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pgbench(1) - phpMan
PGBENCH(1) PostgreSQL 12.3 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
number of transactions per client: 1000
number of transactions actually processed: 10000/10000
tps = 85.184871 (including connections establishing)
tps = 85.296346 (excluding connections establishing)
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 -T mode, only the actual number of
transactions is printed.) The last two lines report the number of transactions per second,
figured with and without counting the time to start database sessions.
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:
-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 (Generate data)
Generate data and load it into the standard tables, replacing any data already
present.
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 100000 rows, which often outputs many lines
per second (especially on good hardware).
-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.
--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 executed scripts. An optional integer
weight after @ allows to adjust the probability of drawing the script. If not
specified, it is set to 1. Available built-in scripts are: tpcb-like, simple-update
and select-only. Unambiguous prefixes of built-in names are accepted. With special
name list, show the list of built-in scripts and exit immediately.
-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
--debug
Print debugging output.
-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 executed scripts. An
optional integer weight after @ allows to adjust the probability of drawing the test.
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.
-M querymode
--protocol=querymode
Protocol to use for submitting queries to the server:
· simple: use simple query protocol.
· extended: use extended query protocol.
· prepared: use extended query protocol with prepared statements.
In the prepared mode, pgbench reuses the parse analysis result starting from the
second query iteration, so pgbench runs faster than in other modes.
The default is simple query protocol. (See Chapter 52 for more information.)
-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 and standard deviation since the last report. Under throttling (-R), the
latency is computed with respect to the transaction scheduled start time, not the
actual transaction beginning time, thus it also includes the average schedule lag
time.
-r
--report-latencies
Report the average per-statement latency (execution time from the perspective of the
client) of each command after the benchmark finishes. See below for details.
-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.
--log-prefix=prefix
Set the filename prefix for the log files created by --log. The default is
pgbench_log.
--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).
Common Options
pgbench accepts the following command-line common arguments:
-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. 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.
ENVIRONMENT
PGHOST
PGPORT
PGUSER
Default connection parameters.
This utility, like most other PostgreSQL utilities, uses the environment variables
supported by libpq (see Section 33.14).
NOTES
What Is the “Transaction” Actually Performed in pgbench?
pgbench executes test scripts chosen randomly from a specified list. They include built-in
scripts with -b and user-provided custom scripts with -f. Each script may be given a
relative weight specified after a @ so as to change its drawing 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 a SQL command does not need one if it is followed by
a meta command). If you need to create a script file that works with both old and new
versions of pgbench, be sure to write each SQL command on a single line ending with a
semicolon.
There is a simple variable-substitution facility for script files. Variable names must
consist of letters (including non-Latin letters), digits, and underscores. 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 257. A value specified for these
variables using -D takes precedence over the automatic presets. Once set, a variable's
value can be inserted into a SQL command by writing :variablename. When running more than
one client session, each session has its own set of variables. pgbench supports up to 255
variable uses in one statement.
Table 257. Automatic Variables
┌─────────────┬────────────────────────────────┐
│Variable │ Description │
├─────────────┼────────────────────────────────┤
│client_id │ unique number identifying the │
│ │ client session (starts from │
│ │ zero) │
├─────────────┼────────────────────────────────┤
│default_seed │ seed used in hash 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]
This command may be used to end SQL queries, taking the place of the terminating
semicolon (;).
When this 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.
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.
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_
\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
Built-in Operators
The arithmetic, bitwise, comparison and logical operators listed in Table 258 are built
into pgbench and may be used in expressions appearing in \set.
Table 258. pgbench Operators by Increasing Precedence
┌──────────────────┬─────────────────────┬───────────┬────────┐
│Operator │ Description │ Example │ Result │
├──────────────────┼─────────────────────┼───────────┼────────┤
│OR │ logical or │ 5 or 0 │ TRUE │
├──────────────────┼─────────────────────┼───────────┼────────┤
│AND │ logical and │ 3 and 0 │ FALSE │
├──────────────────┼─────────────────────┼───────────┼────────┤
│NOT │ logical not │ not false │ TRUE │
├──────────────────┼─────────────────────┼───────────┼────────┤
│IS [NOT] │ value tests │ 1 is null │ FALSE │
│(NULL|TRUE|FALSE) │ │ │ │
├──────────────────┼─────────────────────┼───────────┼────────┤
│ISNULL|NOTNULL │ null tests │ 1 notnull │ TRUE │
├──────────────────┼─────────────────────┼───────────┼────────┤
│= │ is equal │ 5 = 4 │ FALSE │
├──────────────────┼─────────────────────┼───────────┼────────┤
│<> │ is not equal │ 5 <> 4 │ TRUE │
├──────────────────┼─────────────────────┼───────────┼────────┤
│!= │ is not equal │ 5 != 5 │ FALSE │
├──────────────────┼─────────────────────┼───────────┼────────┤
│< │ lower than │ 5 < 4 │ FALSE │
├──────────────────┼─────────────────────┼───────────┼────────┤
│<= │ lower or equal │ 5 <= 4 │ FALSE │
├──────────────────┼─────────────────────┼───────────┼────────┤
│> │ greater than │ 5 > 4 │ TRUE │
├──────────────────┼─────────────────────┼───────────┼────────┤
│>= │ greater or equal │ 5 >= 4 │ TRUE │
├──────────────────┼─────────────────────┼───────────┼────────┤
│| │ integer bitwise OR │ 1 | 2 │ 3 │
├──────────────────┼─────────────────────┼───────────┼────────┤
│# │ integer bitwise XOR │ 1 # 3 │ 2 │
├──────────────────┼─────────────────────┼───────────┼────────┤
│& │ integer bitwise AND │ 1 & 3 │ 1 │
├──────────────────┼─────────────────────┼───────────┼────────┤
│~ │ integer bitwise NOT │ ~ 1 │ -2 │
├──────────────────┼─────────────────────┼───────────┼────────┤
│<< │ integer bitwise │ 1 << 2 │ 4 │
│ │ shift left │ │ │
├──────────────────┼─────────────────────┼───────────┼────────┤
│>> │ integer bitwise │ 8 >> 2 │ 2 │
│ │ shift right │ │ │
├──────────────────┼─────────────────────┼───────────┼────────┤
│+ │ addition │ 5 + 4 │ 9 │
├──────────────────┼─────────────────────┼───────────┼────────┤
│- │ subtraction │ 3 - 2.0 │ 1.0 │
├──────────────────┼─────────────────────┼───────────┼────────┤
│* │ multiplication │ 5 * 4 │ 20 │
├──────────────────┼─────────────────────┼───────────┼────────┤
│/ │ division (integer │ 5 / 3 │ 1 │
│ │ truncates the │ │ │
│ │ results) │ │ │
├──────────────────┼─────────────────────┼───────────┼────────┤
│% │ modulo │ 3 % 2 │ 1 │
├──────────────────┼─────────────────────┼───────────┼────────┤
│- │ opposite │ - 2.0 │ -2.0 │
└──────────────────┴─────────────────────┴───────────┴────────┘
Built-In Functions
The functions listed in Table 259 are built into pgbench and may be used in expressions
appearing in \set.
Table 259. pgbench Functions
┌───────────────────────┬─────────────────┬───────────────────────────┬───────────────────────┬────────────────────────┐
│Function │ Return Type │ Description │ Example │ Result │
├───────────────────────┼─────────────────┼───────────────────────────┼───────────────────────┼────────────────────────┤
│abs(a) │ same as a │ absolute value │ abs(-17) │ 17 │
├───────────────────────┼─────────────────┼───────────────────────────┼───────────────────────┼────────────────────────┤
│debug(a) │ same as a │ print a to │ debug(5432.1) │ 5432.1 │
│ │ │ stderr, │ │ │
│ │ │ and │ │ │
│ │ │ return a │ │ │
├───────────────────────┼─────────────────┼───────────────────────────┼───────────────────────┼────────────────────────┤
│double(i) │ double │ cast to double │ double(5432) │ 5432.0 │
├───────────────────────┼─────────────────┼───────────────────────────┼───────────────────────┼────────────────────────┤
│exp(x) │ double │ exponential │ exp(1.0) │ 2.718281828459045 │
├───────────────────────┼─────────────────┼───────────────────────────┼───────────────────────┼────────────────────────┤
│greatest(a [, │ double if any a │ largest value │ greatest(5, 4, │ 5 │
│... ] ) │ is double, else │ among arguments │ 3, 2) │ │
│ │ integer │ │ │ │
├───────────────────────┼─────────────────┼───────────────────────────┼───────────────────────┼────────────────────────┤
│hash(a [, seed ] │ integer │ alias for │ hash(10, 5432) │ -5817877081768721676 │
│) │ │ hash_murmur2() │ │ │
├───────────────────────┼─────────────────┼───────────────────────────┼───────────────────────┼────────────────────────┤
│hash_fnv1a(a [, │ integer │ FNV-1a hash │ hash_fnv1a(10, │ -7793829335365542153 │
│seed ] ) │ │ │ 5432) │ │
├───────────────────────┼─────────────────┼───────────────────────────┼───────────────────────┼────────────────────────┤
│hash_murmur2(a │ integer │ MurmurHash2 hash │ hash_murmur2(10, │ -5817877081768721676 │
│[, seed ] ) │ │ │ 5432) │ │
├───────────────────────┼─────────────────┼───────────────────────────┼───────────────────────┼────────────────────────┤
│int(x) │ integer │ cast to int │ int(5.4 + 3.8) │ 9 │
├───────────────────────┼─────────────────┼───────────────────────────┼───────────────────────┼────────────────────────┤
│least(a [, ... ] │ double if any a │ smallest value │ least(5, 4, 3, │ 2.1 │
│) │ is double, else │ among arguments │ 2.1) │ │
│ │ integer │ │ │ │
├───────────────────────┼─────────────────┼───────────────────────────┼───────────────────────┼────────────────────────┤
│ln(x) │ double │ natural │ ln(2.718281828459045) │ 1.0 │
│ │ │ logarithm │ │ │
├───────────────────────┼─────────────────┼───────────────────────────┼───────────────────────┼────────────────────────┤
│mod(i, j) │ integer │ modulo │ mod(54, 32) │ 22 │
├───────────────────────┼─────────────────┼───────────────────────────┼───────────────────────┼────────────────────────┤
│pi() │ double │ value of the │ pi() │ 3.14159265358979323846 │
│ │ │ constant PI │ │ │
├───────────────────────┼─────────────────┼───────────────────────────┼───────────────────────┼────────────────────────┤
│pow(x, y), │ double │ exponentiation │ pow(2.0, 10), │ 1024.0 │
│power(x, y) │ │ │ power(2.0, 10) │ │
├───────────────────────┼─────────────────┼───────────────────────────┼───────────────────────┼────────────────────────┤
│random(lb, ub) │ integer │ uniformly-distributed │ random(1, 10) │ an integer between 1 │
│ │ │ random integer │ │ and 10 │
│ │ │ in [lb, ub] │ │ │
├───────────────────────┼─────────────────┼───────────────────────────┼───────────────────────┼────────────────────────┤
│random_exponential(lb, │ integer │ exponentially-distributed │ random_exponential(1, │ an integer between 1 │
│ub, parameter) │ │ random integer in │ 10, 3.0) │ and 10 │
│ │ │ [lb, ub], │ │ │
│ │ │ see │ │ │
│ │ │ below │ │ │
├───────────────────────┼─────────────────┼───────────────────────────┼───────────────────────┼────────────────────────┤
│random_gaussian(lb, │ integer │ Gaussian-distributed │ random_gaussian(1, │ an integer between 1 │
│ub, parameter) │ │ random integer in [lb, │ 10, 2.5) │ and 10 │
│ │ │ ub], │ │ │
│ │ │ see below │ │ │
├───────────────────────┼─────────────────┼───────────────────────────┼───────────────────────┼────────────────────────┤
│random_zipfian(lb, ub, │ integer │ Zipfian-distributed │ random_zipfian(1, 10, │ an integer between 1 │
│parameter) │ │ random integer in [lb, │ 1.5) │ and 10 │
│ │ │ ub], │ │ │
│ │ │ see below │ │ │
├───────────────────────┼─────────────────┼───────────────────────────┼───────────────────────┼────────────────────────┤
│sqrt(x) │ double │ square root │ 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.
· 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.
· 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.
· 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].
Hash functions hash, hash_murmur2 and hash_fnv1a accept an input value and an optional
seed parameter. In case the seed isn't provided the value of :default_seed is used, which
is initialized randomly unless set by the command-line -D option. Hash functions can be
used to scatter the distribution of random functions such as random_zipfian or
random_exponential. For instance, the following pgbench script simulates possible real
world workload typical for social media and blogging platforms where few accounts generate
excessive load:
\set r random_zipfian(0, 100000000, 1.07)
\set k abs(hash(:r)) % 1000000
In some cases several distinct distributions are needed which don't correlate with each
other and this is when implicit seed parameter comes in handy:
\set k1 abs(hash(:r, :default_seed + 123)) % 1000000
\set k2 abs(hash(:r, :default_seed + 321)) % 1000000
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.
The format of the log is:
client_id transaction_no time script_no time_epoch time_us [ schedule_lag ]
where client_id indicates which client session ran the transaction, transaction_no counts
how many transactions have been run by that session, time is the total elapsed transaction
time in microseconds, script_no identifies which script file was used (useful when
multiple scripts were specified with -f or -b), and time_epoch/time_us are a Unix-epoch
time stamp and an offset in microseconds (suitable for creating an ISO 8601 time stamp
with fractional seconds) showing when the transaction completed. The schedule_lag field is
the difference between the transaction's scheduled start time, and the time it actually
started, in microseconds. It is only present when the --rate option is used. When both
--rate and --latency-limit are used, the time for a skipped transaction will be reported
as skipped.
Here is a snippet of a log file generated in a single-client run:
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.
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:
interval_start num_transactions sum_latency sum_latency_2 min_latency max_latency [ sum_lag sum_lag_2 min_lag max_lag [ skipped ] ]
where interval_start is the start of the interval (as a Unix epoch time stamp),
num_transactions is the number of transactions within the interval, sum_latency is the sum
of the transaction latencies within the interval, sum_latency_2 is the sum of squares of
the transaction latencies within the interval, min_latency is the minimum latency within
the interval, and max_latency is the maximum latency within the interval. The next fields,
sum_lag, sum_lag_2, min_lag, and max_lag, are only present if the --rate option is used.
They provide statistics about the time each transaction had to wait for the previous one
to finish, i.e. the difference between each transaction's scheduled start time and the
time it actually started. The very last field, skipped, is only present if the
--latency-limit option is used, too. It counts the number of transactions skipped because
they would have started too late. Each transaction is counted in the interval when it was
committed.
Here is some example output:
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
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.
Per-Statement Latencies
With the -r option, pgbench collects the elapsed transaction time of each statement
executed by every client. It then reports an average of those values, referred to as the
latency for each statement, after the benchmark has finished.
For the default script, the output will look similar to this:
starting vacuum...end.
transaction type: <builtin: TPC-B (sort of)>
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 = 15.844 ms
latency stddev = 2.715 ms
tps = 618.764555 (including connections establishing)
tps = 622.977698 (excluding connections establishing)
statement latencies in milliseconds:
0.002 \set aid random(1, 100000 * :scale)
0.005 \set bid random(1, 1 * :scale)
0.002 \set tid random(1, 10 * :scale)
0.001 \set delta random(-5000, 5000)
0.326 BEGIN;
0.603 UPDATE pgbench_accounts SET abalance = abalance + :delta WHERE aid = :aid;
0.454 SELECT abalance FROM pgbench_accounts WHERE aid = :aid;
5.528 UPDATE pgbench_tellers SET tbalance = tbalance + :delta WHERE tid = :tid;
7.335 UPDATE pgbench_branches SET bbalance = bbalance + :delta WHERE bid = :bid;
0.371 INSERT INTO pgbench_history (tid, bid, aid, delta, mtime) VALUES (:tid, :bid, :aid, :delta, CURRENT_TIMESTAMP);
1.212 END;
If multiple script files are specified, the averages are reported separately for each
script file.
Note that collecting the additional timing information needed for per-statement latency
computation adds some overhead. This will slow average execution speed and lower the
computed TPS. The amount of slowdown varies significantly depending on platform and
hardware. Comparing average TPS values with and without latency reporting enabled is a
good way to measure if the timing overhead is significant.
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 12.3 2020 PGBENCH(1)
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