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pgbench

   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 to load data into ordinary
                (non-partition) tables with version 14 or later of
                PostgreSQL to speed up subsequent VACUUM. 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:

          + 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 54 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:

          + serialization failures;
          + 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 301. 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 301. 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_s
tarting_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 302 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 302. pgbench Operators

   Operator

   Description

   Example(s)

   boolean OR boolean → boolean

   Logical OR

   5 or 0 → TRUE

   boolean AND boolean → boolean

   Logical AND

   3 and 0 → FALSE

   NOT boolean → boolean

   Logical NOT

   not false → TRUE

   boolean IS [NOT] (NULL|TRUE|FALSE) → boolean

   Boolean value tests

   1 is null → FALSE

   value ISNULL|NOTNULL → boolean

   Nullness tests

   1 notnull → TRUE

   number = number → boolean

   Equal

   5 = 4 → FALSE

   number <> number → boolean

   Not equal

   5 <> 4 → TRUE

   number != number → boolean

   Not equal

   5 != 5 → FALSE

   number < number → boolean

   Less than

   5 < 4 → FALSE

   number <= number → boolean

   Less than or equal to

   5 <= 4 → FALSE

   number > number → boolean

   Greater than

   5 > 4 → TRUE

   number >= number → boolean

   Greater than or equal to

   5 >= 4 → TRUE

   integer | integer → integer

   Bitwise OR

   1 | 2 → 3

   integer # integer → integer

   Bitwise XOR

   1 # 3 → 2

   integer & integer → integer

   Bitwise AND

   1 & 3 → 1

   ~ integer → integer

   Bitwise NOT

   ~ 1 → -2

   integer << integer → integer

   Bitwise shift left

   1 << 2 → 4

   integer >> integer → integer

   Bitwise shift right

   8 >> 2 → 2

   number + number → number

   Addition

   5 + 4 → 9

   number - number → number

   Subtraction

   3 - 2.0 → 1.0

   number * number → number

   Multiplication

   5 * 4 → 20

   number / number → number

   Division (truncates the result towards zero if both inputs are
   integers)

   5 / 3 → 1

   integer % integer → integer

   Modulo (remainder)

   3 % 2 → 1

   - number → number

   Negation

   - 2.0 → -2.0

Built-In Functions

   The functions listed in Table 303 are built into pgbench and may be
   used in expressions appearing in \set.

   Table 303. pgbench Functions

   Function

   Description

   Example(s)

   abs ( number ) → same type as input

   Absolute value

   abs(-17) → 17

   debug ( number ) → same type as input

   Prints the argument to stderr, and returns the argument.

   debug(5432.1) → 5432.1

   double ( number ) → double

   Casts to double.

   double(5432) → 5432.0

   exp ( number ) → double

   Exponential (e raised to the given power)

   exp(1.0) → 2.718281828459045

   greatest ( number [, ... ] ) → double if any argument is double, else
   integer

   Selects the largest value among the arguments.

   greatest(5, 4, 3, 2) → 5

   hash ( value [, seed ] ) → integer

   This is an alias for hash_murmur2.

   hash(10, 5432) → -5817877081768721676

   hash_fnv1a ( value [, seed ] ) → integer

   Computes FNV-1a hash.

   hash_fnv1a(10, 5432) → -7793829335365542153

   hash_murmur2 ( value [, seed ] ) → integer

   Computes MurmurHash2 hash.

   hash_murmur2(10, 5432) → -5817877081768721676

   int ( number ) → integer

   Casts to integer.

   int(5.4 + 3.8) → 9

   least ( number [, ... ] ) → double if any argument is double, else
   integer

   Selects the smallest value among the arguments.

   least(5, 4, 3, 2.1) → 2.1

   ln ( number ) → double

   Natural logarithm

   ln(2.718281828459045) → 1.0

   mod ( integer, integer ) → integer

   Modulo (remainder)

   mod(54, 32) → 22

   permute ( i, size [, seed ] ) → integer

   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.

   permute(0, 4) → an integer between 0 and 3

   pi () → double

   Approximate value of π

   pi() → 3.14159265358979323846

   pow ( x, y ) → double

   power ( x, y ) → double

   x raised to the power of y

   pow(2.0, 10) → 1024.0

   random ( lb, ub ) → integer

   Computes a uniformly-distributed random integer in [lb, ub].

   random(1, 10) → an integer between 1 and 10

   random_exponential ( lb, ub, parameter ) → integer

   Computes an exponentially-distributed random integer in [lb, ub], see
   below.

   random_exponential(1, 10, 3.0) → an integer between 1 and 10

   random_gaussian ( lb, ub, parameter ) → integer

   Computes a Gaussian-distributed random integer in [lb, ub], see below.

   random_gaussian(1, 10, 2.5) → an integer between 1 and 10

   random_zipfian ( lb, ub, parameter ) → integer

   Computes a Zipfian-distributed random integer in [lb, ub], see below.

   random_zipfian(1, 10, 1.5) → an integer between 1 and 10

   sqrt ( number ) → 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(-pa
       rameter))
       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].

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, :a
id, :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 --latenc
y-limit=10 --failures-detailed --max-tries=10 test

1650260552 5178 26171317 177284491527 1136 44462 2647617 7321113867 0 9866 64 75
64 28340 4148 0
1650260562 4808 25573984 220121792172 1171 62083 3037380 9666800914 0 9998 598 7
392 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:
     * latency — elapsed transaction time for each statement. pgbench
       reports an average value of all successful runs of the statement.
     * The number of failures in this statement. See Failures and
       Serialization/Deadlock Retries for more information.
     * 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 W
HERE 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 WH
ERE tid = :tid;
  0.582  3363  41576  UPDATE pgbench_branches SET bbalance = bbalance + :delta W
HERE 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:
     * 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:
          + errors at the beginning of pgbench (e.g. an invalid option
            value);
          + errors in the initialization mode (e.g. the query to create
            tables for built-in scripts fails);
          + errors before starting threads (e.g. could not connect to the
            database server, syntax error in the meta command, thread
            creation failure);
          + internal pgbench errors (which are supposed to never
            occur...).
     * 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.
     * 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.