>TRUE</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<replaceable>integer</replaceable> <literal>|</literal> <replaceable>integer</replaceable>
<returnvalue><replaceable>integer</replaceable></returnvalue>
</para>
<para>
Bitwise OR
</para>
<para>
<literal>1 | 2</literal>
<returnvalue>3</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<replaceable>integer</replaceable> <literal>#</literal> <replaceable>integer</replaceable>
<returnvalue><replaceable>integer</replaceable></returnvalue>
</para>
<para>
Bitwise XOR
</para>
<para>
<literal>1 # 3</literal>
<returnvalue>2</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<replaceable>integer</replaceable> <literal>&amp;</literal> <replaceable>integer</replaceable>
<returnvalue><replaceable>integer</replaceable></returnvalue>
</para>
<para>
Bitwise AND
</para>
<para>
<literal>1 &amp; 3</literal>
<returnvalue>1</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<literal>~</literal> <replaceable>integer</replaceable>
<returnvalue><replaceable>integer</replaceable></returnvalue>
</para>
<para>
Bitwise NOT
</para>
<para>
<literal>~ 1</literal>
<returnvalue>-2</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<replaceable>integer</replaceable> <literal>&lt;&lt;</literal> <replaceable>integer</replaceable>
<returnvalue><replaceable>integer</replaceable></returnvalue>
</para>
<para>
Bitwise shift left
</para>
<para>
<literal>1 &lt;&lt; 2</literal>
<returnvalue>4</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<replaceable>integer</replaceable> <literal>&gt;&gt;</literal> <replaceable>integer</replaceable>
<returnvalue><replaceable>integer</replaceable></returnvalue>
</para>
<para>
Bitwise shift right
</para>
<para>
<literal>8 &gt;&gt; 2</literal>
<returnvalue>2</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<replaceable>number</replaceable> <literal>+</literal> <replaceable>number</replaceable>
<returnvalue><replaceable>number</replaceable></returnvalue>
</para>
<para>
Addition
</para>
<para>
<literal>5 + 4</literal>
<returnvalue>9</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<replaceable>number</replaceable> <literal>-</literal> <replaceable>number</replaceable>
<returnvalue><replaceable>number</replaceable></returnvalue>
</para>
<para>
Subtraction
</para>
<para>
<literal>3 - 2.0</literal>
<returnvalue>1.0</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<replaceable>number</replaceable> <literal>*</literal> <replaceable>number</replaceable>
<returnvalue><replaceable>number</replaceable></returnvalue>
</para>
<para>
Multiplication
</para>
<para>
<literal>5 * 4</literal>
<returnvalue>20</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<replaceable>number</replaceable> <literal>/</literal> <replaceable>number</replaceable>
<returnvalue><replaceable>number</replaceable></returnvalue>
</para>
<para>
Division (truncates the result towards zero if both inputs are integers)
</para>
<para>
<literal>5 / 3</literal>
<returnvalue>1</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<replaceable>integer</replaceable> <literal>%</literal> <replaceable>integer</replaceable>
<returnvalue><replaceable>integer</replaceable></returnvalue>
</para>
<para>
Modulo (remainder)
</para>
<para>
<literal>3 % 2</literal>
<returnvalue>1</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<literal>-</literal> <replaceable>number</replaceable>
<returnvalue><replaceable>number</replaceable></returnvalue>
</para>
<para>
Negation
</para>
<para>
<literal>- 2.0</literal>
<returnvalue>-2.0</returnvalue>
</para></entry>
</row>
</tbody>
</tgroup>
</table>
</refsect2>
<refsect2 id="pgbench-builtin-functions">
<title>Built-In Functions</title>
<para>
The functions listed in <xref linkend="pgbench-functions"/> are built
into <application>pgbench</application> and may be used in expressions appearing in
<link linkend="pgbench-metacommand-set"><literal>\set</literal></link>.
</para>
<!-- list pgbench functions in alphabetical order -->
<table id="pgbench-functions">
<title>pgbench Functions</title>
<tgroup cols="1">
<thead>
<row>
<entry role="func_table_entry"><para role="func_signature">
Function
</para>
<para>
Description
</para>
<para>
Example(s)
</para></entry>
</row>
</thead>
<tbody>
<row>
<entry role="func_table_entry"><para role="func_signature">
<function>abs</function> ( <replaceable>number</replaceable> )
<returnvalue></returnvalue> same type as input
</para>
<para>
Absolute value
</para>
<para>
<literal>abs(-17)</literal>
<returnvalue>17</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<function>debug</function> ( <replaceable>number</replaceable> )
<returnvalue></returnvalue> same type as input
</para>
<para>
Prints the argument to <systemitem>stderr</systemitem>,
and returns the argument.
</para>
<para>
<literal>debug(5432.1)</literal>
<returnvalue>5432.1</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<function>double</function> ( <replaceable>number</replaceable> )
<returnvalue>double</returnvalue>
</para>
<para>
Casts to double.
</para>
<para>
<literal>double(5432)</literal>
<returnvalue>5432.0</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<function>exp</function> ( <replaceable>number</replaceable> )
<returnvalue>double</returnvalue>
</para>
<para>
Exponential (<literal>e</literal> raised to the given power)
</para>
<para>
<literal>exp(1.0)</literal>
<returnvalue>2.718281828459045</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<function>greatest</function> ( <replaceable>number</replaceable> <optional>, <literal>...</literal> </optional> )
<returnvalue></returnvalue> <type>double</type> if any argument is double, else <type>integer</type>
</para>
<para>
Selects the largest value among the arguments.
</para>
<para>
<literal>greatest(5, 4, 3, 2)</literal>
<returnvalue>5</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<function>hash</function> ( <parameter>value</parameter> <optional>, <parameter>seed</parameter> </optional> )
<returnvalue>integer</returnvalue>
</para>
<para>
This is an alias for <function>hash_murmur2</function>.
</para>
<para>
<literal>hash(10, 5432)</literal>
<returnvalue>-5817877081768721676</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<function>hash_fnv1a</function> ( <parameter>value</parameter> <optional>, <parameter>seed</parameter> </optional> )
<returnvalue>integer</returnvalue>
</para>
<para>
Computes <ulink url="https://en.wikipedia.org/wiki/Fowler%E2%80%93Noll%E2%80%93Vo_hash_function">FNV-1a hash</ulink>.
</para>
<para>
<literal>hash_fnv1a(10, 5432)</literal>
<returnvalue>-7793829335365542153</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<function>hash_murmur2</function> ( <parameter>value</parameter> <optional>, <parameter>seed</parameter> </optional> )
<returnvalue>integer</returnvalue>
</para>
<para>
Computes <ulink url="https://en.wikipedia.org/wiki/MurmurHash">MurmurHash2 hash</ulink>.
</para>
<para>
<literal>hash_murmur2(10, 5432)</literal>
<returnvalue>-5817877081768721676</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<function>int</function> ( <replaceable>number</replaceable> )
<returnvalue>integer</returnvalue>
</para>
<para>
Casts to integer.
</para>
<para>
<literal>int(5.4 + 3.8)</literal>
<returnvalue>9</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<function>least</function> ( <replaceable>number</replaceable> <optional>, <literal>...</literal> </optional> )
<returnvalue></returnvalue> <type>double</type> if any argument is double, else <type>integer</type>
</para>
<para>
Selects the smallest value among the arguments.
</para>
<para>
<literal>least(5, 4, 3, 2.1)</literal>
<returnvalue>2.1</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<function>ln</function> ( <replaceable>number</replaceable> )
<returnvalue>double</returnvalue>
</para>
<para>
Natural logarithm
</para>
<para>
<literal>ln(2.718281828459045)</literal>
<returnvalue>1.0</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<function>mod</function> ( <replaceable>integer</replaceable>, <replaceable>integer</replaceable> )
<returnvalue>integer</returnvalue>
</para>
<para>
Modulo (remainder)
</para>
<para>
<literal>mod(54, 32)</literal>
<returnvalue>22</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<function>permute</function> ( <parameter>i</parameter>, <parameter>size</parameter> [, <parameter>seed</parameter> ] )
<returnvalue>integer</returnvalue>
</para>
<para>
Permuted value of <parameter>i</parameter>, in the range
<literal>[0, size)</literal>. This is the new position of
<parameter>i</parameter> (modulo <parameter>size</parameter>) in a
pseudorandom permutation of the integers <literal>0...size-1</literal>,
parameterized by <parameter>seed</parameter>, see below.
</para>
<para>
<literal>permute(0, 4)</literal>
<returnvalue>an integer between 0 and 3</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<function>pi</function> ()
<returnvalue>double</returnvalue>
</para>
<para>
Approximate value of <phrase role="symbol_font">&pi;</phrase>
</para>
<para>
<literal>pi()</literal>
<returnvalue>3.14159265358979323846</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<function>pow</function> ( <parameter>x</parameter>, <parameter>y</parameter> )
<returnvalue>double</returnvalue>
</para>
<para role="func_signature">
<function>power</function> ( <parameter>x</parameter>, <parameter>y</parameter> )
<returnvalue>double</returnvalue>
</para>
<para>
<parameter>x</parameter> raised to the power of <parameter>y</parameter>
</para>
<para>
<literal>pow(2.0, 10)</literal>
<returnvalue>1024.0</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<function>random</function> ( <parameter>lb</parameter>, <parameter>ub</parameter> )
<returnvalue>integer</returnvalue>
</para>
<para>
Computes a uniformly-distributed random integer in <literal>[lb,
ub]</literal>.
</para>
<para>
<literal>random(1, 10)</literal>
<returnvalue>an integer between 1 and 10</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<function>random_exponential</function> ( <parameter>lb</parameter>, <parameter>ub</parameter>, <parameter>parameter</parameter> )
<returnvalue>integer</returnvalue>
</para>
<para>
Computes an exponentially-distributed random integer in <literal>[lb,
ub]</literal>, see below.
</para>
<para>
<literal>random_exponential(1, 10, 3.0)</literal>
<returnvalue>an integer between 1 and 10</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<function>random_gaussian</function> ( <parameter>lb</parameter>, <parameter>ub</parameter>, <parameter>parameter</parameter> )
<returnvalue>integer</returnvalue>
</para>
<para>
Computes a Gaussian-distributed random integer in <literal>[lb,
ub]</literal>, see below.
</para>
<para>
<literal>random_gaussian(1, 10, 2.5)</literal>
<returnvalue>an integer between 1 and 10</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<function>random_zipfian</function> ( <parameter>lb</parameter>, <parameter>ub</parameter>, <parameter>parameter</parameter> )
<returnvalue>integer</returnvalue>
</para>
<para>
Computes a Zipfian-distributed random integer in <literal>[lb,
ub]</literal>, see below.
</para>
<para>
<literal>random_zipfian(1, 10, 1.5)</literal>
<returnvalue>an integer between 1 and 10</returnvalue>
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<function>sqrt</function> ( <replaceable>number</replaceable> )
<returnvalue>double</returnvalue>
</para>
<para>
Square root
</para>
<para>
<literal>sqrt(2.0)</literal>
<returnvalue>1.414213562</returnvalue>
</para></entry>
</row>
</tbody>
</tgroup>
</table>
<para>
The <literal>random</literal> function generates values using a uniform
distribution, that is all the values are drawn within the specified
range with equal probability. The <literal>random_exponential</literal>,
<literal>random_gaussian</literal> and <literal>random_zipfian</literal>
functions require an additional double parameter which determines the precise
shape of the distribution.
</para>
<itemizedlist>
<listitem>
<para>
For an exponential distribution, <replaceable>parameter</replaceable>
controls the distribution by truncating a quickly-decreasing
exponential distribution at <replaceable>parameter</replaceable>, and then
projecting onto integers between the bounds.
To be precise, with
<literallayout>
f(x) = exp(-parameter * (x - min) / (max - min + 1)) / (1 - exp(-parameter))
</literallayout>
Then value <replaceable>i</replaceable> between <replaceable>min</replaceable> and
<replaceable>max</replaceable> inclusive is drawn with probability:
<literal>f(i) - f(i + 1)</literal>.
</para>
<para>
Intuitively, the larger the <replaceable>parameter</replaceable>, the more
frequently values close to <replaceable>min</replaceable> are accessed, and the
less frequently values close to <replaceable>max</replaceable> are accessed.
The closer to 0 <replaceable>parameter</replaceable> is, the flatter (more
uniform) the access distribution.
A crude approximation of the distribution is that the most frequent 1%
values in the range, close to <replaceable>min</replaceable>, are drawn
<replaceable>parameter</replaceable>% of the time.
The <replaceable>parameter</replaceable> value must be strictly positive.
</para>
</listitem>
<listitem>
<para>
For a Gaussian distribution, the interval is mapped onto a standard
normal distribution (the classical bell-shaped Gaussian curve) truncated
at <literal>-parameter</literal> on the left and <literal>+parameter</literal>
on the right.
Values in the middle of the interval are more likely to be drawn.
To be precise, if <literal>PHI(x)</literal> is the cumulative distribution
function of the standard normal distribution, with mean <literal>mu</literal>
defined as <literal>(max + min) / 2.0</literal>, with
<literallayout>
f(x) = PHI(2.0 * parameter * (x - mu) / (max - min + 1)) /
(2.0 * PHI(parameter) - 1)
</literallayout>
then value <replaceable>i</replaceable> between <replaceable>min</replaceable> and
<replaceable>max</replaceable> inclusive is drawn with probability:
<literal>f(i + 0.5) - f(i - 0.5)</literal>.
Intuitively, the larger the <replaceable>parameter</replaceable>, the more
frequently values close to the middle of the interval are drawn, and the
less frequently values close to the <replaceable>min</replaceable> and
<replaceable>max</replaceable> bounds. About 67% of values are drawn from the
middle <literal>1.0 / parameter</literal>, that is a relative
<literal>0.5 / parameter</literal> around the mean, and 95% in the middle
<literal>2.0 / parameter</literal>, that is a relative
<literal>1.0 / parameter</literal> around the mean; for instance, if
<replaceable>parameter</replaceable> is 4.0, 67% of values are drawn from the
middle quarter (1.0 / 4.0) of the interval (i.e., from
<literal>3.0 / 8.0</literal> to <literal>5.0 / 8.0</literal>) and 95% from
the middle half (<literal>2.0 / 4.0</literal>) of the interval (second and third
quartiles). The minimum allowed <replaceable>parameter</replaceable>
value is 2.0.
</para>
</listitem>
<listitem>
<para>
<literal>random_zipfian</literal> generates a bounded Zipfian
distribution.
<replaceable>parameter</replaceable> defines how skewed the distribution
is. The larger the <replaceable>parameter</replaceable>, the more
frequently values closer to the beginning of the interval are drawn.
The distribution is such that, assuming the range starts from 1,
the ratio of the probability of drawing <replaceable>k</replaceable>
versus drawing <replaceable>k+1</replaceable> is
<literal>((<replaceable>k</replaceable>+1)/<replaceable>k</replaceable>)**<replaceable>parameter</replaceable></literal>.
For example, <literal>random_zipfian(1, ..., 2.5)</literal> produces
the value <literal>1</literal> about <literal>(2/1)**2.5 =
5.66</literal> times more frequently than <literal>2</literal>, which
itself is produced <literal>(3/2)**2.5 = 2.76</literal> times more
frequently than <literal>3</literal>, and so on.
</para>
<para>
<application>pgbench</application>'s implementation is based on
"Non-Uniform Random Variate Generation", Luc Devroye, p. 550-551,
Springer 1986. Due to limitations of that algorithm,
the <replaceable>parameter</replaceable> value is restricted to
the range [1.001, 1000].
</para>
</listitem>
</itemizedlist>
<note>
<para>
When designing a benchmark which selects rows non-uniformly, be aware
that the rows chosen may be correlated with other data such as IDs from
a sequence or the physical row ordering, which may skew performance
measurements.
</para>
<para>
To avoid this, you may wish to use the <function>permute</function>
function, or some other additional step with similar effect, to shuffle
the selected rows and remove such correlations.
</para>
</note>
<para>
Hash functions <literal>hash</literal>, <literal>hash_murmur2</literal> and
<literal>hash_fnv1a</literal> accept an input value and an optional seed parameter.
In case the seed isn't provided the value of <literal>:default_seed</literal>
is used, which is initialized randomly unless set by the command-line
<literal>-D</literal> option.
</para>
<para>
<literal>permute</literal> accepts an input value, a size, and an optional
seed parameter. It generates a pseudorandom permutation of integers in
the range <literal>[0, size)</literal>, and returns the index of the input
value in the permuted values. The permutation chosen is parameterized by
the seed, which defaults to <literal>:default_seed</literal>, if not
specified. Unlike the hash functions, <literal>permute</literal> ensures
that there are no collisions or holes in the output values. Input values
outside the interval are interpreted modulo the size. The function raises
an error if the size is not positive. <function>permute</function> can be
used to scatter the distribution of non-uniform random functions such as
<literal>random_zipfian</literal> or <literal>random_exponential</literal>
so that values drawn more often are not trivially correlated. For
instance, the following <application>pgbench</application> script
simulates a possible real world workload typical for social media and
blogging platforms where a few accounts generate excessive load:
<programlisting>
\set size 1000000
\set r random_zipfian(1, :size, 1.07)
\set k 1 + permute(:r, :size)
</programlisting>
In some cases several distinct distributions are needed which don't correlate
with each other and this is when the optional seed parameter comes in handy:
<programlisting>
\set k1 1 + permute(:r, :size, :default_seed + 123)
\set k2 1 + permute(:r, :size, :default_seed + 321)
</programlisting>
A similar behavior can also be approximated with <function>hash</function>:
<programlisting>
\set size 1000000
\set r random_zipfian(1, 100 * :size, 1.07)
\set k 1 + abs(hash(:r)) % :size
</programlisting>
However, since <function>hash</function> generates collisions, some values
will not be reachable and others will be more frequent than expected from
the original distribution.
</para>
<para>
As an example, the full definition of the built-in TPC-B-like
transaction is:
<programlisting>
\set aid random(1, 100000 * :scale)
\set bid random(1, 1 * :scale)
\set tid random(1, 10 * :scale)
\set delta random(-5000, 5000)
BEGIN;
UPDATE pgbench_accounts SET abalance = abalance + :delta WHERE aid = :aid;
SELECT abalance FROM pgbench_accounts WHERE aid = :aid;
UPDATE pgbench_tellers SET tbalance = tbalance + :delta WHERE tid = :tid;
UPDATE pgbench_branches SET bbalance = bbalance + :delta WHERE bid = :bid;
INSERT INTO pgbench_history (tid, bid, aid, delta, mtime) VALUES (:tid, :bid, :aid, :delta, CURRENT_TIMESTAMP);
END;
</programlisting>
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 &mdash;
otherwise they'd not be independently touching different rows.)
</para>
</refsect2>
<refsect2>
<title>Per-Transaction Logging</title>
<para>
With the <option>-l</option> option (but without
the <option>--aggregate-interval</option> option),
<application>pgbench</application> writes information about each transaction
to a log file. The log file will be named
<filename><replaceable>prefix</replaceable>.<replaceable>nnn</replaceable></filename>,
where <replaceable>prefix</replaceable> defaults to <literal>pgbench_log</literal>, and
<replaceable>nnn</replaceable> is the PID of the
<application>pgbench</application> process.
The prefix can be changed by using the <option>--log-prefix</option> option.
If the <option>-j</option> 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
<filename><replaceable>prefix</replaceable>.<replaceable>nnn</replaceable>.<replaceable>mmm</replaceable></filename>,
where <replaceable>mmm</replaceable> is a sequential number for each worker starting
with 1.
</para>
<para>
The format of the log is:
<synopsis>
<replaceable>client_id</replaceable> <replaceable>transaction_no</replaceable> <replaceable>time</replaceable> <replaceable>script_no</replaceable> <replaceable>time_epoch</replaceable> <replaceable>time_us</replaceable> <optional> <replaceable>schedule_lag</replaceable> </optional>
</synopsis>
where
<replaceable>client_id</replaceable> indicates which client session ran the transaction,
<replaceable>transaction_no</replaceable> counts how many transactions have been
run by that session,
<replaceable>time</replaceable> is the total elapsed transaction time in microseconds,
<replaceable>script_no</replaceable> identifies which script file was used (useful when
multiple scripts were specified with <option>-f</option> or <option>-b</option>),
and <replaceable>time_epoch</replaceable>/<replaceable>time_us</replaceable> 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 <replaceable>schedule_lag</replaceable> 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 <option>--rate</option> option is used.
When both <option>--rate</option> and <option>--latency-limit</option> are used,
the <replaceable>time</replaceable> for a skipped transaction will be reported as
<literal>skipped</literal>.
</para>
<para>
Here is a snippet of a log file generated in a single-client run:
<screen>
0 199 2241 0 1175850568 995598
0 200 2465 0 1175850568 998079
0 201 2513 0 1175850569 608
0 202 2038 0 1175850569 2663
</screen>
Another example with <literal>--rate=100</literal>
and <literal>--latency-limit=5</literal> (note the additional
<replaceable>schedule_lag</replaceable> column):
<screen>
0 81 4621 0 1412881037 912698 3005
0 82 6173 0 1412881037 914578 4304
0 83 skipped 0 1412881037 914578 5217
0 83 skipped 0 1412881037 914578 5099
0 83 4722 0 1412881037 916203 3108
0 84 4142 0 1412881037 918023 2333
0 85 2465 0 1412881037 919759 740
</screen>
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.
</para>
<para>
When running a long test on hardware that can handle a lot of transactions,
the log files can become very large. The <option>--sampling-rate</option> option
can be used to log only a random sample of transactions.
</para>
</refsect2>
<refsect2>
<title>Aggregated Logging</title>
<para>
With the <option>--aggregate-interval</option> option, a different
format is used for the log files:
<synopsis>
<replaceable>interval_start</replaceable> <replaceable>num_transactions</replaceable>&zwsp; <replaceable>sum_latency</replaceable> <replaceable>sum_latency_2</replaceable> <replaceable>min_latency</replaceable> <replaceable>max_latency</replaceable>&zwsp; <optional> <replaceable>sum_lag</replaceable> <replaceable>sum_lag_2</replaceable> <replaceable>min_lag</replaceable> <replaceable>max_lag</replaceable> <optional> <replaceable>skipped</replaceable> </optional> </optional>
</synopsis>
where
<replaceable>interval_start</replaceable> is the start of the interval (as a Unix
epoch time stamp),
<replaceable>num_transactions</replaceable> is the number of transactions
within the interval,
<replaceable>sum_latency</replaceable> is the sum of the transaction
latencies within the interval,
<replaceable>sum_latency_2</replaceable> is the sum of squares of the
transaction latencies within the interval,
<replaceable>min_latency</replaceable> is the minimum latency within the interval,
and
<replaceable>max_latency</replaceable> is the maximum latency within the interval.
The next fields,
<replaceable>sum_lag</replaceable>, <replaceable>sum_lag_2</replaceable>, <replaceable>min_lag</replaceable>,
and <replaceable>max_lag</replaceable>, are only present if the <option>--rate</option>
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, <replaceable>skipped</replaceable>,
is only present if the <option>--latency-limit</option> 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.
</para>
<para>
Here is some example output:
<screen>
1345828501 5601 1542744 483552416 61 2573
1345828503 7884 1979812 565806736 60 1479
1345828505 7208 1979422 567277552 59 1391
1345828507 7685 1980268 569784714 60 1398
1345828509 7073 1979779 573489941 236 1411
</screen></para>
<para>
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.
</para>
</refsect2>
<refsect2>
<title>Per-Statement Latencies</title>
<para>
With the <option>-r</option> option, <application>pgbench</application> 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.
</para>
<para>
For the default script, the output will look similar to this:
<screen>
starting vacuum...end.
transaction type: &lt;builtin: TPC-B (sort of)&gt;
scaling factor: 1
query mode: simple
number of clients: 10
number of threads: 1
number of transactions per client: 1000
number of transactions actually processed: 10000/10000
latency average = 10.870 ms
latency stddev = 7.341 ms
initial connection time = 30.954 ms
tps = 907.949122 (without initial connection time)
statement latencies in milliseconds:
0.001 \set aid random(1, 100000 * :scale)
0.001 \set bid random(1, 1 * :scale)
0.001 \set tid random(1, 10 * :scale)
0.000 \set delta random(-5000, 5000)
0.046 BEGIN;
0.151 UPDATE pgbench_accounts SET abalance = abalance + :delta WHERE aid = :aid;
0.107 SELECT abalance FROM pgbench_accounts WHERE aid = :aid;
4.241 UPDATE pgbench_tellers SET tbalance = tbalance + :delta WHERE tid = :tid;
5.245 UPDATE pgbench_branches SET bbalance = bbalance + :delta WHERE bid = :bid;
0.102 INSERT INTO pgbench_history (tid, bid, aid, delta, mtime) VALUES (:tid, :bid, :aid, :delta, CURRENT_TIMESTAMP);
0.974 END;
</screen>
</para>
<para>
If multiple script files are specified, the averages are reported
separately for each script file.
</para>
<para>
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.
</para>
</refsect2>
<refsect2>
<title>Good Practices</title>
<para>
It is very easy to use <application>pgbench</application> to produce completely
meaningless numbers. Here are some guidelines to help you get useful
results.
</para>
<para>
In the first place, <emphasis>never</emphasis> believe any test that runs
for only a few seconds. Use the <option>-t</option> or <option>-T</option> 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.
</para>
<para>
For the default TPC-B-like test scenario, the initialization scale factor
(<option>-s</option>) should be at least as large as the largest number of
clients you intend to test (<option>-c</option>); else you'll mostly be
measuring update contention. There are only <option>-s</option> rows in
the <structname>pgbench_branches</structname> table, and every transaction wants to
update one of them, so <option>-c</option> values in excess of <option>-s</option>
will undoubtedly result in lots of transactions blocked waiting for
other transactions.
</para>
<para>
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.
</para>
<para>
A limitation of <application>pgbench</application> is that it can itself become
the bottleneck when trying to test a large number of client sessions.
This can be alleviated by running <application>pgbench</application> on a different
machine from the database server, although low network latency will be
essential. It might even be useful to run several <application>pgbench</application>
instances concurrently, on several client machines, against the same
database server.
</para>
</refsect2>
<refsect2>
<title>Security</title>
<para>
If untrusted users have access to a database that has not adopted a
<link linkend="ddl-schemas-patterns">secure schema usage pattern</link>,
do not run <application>pgbench</application> in that
database. <application>pgbench</application> uses unqualified names and
does not manipulate the search path.
</para>
</refsect2>
</refsect1>
</refentry>