F.18. pgbench

F.18.1. Overview

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.)

At the default "scale factor" of 1, the tables initially contain this many rows:

table           # of rows
-------------------------
branches        1
tellers         10
accounts        100000
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), and -f (specify a custom script file). See below for a full list.

Table F-14 shows options that are used during database initialization, while Table F-15 shows options that are used while running benchmarks, and Table F-16 shows options that are useful in both cases.

F.18.2. What is the "transaction" actually performed in pgbench?

The default transaction script issues seven commands per transaction:

1. BEGIN;

2. UPDATE accounts SET abalance = abalance + :delta WHERE aid = :aid;

3. SELECT abalance FROM accounts WHERE aid = :aid;

4. UPDATE tellers SET tbalance = tbalance + :delta WHERE tid = :tid;

5. UPDATE branches SET bbalance = bbalance + :delta WHERE bid = :bid;

6. INSERT INTO history (tid, bid, aid, delta, mtime) VALUES (:tid, :bid, :aid, :delta, CURRENT_TIMESTAMP);

7. END;

If you specify -N, steps 4 and 5 aren't included in the transaction. If you specify -S, only the SELECT is issued.

F.18.3. 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. You can even specify multiple scripts (multiple -f options), in which case a random one of the scripts is chosen each time a client session starts a new transaction.

The format of a script file is one SQL command per line; multi-line SQL commands are not supported. Empty lines and lines beginning with -- are ignored. Script file lines can also be "meta commands", which are interpreted by pgbench itself, as described below.

There is a simple variable-substitution facility for script files. 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, the variable scale is preset to the current scale factor. 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.

Script file meta commands begin with a backslash (\). Arguments to a meta command are separated by white space. These meta commands are supported:

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

\set nbranches :scale
\set ntellers 10 * :scale
\set naccounts 100000 * :scale
\setrandom aid 1 :naccounts
\setrandom bid 1 :nbranches
\setrandom tid 1 :ntellers
\setrandom delta -5000 5000
BEGIN;
UPDATE accounts SET abalance = abalance + :delta WHERE aid = :aid;
SELECT abalance FROM accounts WHERE aid = :aid;
UPDATE tellers SET tbalance = tbalance + :delta WHERE tid = :tid;
UPDATE branches SET bbalance = bbalance + :delta WHERE bid = :bid;
INSERT INTO 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.)

F.18.4. Per-transaction logging

With the -l option, pgbench writes the time taken by each transaction to a logfile. The logfile will be named pgbench_log.nnn, where nnn is the PID of the pgbench process. The format of the log is:

    client_id transaction_no time file_no time_epoch time_us
   

where time is the elapsed transaction time in microseconds, file_no identifies which script file was used (useful when multiple scripts were specified with -f), and time_epoch/time_us are a UNIX epoch format timestamp and an offset in microseconds (suitable for creating a ISO 8601 timestamp with fractional seconds) showing when the transaction completed.

Here are example outputs:

 0 199 2241 0 1175850568 995598
 0 200 2465 0 1175850568 998079
 0 201 2513 0 1175850569 608
 0 202 2038 0 1175850569 2663
   

F.18.5. 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. Increase the -t setting enough 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 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.