pgbench(1) PostgreSQL 17.4 Documentation pgbench(1)
NAME
pgbench - run a benchmark test on PostgreSQL
SYNOPSIS
pgbench -i [option...] [dbname]
pgbench [option...] [dbname]
DESCRIPTION
pgbench is a simple program for running benchmark tests on PostgreSQL.
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, pgbench tests a
scenario that is loosely based on TPC-B, involving five SELECT, UPDATE,
and INSERT commands per transaction. However, it is easy to test other
cases by writing your own transaction script files.
Typical output from pgbench looks like:
transaction type: <builtin: TPC-B (sort of)>
scaling factor: 10
query mode: simple
number of clients: 10
number of threads: 1
maximum number of tries: 1
number of transactions per client: 1000
number of transactions actually processed: 10000/10000
number of failed transactions: 0 (0.000%)
latency average = 11.013 ms
latency stddev = 7.351 ms
initial connection time = 45.758 ms
tps = 896.967014 (without initial connection time)
The first seven lines report some of the most important parameter
settings. The sixth line reports the maximum number of tries for
transactions with serialization or deadlock errors (see Failures and
Serialization/Deadlock Retries for more information). The eighth 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 or some SQL command(s) failed. (In -T mode, only the actual
number of transactions is printed.) The next line reports the number of
failed transactions due to serialization or deadlock errors (see
Failures and Serialization/Deadlock Retries for more information). The
last line reports the number of transactions per second.
The default TPC-B-like transaction test requires specific tables to be
set up beforehand. pgbench should be invoked with the -i (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:
pgbench -i [ other-options ] dbname
where dbname is the name of the already-created database to test in.
(You may also need -h, -p, and/or -U options to specify how to connect
to the database server.)
Caution
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 "scale factor" of 1, the tables initially contain this
many rows:
table # of rows
---------------------------------
pgbench_branches 1
pgbench_tellers 10
pgbench_accounts 100000
pgbench_history 0
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
pgbench [ options ] dbname
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.
OPTIONS
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.
Initialization Options
pgbench accepts the following command-line initialization arguments:
[-d] dbname
[--dbname=]dbname
Specifies the name of the database to test in. If this is not
specified, the environment variable PGDATABASE is used. If that is
not set, the user name specified for the connection is used.
-i
--initialize
Required to invoke initialization mode.
-I init_steps
--init-steps=init_steps
Perform just a selected set of the normal initialization steps.
init_steps specifies the initialization steps to be performed,
using one character per step. Each step is invoked in the specified
order. The default is dtgvp. The available steps are:
d (Drop)
Drop any existing pgbench tables.
t (create Tables)
Create the tables used by the standard pgbench scenario, namely
pgbench_accounts, pgbench_branches, pgbench_history, and
pgbench_tellers.
g or G (Generate data, client-side or server-side)
Generate data and load it into the standard tables, replacing
any data already present.
With g (client-side data generation), data is generated in
pgbench client and then sent to the server. This uses the
client/server bandwidth extensively through a COPY. pgbench
uses the FREEZE option with version 14 or later of PostgreSQL
to speed up subsequent VACUUM, except on the pgbench_accounts
table if partitions are enabled. Using g causes logging to
print one message every 100,000 rows while generating data for
all tables.
With G (server-side data generation), only small queries are
sent from the pgbench 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 G
causes logging not to print any progress message while
generating data.
The default initialization behavior uses client-side data
generation (equivalent to g).
v (Vacuum)
Invoke VACUUM on the standard tables.
p (create Primary keys)
Create primary key indexes on the standard tables.
f (create Foreign keys)
Create foreign key constraints between the standard tables.
(Note that this step is not performed by default.)
-F fillfactor
--fillfactor=fillfactor
Create the pgbench_accounts, pgbench_tellers and pgbench_branches
tables with the given fillfactor. Default is 100.
-n
--no-vacuum
Perform no vacuuming during initialization. (This option suppresses
the v initialization step, even if it was specified in -I.)
-q
--quiet
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).
This setting has no effect if G is specified in -I.
-s scale_factor
--scale=scale_factor
Multiply the number of rows generated by the scale factor. For
example, -s 100 will create 10,000,000 rows in the pgbench_accounts
table. Default is 1. When the scale is 20,000 or larger, the
columns used to hold account identifiers (aid columns) will switch
to using larger integers (bigint), in order to be big enough to
hold the range of account identifiers.
--foreign-keys
Create foreign key constraints between the standard tables. (This
option adds the f step to the initialization step sequence, if it
is not already present.)
--index-tablespace=index_tablespace
Create indexes in the specified tablespace, rather than the default
tablespace.
--partition-method=NAME
Create a partitioned pgbench_accounts table with NAME method.
Expected values are range or hash. This option requires that
--partitions is set to non-zero. If unspecified, default is range.
--partitions=NUM
Create a partitioned pgbench_accounts table with NUM partitions of
nearly equal size for the scaled number of accounts. Default is 0,
meaning no partitioning.
--tablespace=tablespace
Create tables in the specified tablespace, rather than the default
tablespace.
--unlogged-tables
Create all tables as unlogged tables, rather than permanent tables.
Benchmarking Options
pgbench accepts the following command-line benchmarking arguments:
-b scriptname[@weight]
--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.
-c clients
--client=clients
Number of clients simulated, that is, number of concurrent database
sessions. Default is 1.
-C
--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.
-D varname=value
--define=varname=value
Define a variable for use by a custom script (see below). Multiple
-D options are allowed.
-f filename[@weight]
--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.
-j threads
--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.
-l
--log
Write information about each transaction to a log file. See below
for details.
-L limit
--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.
When the --max-tries option is used, a transaction which fails due
to a serialization anomaly or from a deadlock will not be retried
if the total time of all its tries is greater than limit ms. To
limit only the time of tries and not their number, use
--max-tries=0. By default, the option --max-tries is set to 1 and
transactions with serialization/deadlock errors are not retried.
See Failures and Serialization/Deadlock Retries for more
information about retrying such transactions.
-M querymode
--protocol=querymode
Protocol to use for submitting queries to the server:
o simple: use simple query protocol.
o extended: use extended query protocol.
o 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 53 for more
information.)
-n
--no-vacuum
Perform no vacuuming before running the test. This option is
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
--skip-some-updates
Run built-in simple-update script. Shorthand for -b simple-update.
-P sec
--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, standard deviation, and the
number of failed transactions 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. When --max-tries is used to enable transaction retries after
serialization/deadlock errors, the report includes the number of
retried transactions and the sum of all retries.
-r
--report-per-command
Report the following statistics for each command after the
benchmark finishes: the average per-statement latency (execution
time from the perspective of the client), the number of failures,
and the number of retries after serialization or deadlock errors in
this command. The report displays retry statistics only if the
--max-tries option is not equal to 1.
-R rate
--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.
-s scale_factor
--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.
-S
--select-only
Run built-in select-only script. Shorthand for -b select-only.
-t transactions
--transactions=transactions
Number of transactions each client runs. Default is 10.
-T seconds
--time=seconds
Run the test for this many seconds, rather than a fixed number of
transactions per client. -t and -T are mutually exclusive.
-v
--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.
--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.
--exit-on-abort
Exit immediately when any client is aborted due to some error.
Without this option, even when a client is aborted, other clients
could continue their run as specified by -t or -T option, and
pgbench will print an incomplete results in this case.
Note that serialization failures or deadlock failures do not abort
the client, so they are not affected by this option. See Failures
and Serialization/Deadlock Retries for more information.
--failures-detailed
Report failures in per-transaction and aggregation logs, as well as
in the main and per-script reports, grouped by the following types:
o serialization failures;
o deadlock failures;
See Failures and Serialization/Deadlock Retries for more
information.
--log-prefix=prefix
Set the filename prefix for the log files created by --log. The
default is pgbench_log.
--max-tries=number_of_tries
Enable retries for transactions with serialization/deadlock errors
and set the maximum number of these tries. This option can be
combined with the --latency-limit option which limits the total
time of all transaction tries; moreover, you cannot use an
unlimited number of tries (--max-tries=0) without --latency-limit
or --time. The default value is 1 and transactions with
serialization/deadlock errors are not retried. See Failures and
Serialization/Deadlock Retries for more information about retrying
such transactions.
--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.
--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.
--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).
--show-script=scriptname
Show the actual code of builtin script scriptname on stderr, and
exit immediately.
--verbose-errors
Print messages about all errors and failures (errors without
retrying) including which limit for retries was exceeded and how
far it was exceeded for the serialization/deadlock failures. (Note
that in this case the output can be significantly increased.) See
Failures and Serialization/Deadlock Retries for more information.
Common Options
pgbench also accepts the following common command-line arguments for
connection parameters and other common settings:
--debug
Print debugging output.
-h hostname
--host=hostname
The database server's host name
-p port
--port=port
The database server's port number
-U login
--username=login
The user name to connect as
-V
--version
Print the pgbench version and exit.
-?
--help
Show help about pgbench command line arguments, and exit.
EXIT STATUS
A successful run will exit with status 0. Exit status 1 indicates
static problems such as invalid command-line options or internal errors
which are supposed to never occur. Early errors that occur when
starting benchmark such as initial connection failures also exit with
status 1. Errors during the run such as database errors or problems in
the script will result in exit status 2. In the latter case, pgbench
will print partial results if --exit-on-abort option is not specified.
ENVIRONMENT
PGDATABASE
PGHOST
PGPORT
PGUSER
Default connection parameters.
This utility, like most other PostgreSQL utilities, uses the
environment variables supported by libpq (see Section 32.15).
The environment variable PG_COLOR specifies whether to use color in
diagnostic messages. Possible values are always, auto and never.
NOTES
What Is the "Transaction" Actually Performed in pgbench?
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.
1. BEGIN;
2. UPDATE pgbench_accounts SET abalance = abalance + :delta WHERE aid
= :aid;
3. SELECT abalance FROM pgbench_accounts WHERE aid = :aid;
4. UPDATE pgbench_tellers SET tbalance = tbalance + :delta WHERE tid =
:tid;
5. UPDATE pgbench_branches SET bbalance = bbalance + :delta WHERE bid
= :bid;
6. INSERT INTO pgbench_history (tid, bid, aid, delta, mtime) VALUES
(:tid, :bid, :aid, :delta, CURRENT_TIMESTAMP);
7. END;
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.
Custom Scripts
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 "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 "meta commands", which are interpreted by
pgbench itself, as described below.
Note
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
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 pgbench, be sure to write each SQL command
on a single line ending with a semicolon.
It is assumed that pgbench scripts do not contain incomplete blocks
of SQL transactions. If at runtime the client reaches the end of
the script without completing the last transaction block, it will
be aborted.
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 298. A value specified for these
variables using -D takes precedence over the automatic presets. Once
set, a variable's value can be inserted into an 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 298. pgbench Automatic Variables
+-------------+----------------------------+
|Variable | Description |
+-------------+----------------------------+
|client_id | unique number identifying |
| | the client session (starts |
| | from zero) |
+-------------+----------------------------+
|default_seed | seed used in hash and |
| | pseudorandom permutation |
| | functions by default |
+-------------+----------------------------+
|random_seed | random generator seed |
| | (unless overwritten with |
| | -D) |
+-------------+----------------------------+
|scale | current scale factor |
+-------------+----------------------------+
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:
\gset [prefix] \aset [prefix]
These commands may be used to end SQL queries, taking the place of
the terminating semicolon (;).
When the \gset 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 prefix if provided.
When the \aset command is used, all combined SQL queries (separated
by \;) have their columns stored into variables named after column
names, and prefixed with prefix 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.
\gset and \aset cannot be used in pipeline mode, since the query
results are not yet available by the time the commands would need
them.
The following example puts the final account balance from the first
query into variable abalance, and fills variables p_two and p_three
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 four and five.
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
\if expression
\elif expression
\else
\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.
\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:
\set ntellers 10 * :scale
\set aid (1021 * random(1, 100000 * :scale)) % \
(100000 * :scale) + 1
\set divx CASE WHEN :x <> 0 THEN :y/:x ELSE NULL END
\sleep number [ us | ms | s ]
Causes script execution to sleep for the specified duration in
microseconds (us), milliseconds (ms) or seconds (s). If the unit is
omitted then seconds are the default. number can be either an
integer constant or a :variablename reference to a variable having
an integer value.
Example:
\sleep 10 ms
\setshell varname command [ argument ... ]
Sets variable varname to the result of the shell command command
with the given argument(s). The command must return an integer
value through its standard output.
command and each argument can be either a text constant or a
:variablename reference to a variable. If you want to use an
argument starting with a colon, write an additional colon at the
beginning of argument.
Example:
\setshell variable_to_be_assigned command literal_argument :variable ::literal_starting_with_colon
\shell command [ argument ... ]
Same as \setshell, but the result of the command is discarded.
Example:
\shell command literal_argument :variable ::literal_starting_with_colon
\startpipeline
\syncpipeline
\endpipeline
This group of commands implements pipelining of SQL statements. A
pipeline must begin with a \startpipeline and end with an
\endpipeline. In between there may be any number of \syncpipeline
commands, which sends a sync message without ending the ongoing
pipeline and flushing the send buffer. In pipeline mode, statements
are sent to the server without waiting for the results of previous
statements. See Section 32.5 for more details. Pipeline mode
requires the use of extended query protocol.
Built-in Operators
The arithmetic, bitwise, comparison and logical operators listed in
Table 299 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 299. pgbench Operators
+----------------------------------------+
| Operator .PP Description |
|.PP Example(s) |
+----------------------------------------+
| boolean OR boolean -> boolean |
|.PP Logical OR .PP 5 or 0 -> |
|TRUE |
+----------------------------------------+
| boolean AND boolean -> boolean |
|.PP Logical AND .PP 3 and 0 -> |
|FALSE |
+----------------------------------------+
| NOT boolean -> boolean .PP |
|Logical NOT .PP not false -> |
|TRUE |
+----------------------------------------+
| boolean IS [NOT] (NULL|TRUE|FALSE) -> |
|boolean .PP Boolean value tests |
|.PP 1 is null -> FALSE |
+----------------------------------------+
| value ISNULL|NOTNULL -> boolean |
|.PP Nullness tests .PP 1 |
|notnull -> TRUE |
+----------------------------------------+
| number = number -> boolean .PP |
|Equal .PP 5 = 4 -> FALSE |
+----------------------------------------+
| number <> number -> boolean |
|.PP Not equal .PP 5 <> 4 -> |
|TRUE |
+----------------------------------------+
| number != number -> boolean |
|.PP Not equal .PP 5 != 5 -> |
|FALSE |
+----------------------------------------+
| number < number -> boolean .PP |
|Less than .PP 5 < 4 -> FALSE |
+----------------------------------------+
| number <= number -> boolean |
|.PP Less than or equal to .PP 5 |
|<= 4 -> FALSE |
+----------------------------------------+
| number > number -> boolean .PP |
|Greater than .PP 5 > 4 -> TRUE |
+----------------------------------------+
| number >= number -> boolean |
|.PP Greater than or equal to |
|.PP 5 >= 4 -> TRUE |
+----------------------------------------+
| integer | integer -> integer |
|.PP Bitwise OR .PP 1 | 2 -> 3 |
+----------------------------------------+
| integer # integer -> integer |
|.PP Bitwise XOR .PP 1 # 3 -> 2 |
+----------------------------------------+
| integer & integer -> integer |
|.PP Bitwise AND .PP 1 & 3 -> 1 |
+----------------------------------------+
| ~ integer -> integer .PP |
|Bitwise NOT .PP ~ 1 -> -2 |
+----------------------------------------+
| integer << integer -> integer |
|.PP Bitwise shift left .PP 1 << |
|2 -> 4 |
+----------------------------------------+
| integer >> integer -> integer |
|.PP Bitwise shift right .PP 8 |
|>> 2 -> 2 |
+----------------------------------------+
| number + number -> number .PP |
|Addition .PP 5 + 4 -> 9 |
+----------------------------------------+
| number - number -> number .PP |
|Subtraction .PP 3 - 2.0 -> 1.0 |
+----------------------------------------+
| number * number -> number .PP |
|Multiplication .PP 5 * 4 -> 20 |
+----------------------------------------+
| number / number -> number .PP |
|Division (truncates the result towards |
|zero if both inputs are integers) |
|.PP 5 / 3 -> 1 |
+----------------------------------------+
| integer % integer -> integer |
|.PP Modulo (remainder) .PP 3 % |
|2 -> 1 |
+----------------------------------------+
| - number -> number .PP |
|Negation .PP - 2.0 -> -2.0 |
+----------------------------------------+
Built-In Functions
The functions listed in
Table 300 are built into pgbench and may be used in expressions
appearing in \set.
Table 300. pgbench Functions
+----------------------------------------+
| Function .PP Description |
|.PP Example(s) |
+----------------------------------------+
| abs ( number ) -> same type as input |
|.PP Absolute value .PP abs(-17) |
|-> 17 |
+----------------------------------------+
| debug ( number ) -> same type as input |
|.PP Prints the argument to stderr, and |
|returns the argument. .PP |
|debug(5432.1) -> 5432.1 |
+----------------------------------------+
| double ( number ) -> double |
|.PP Casts to double. .PP |
|double(5432) -> 5432.0 |
+----------------------------------------+
| exp ( number ) -> double .PP |
|Exponential (e raised to the given |
|power) .PP exp(1.0) -> |
|2.718281828459045 |
+----------------------------------------+
| greatest ( number [, ... ] ) -> double |
|if any argument is double, else integer |
|.PP Selects the largest value among the |
|arguments. .PP greatest(5, 4, |
|3, 2) -> 5 |
+----------------------------------------+
| hash ( value [, seed ] ) -> integer |
|.PP This is an alias for hash_murmur2. |
|.PP hash(10, 5432) -> |
|-5817877081768721676 |
+----------------------------------------+
| hash_fnv1a ( value [, seed ] ) -> |
|integer .PP Computes FNV-1a |
|hash. .PP hash_fnv1a(10, 5432) |
|-> -7793829335365542153 |
+----------------------------------------+
| hash_murmur2 ( value [, seed ] ) -> |
|integer .PP Computes |
|MurmurHash2 hash. .PP |
|hash_murmur2(10, 5432) -> |
|-5817877081768721676 |
+----------------------------------------+
| int ( number ) -> integer .PP |
|Casts to integer. .PP int(5.4 + |
|3.8) -> 9 |
+----------------------------------------+
| least ( number [, ... ] ) -> double if |
|any argument is double, else integer |
|.PP Selects the smallest value among |
|the arguments. .PP least(5, 4, |
|3, 2.1) -> 2.1 |
+----------------------------------------+
| ln ( number ) -> double .PP |
|Natural logarithm .PP |
|ln(2.718281828459045) -> 1.0 |
+----------------------------------------+
| mod ( integer, integer ) -> integer |
|.PP Modulo (remainder) .PP |
|mod(54, 32) -> 22 |
+----------------------------------------+
| permute ( i, size [, seed ] ) -> |
|integer .PP Permuted value of |
|i, in the range [0, size). This is the |
|new position of i (modulo size) in a |
|pseudorandom permutation of the |
|integers 0...size-1, parameterized by |
|seed, see below. .PP permute(0, |
|4) -> an integer between 0 and 3 |
+----------------------------------------+
| pi () -> double .PP |
|Approximate value of <pi> .PP |
|pi() -> 3.14159265358979323846 |
+----------------------------------------+
| pow ( x, y ) -> double .PP |
|power ( x, y ) -> double .PP x |
|raised to the power of y .PP |
|pow(2.0, 10) -> 1024.0 |
+----------------------------------------+
| random ( lb, ub ) -> integer |
|.PP Computes a uniformly-distributed |
|random integer in [lb, ub]. .PP |
|random(1, 10) -> an integer between 1 |
|and 10 |
+----------------------------------------+
| random_exponential ( lb, ub, parameter |
|) -> integer .PP Computes an |
|exponentially-distributed random |
|integer in [lb, ub], see below. |
|.PP random_exponential(1, 10, 3.0) -> |
|an integer between 1 and 10 |
+----------------------------------------+
| random_gaussian ( lb, ub, parameter ) |
|-> integer .PP Computes a |
|Gaussian-distributed random integer in |
|[lb, ub], see below. .PP |
|random_gaussian(1, 10, 2.5) -> an |
|integer between 1 and 10 |
+----------------------------------------+
| random_zipfian ( lb, ub, parameter ) |
|-> integer .PP Computes a |
|Zipfian-distributed random integer in |
|[lb, ub], see below. .PP |
|random_zipfian(1, 10, 1.5) -> an |
|integer between 1 and 10 |
+----------------------------------------+
| sqrt ( number ) -> double .PP |
|Square root .PP sqrt(2.0) -> |
|1.414213562 |
+----------------------------------------+
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.
o For an exponential distribution, parameter controls the
distribution by truncating a quickly-decreasing exponential
distribution at parameter, and then projecting onto integers
between the bounds. To be precise, with
f(x) = exp(-parameter * (x - min) / (max - min + 1)) / (1 - exp(-parameter))
Then value i between min and max inclusive is drawn with
probability: f(i) - f(i + 1).
Intuitively, the larger the parameter, the more frequently values
close to min are accessed, and the less frequently values close to
max are accessed. The closer to 0 parameter 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 min, are drawn parameter% of the time. The parameter value
must be strictly positive.
o For a Gaussian distribution, the interval is mapped onto a standard
normal distribution (the classical bell-shaped Gaussian curve)
truncated at -parameter on the left and +parameter on the right.
Values in the middle of the interval are more likely to be drawn.
To be precise, if PHI(x) is the cumulative distribution function of
the standard normal distribution, with mean mu defined as (max +
min) / 2.0, with
f(x) = PHI(2.0 * parameter * (x - mu) / (max - min + 1)) /
(2.0 * PHI(parameter) - 1)
then value i between min and max inclusive is drawn with
probability: f(i + 0.5) - f(i - 0.5). Intuitively, the larger the
parameter, the more frequently values close to the middle of the
interval are drawn, and the less frequently values close to the min
and max bounds. About 67% of values are drawn from the middle 1.0 /
parameter, that is a relative 0.5 / parameter around the mean, and
95% in the middle 2.0 / parameter, that is a relative 1.0 /
parameter around the mean; for instance, if parameter is 4.0, 67%
of values are drawn from the middle quarter (1.0 / 4.0) of the
interval (i.e., from 3.0 / 8.0 to 5.0 / 8.0) and 95% from the
middle half (2.0 / 4.0) of the interval (second and third
quartiles). The minimum allowed parameter value is 2.0.
o random_zipfian generates a bounded Zipfian distribution. parameter
defines how skewed the distribution is. The larger the parameter,
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 k versus drawing
k+1 is ((k+1)/k)**parameter. For example, random_zipfian(1, ...,
2.5) produces the value 1 about (2/1)**2.5 = 5.66 times more
frequently than 2, which itself is produced (3/2)**2.5 = 2.76 times
more frequently than 3, and so on.
pgbench's implementation is based on "Non-Uniform Random Variate
Generation", Luc Devroye, p. 550-551, Springer 1986. Due to
limitations of that algorithm, the parameter value is restricted to
the range [1.001, 1000].
Note
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.
To avoid this, you may wish to use the permute function, or some
other additional step with similar effect, to shuffle the selected
rows and remove such correlations.
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.
permute accepts an input value, a size, and an optional seed parameter.
It generates a pseudorandom permutation of integers in the range [0,
size), and returns the index of the input value in the permuted values.
The permutation chosen is parameterized by the seed, which defaults to
:default_seed, if not specified. Unlike the hash functions, permute
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. permute can be
used to scatter the distribution of non-uniform random functions such
as random_zipfian or random_exponential so that values drawn more often
are not trivially correlated. For instance, the following pgbench
script simulates a possible real world workload typical for social
media and blogging platforms where a few accounts generate excessive
load:
\set size 1000000
\set r random_zipfian(1, :size, 1.07)
\set k 1 + permute(:r, :size)
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:
\set k1 1 + permute(:r, :size, :default_seed + 123)
\set k2 1 + permute(:r, :size, :default_seed + 321)
A similar behavior can also be approximated with hash:
\set size 1000000
\set r random_zipfian(1, 100 * :size, 1.07)
\set k 1 + abs(hash(:r)) % :size
However, since hash generates collisions, some values will not be
reachable and others will be more frequent than expected from the
original distribution.
As an example, the full definition of the built-in TPC-B-like
transaction is:
\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;
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.)
Per-Transaction Logging
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.
Each line in a log file describes one transaction. It contains the
following space-separated fields:
client_id
identifies the client session that ran the transaction
transaction_no
counts how many transactions have been run by that session
time
transaction's elapsed time, in microseconds
script_no
identifies the script file that was used for the transaction
(useful when multiple scripts are specified with -f or -b)
time_epoch
transaction's completion time, as a Unix-epoch time stamp
time_us
fractional-second part of transaction's completion time, in
microseconds
schedule_lag
transaction start delay, that is the difference between the
transaction's scheduled start time and the time it actually
started, in microseconds (present only if --rate is specified)
retries
count of retries after serialization or deadlock errors during the
transaction (present only if --max-tries is not equal to one)
When both --rate and --latency-limit are used, the time for a skipped
transaction will be reported as skipped. If the transaction ends with a
failure, its time will be reported as failed. If you use the
--failures-detailed option, the time of the failed transaction will be
reported as serialization or deadlock depending on the type of failure
(see Failures and Serialization/Deadlock Retries for more information).
Here is a snippet of a log file generated in a single-client run:
0 199 2241 0 1175850568 995598
0 200 2465 0 1175850568 998079
0 201 2513 0 1175850569 608
0 202 2038 0 1175850569 2663
Another example with --rate=100 and --latency-limit=5 (note the
additional schedule_lag column):
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
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.
The following example shows a snippet of a log file with failures and
retries, with the maximum number of tries set to 10 (note the
additional retries column):
3 0 47423 0 1499414498 34501 3
3 1 8333 0 1499414498 42848 0
3 2 8358 0 1499414498 51219 0
4 0 72345 0 1499414498 59433 6
1 3 41718 0 1499414498 67879 4
1 4 8416 0 1499414498 76311 0
3 3 33235 0 1499414498 84469 3
0 0 failed 0 1499414498 84905 9
2 0 failed 0 1499414498 86248 9
3 4 8307 0 1499414498 92788 0
If the --failures-detailed option is used, the type of failure is
reported in the time like this:
3 0 47423 0 1499414498 34501 3
3 1 8333 0 1499414498 42848 0
3 2 8358 0 1499414498 51219 0
4 0 72345 0 1499414498 59433 6
1 3 41718 0 1499414498 67879 4
1 4 8416 0 1499414498 76311 0
3 3 33235 0 1499414498 84469 3
0 0 serialization 0 1499414498 84905 9
2 0 serialization 0 1499414498 86248 9
3 4 8307 0 1499414498 92788 0
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.
Aggregated Logging
With the --aggregate-interval option, a different format is used for
the log files. Each log line describes one aggregation interval. It
contains the following space-separated fields:
interval_start
start time of the interval, as a Unix-epoch time stamp
num_transactions
number of transactions within the interval
sum_latency
sum of transaction latencies
sum_latency_2
sum of squares of transaction latencies
min_latency
minimum transaction latency
max_latency
maximum transaction latency
sum_lag
sum of transaction start delays (zero unless --rate is specified)
sum_lag_2
sum of squares of transaction start delays (zero unless --rate is
specified)
min_lag
minimum transaction start delay (zero unless --rate is specified)
max_lag
maximum transaction start delay (zero unless --rate is specified)
skipped
number of transactions skipped because they would have started too
late (zero unless --rate and --latency-limit are specified)
retried
number of retried transactions (zero unless --max-tries is not
equal to one)
retries
number of retries after serialization or deadlock errors (zero
unless --max-tries is not equal to one)
serialization_failures
number of transactions that got a serialization error and were not
retried afterwards (zero unless --failures-detailed is specified)
deadlock_failures
number of transactions that got a deadlock error and were not
retried afterwards (zero unless --failures-detailed is specified)
Here is some example output generated with this option:
pgbench --aggregate-interval=10 --time=20 --client=10 --log --rate=1000 --latency-limit=10 --failures-detailed --max-tries=10 test
1650260552 5178 26171317 177284491527 1136 44462 2647617 7321113867 0 9866 64 7564 28340 4148 0
1650260562 4808 25573984 220121792172 1171 62083 3037380 9666800914 0 9998 598 7392 26621 4527 0
Notice that while the plain (unaggregated) log format shows which
script was used for each transaction, the aggregated format does not.
Therefore if you need per-script data, you need to aggregate the data
on your own.
Per-Statement Report
With the -r option, pgbench collects the following statistics for each
statement:
o latency -- elapsed transaction time for each statement. pgbench
reports an average value of all successful runs of the statement.
o The number of failures in this statement. See Failures and
Serialization/Deadlock Retries for more information.
o The number of retries after a serialization or a deadlock error in
this statement. See Failures and Serialization/Deadlock Retries for
more information.
The report displays retry statistics only if the --max-tries option is
not equal to 1.
All values are computed for each statement executed by every client and
are reported after the benchmark has finished.
For the default script, the output will look similar to this:
starting vacuum...end.
transaction type: <builtin: TPC-B (sort of)>
scaling factor: 1
query mode: simple
number of clients: 10
number of threads: 1
maximum number of tries: 1
number of transactions per client: 1000
number of transactions actually processed: 10000/10000
number of failed transactions: 0 (0.000%)
number of transactions above the 50.0 ms latency limit: 1311/10000 (13.110 %)
latency average = 28.488 ms
latency stddev = 21.009 ms
initial connection time = 69.068 ms
tps = 346.224794 (without initial connection time)
statement latencies in milliseconds and failures:
0.012 0 \set aid random(1, 100000 * :scale)
0.002 0 \set bid random(1, 1 * :scale)
0.002 0 \set tid random(1, 10 * :scale)
0.002 0 \set delta random(-5000, 5000)
0.319 0 BEGIN;
0.834 0 UPDATE pgbench_accounts SET abalance = abalance + :delta WHERE aid = :aid;
0.641 0 SELECT abalance FROM pgbench_accounts WHERE aid = :aid;
11.126 0 UPDATE pgbench_tellers SET tbalance = tbalance + :delta WHERE tid = :tid;
12.961 0 UPDATE pgbench_branches SET bbalance = bbalance + :delta WHERE bid = :bid;
0.634 0 INSERT INTO pgbench_history (tid, bid, aid, delta, mtime) VALUES (:tid, :bid, :aid, :delta, CURRENT_TIMESTAMP);
1.957 0 END;
Another example of output for the default script using serializable
default transaction isolation level (PGOPTIONS='-c
default_transaction_isolation=serializable' pgbench ...):
starting vacuum...end.
transaction type: <builtin: TPC-B (sort of)>
scaling factor: 1
query mode: simple
number of clients: 10
number of threads: 1
maximum number of tries: 10
number of transactions per client: 1000
number of transactions actually processed: 6317/10000
number of failed transactions: 3683 (36.830%)
number of transactions retried: 7667 (76.670%)
total number of retries: 45339
number of transactions above the 50.0 ms latency limit: 106/6317 (1.678 %)
latency average = 17.016 ms
latency stddev = 13.283 ms
initial connection time = 45.017 ms
tps = 186.792667 (without initial connection time)
statement latencies in milliseconds, failures and retries:
0.006 0 0 \set aid random(1, 100000 * :scale)
0.001 0 0 \set bid random(1, 1 * :scale)
0.001 0 0 \set tid random(1, 10 * :scale)
0.001 0 0 \set delta random(-5000, 5000)
0.385 0 0 BEGIN;
0.773 0 1 UPDATE pgbench_accounts SET abalance = abalance + :delta WHERE aid = :aid;
0.624 0 0 SELECT abalance FROM pgbench_accounts WHERE aid = :aid;
1.098 320 3762 UPDATE pgbench_tellers SET tbalance = tbalance + :delta WHERE tid = :tid;
0.582 3363 41576 UPDATE pgbench_branches SET bbalance = bbalance + :delta WHERE bid = :bid;
0.465 0 0 INSERT INTO pgbench_history (tid, bid, aid, delta, mtime) VALUES (:tid, :bid, :aid, :delta, CURRENT_TIMESTAMP);
1.933 0 0 END;
If multiple script files are specified, all statistics 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.
Failures and Serialization/Deadlock Retries
When executing pgbench, there are three main types of errors:
o Errors of the main program. They are the most serious and always
result in an immediate exit from pgbench with the corresponding
error message. They include:
o errors at the beginning of pgbench (e.g. an invalid option
value);
o errors in the initialization mode (e.g. the query to create
tables for built-in scripts fails);
o errors before starting threads (e.g. could not connect to the
database server, syntax error in the meta command, thread
creation failure);
o internal pgbench errors (which are supposed to never occur...).
o Errors when the thread manages its clients (e.g. the client could
not start a connection to the database server / the socket for
connecting the client to the database server has become invalid).
In such cases all clients of this thread stop while other threads
continue to work. However, --exit-on-abort is specified, all of the
threads stop immediately in this case.
o Direct client errors. They lead to immediate exit from pgbench with
the corresponding error message in the case of an internal pgbench
error (which are supposed to never occur...) or when
--exit-on-abort is specified. Otherwise in the worst case they only
lead to the abortion of the failed client while other clients
continue their run (but some client errors are handled without an
abortion of the client and reported separately, see below). Later
in this section it is assumed that the discussed errors are only
the direct client errors and they are not internal pgbench errors.
A client's run is aborted in case of a serious error; for example, the
connection with the database server was lost or the end of script was
reached without completing the last transaction. In addition, if
execution of an SQL or meta command fails for reasons other than
serialization or deadlock errors, the client is aborted. Otherwise, if
an SQL command fails with serialization or deadlock errors, the client
is not aborted. In such cases, the current transaction is rolled back,
which also includes setting the client variables as they were before
the run of this transaction (it is assumed that one transaction script
contains only one transaction; see What Is the "Transaction" Actually
Performed in pgbench? for more information). Transactions with
serialization or deadlock errors are repeated after rollbacks until
they complete successfully or reach the maximum number of tries
(specified by the --max-tries option) / the maximum time of retries
(specified by the --latency-limit option) / the end of benchmark
(specified by the --time option). If the last trial run fails, this
transaction will be reported as failed but the client is not aborted
and continues to work.
Note
Without specifying the --max-tries option, a transaction will never
be retried after a serialization or deadlock error because its
default value is 1. Use an unlimited number of tries
(--max-tries=0) and the --latency-limit option to limit only the
maximum time of tries. You can also use the --time option to limit
the benchmark duration under an unlimited number of tries.
Be careful when repeating scripts that contain multiple
transactions: the script is always retried completely, so
successful transactions can be performed several times.
Be careful when repeating transactions with shell commands. Unlike
the results of SQL commands, the results of shell commands are not
rolled back, except for the variable value of the \setshell
command.
The latency of a successful transaction includes the entire time of
transaction execution with rollbacks and retries. The latency is
measured only for successful transactions and commands but not for
failed transactions or commands.
The main report contains the number of failed transactions. If the
--max-tries option is not equal to 1, the main report also contains
statistics related to retries: the total number of retried transactions
and total number of retries. The per-script report inherits all these
fields from the main report. The per-statement report displays retry
statistics only if the --max-tries option is not equal to 1.
If you want to group failures by basic types in per-transaction and
aggregation logs, as well as in the main and per-script reports, use
the --failures-detailed option. If you also want to distinguish all
errors and failures (errors without retrying) by type including which
limit for retries was exceeded and how much it was exceeded by for the
serialization/deadlock failures, use the --verbose-errors option.
Table Access Methods
You may specify the Table Access Method for the pgbench tables. The
environment variable PGOPTIONS specifies database configuration options
that are passed to PostgreSQL via the command line (See
Section 19.1.4). For example, a hypothetical default Table Access
Method for the tables that pgbench creates called wuzza can be
specified with:
PGOPTIONS='-c default_table_access_method=wuzza'
Good Practices
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.
Security
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.
PostgreSQL 17.4 2025 pgbench(1)
postgresql 17.4 - Generated Sat Mar 22 11:54:47 CDT 2025