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pg_sampletolog is a PostgreSQL extension which allows to sample statements and/or transactions to logs. It adds in PostgreSQL (from 9.4 to 11) same kind of statement sampling added in PostgreSQL 12 (currently not released).

pg_sampletolog allows to:

  • Log a sample of statements
  • Log a sample of transactions
  • Log before or after execution (in order to be compatible with pgreplay)
  • Log all DDL or MOD statements, same as log_statement
  • Log statement's queryid if pg_stat_statements is installed



The module can be built using the standard PGXS infrastructure. For this to work, the pg_config program must be available in your $PATH. Instruction to install follows:

git clone
cd pg_sampletolog
make install

Postgres setup

Extension must me loaded either:

  • In local session with LOAD 'pg_sampletolog'; order.
  • In session_preload_libraries. To be loaded at connection start.

New settings should be visible in pg_settings view:

select name from pg_settings where name like 'pg_sampletolog%';
(6 rows)


pg_sampletolog is controlled by specifing GUC settings. So you can apply them in your postgresql.conf or change them inside the session with SET order.

By default, pg_sampletolog is disabled, you have to change theses settings (default values in parenthesis):

  • pg_sampletolog.log_level (LOG): Control log severity level. Refer to PostgreSQL documentation for their meaning : Message Severity Levels
  • pg_sampletolog.statement_sample_rate (0): Control fraction of statements to report in log.
  • pg_sampletolog.transaction_sample_rate (0): Control fraction of transactions to report in log.
  • pg_sampletolog.statement_sample_limit (-1): Always log statements whose duration exceed this limit.
  • pg_sampletolog.log_statement none): Allow to log all DDL and/or MOD statements.
  • pg_sampletolog.log_before_execution (false): Control to log statements before or after execution.
  • pg_sampletolog.disable_log_duration (false): Disable duration reporting (mainly used for tests).


  • Log only 10% of statements: pg_sampletolog.statement_sample_rate = 0.1

pg_sampelog will log arround 10% of statements. It will do computation for each statement by calling random() function whose cost is negligible. Message output will look like (depending of your log_line_prefix) :

2019-01-27 12:50:39.361 CET [27047] LOG:  duration: 0.014 ms  statement: SELECT 1;
  • Log only 10% of transactions: pg_sampletolog.transaction_sample_rate = 0.1

Same as previous, with the difference that, when a transaction starts, pg_sampletolog will choose if the transaction will be logged. When a transaction is logged, all statements of the transaction are logged.

Message output will look like (depending of your log_line_prefix) :

Transaction : BEGIN; SELECT 1; SELECT 1; COMMIT;

2019-01-27 12:51:40.562 CET [27069] LOG:  duration: 0.008 ms  statement: SELECT 1;
2019-01-27 12:51:40.562 CET [27069] LOG:  duration: 0.005 ms  statement: SELECT 1;
  • Do not sample statement if execution exceed 100ms: pg_sampletolog.statement_sample_limit = '100ms'

It can be useful if you do not want sampling (i.e log all occurences) for long running queries. Note that it is possible only if either pg_sampletolog.statement_sample_rate or pg_sampletolog.transaction_sample_rate are > 0.

  • Log all DDL statements: pg_sampletolog.log_statement = 'ddl':

pg_sampletolog will log all DDL statements, it is similar to log_statement = ddl parameter in Postgres. It could be useful if you want a sample of read queries but catch all ddl changes.

2019-01-27 12:53:47.564 CET [27103] LOG:  statement: CREATE TABLE t1(c1 int);
  • Log all data-modifying statements: pg_sampletolog.log_statement = 'mod':

pg_sampletolog will log all data-modifying statements (including ddl), it is similar to log_statement = mod parameter in Postgres. It could be useful if you want a sample of read queries but catch all write changes.

2019-01-27 12:59:54.043 CET [27160] LOG:  duration: 0.246 ms  statement: INSERT INTO t1 VALUES(1);
2019-01-27 13:00:16.468 CET [27160] LOG:  duration: 126.851 ms  statement: CREATE INDEX ON t1(c1);
  • Log before execution: pg_sampletolog.log_before_execution = on

By default, pg_sampletolog, will log after query is executed. It could be useful if you want to use results with pgreplay.

pg_sampletolog works on primary, but also on standby. If you call an SQL function containing several queries, pg_sampletolog will only log function call.

With a simple test with pgbench and log_min_duration = 0:

pgbench -c8 -j2 -t 30000 -P5 bench
starting vacuum...end.
progress: 5.0 s, 10618.2 tps, lat 0.750 ms stddev 0.170
progress: 10.0 s, 10690.9 tps, lat 0.746 ms stddev 0.168
progress: 15.0 s, 10596.4 tps, lat 0.752 ms stddev 0.149
progress: 20.0 s, 9547.6 tps, lat 0.836 ms stddev 0.176
transaction type: <builtin: TPC-B (sort of)>
scaling factor: 100
query mode: simple
number of clients: 8
number of threads: 2
number of transactions per client: 30000
number of transactions actually processed: 240000/240000
latency average = 0.774 ms
latency stddev = 0.175 ms
tps = 9836.617690 (including connections establishing)
tps = 9837.334350 (excluding connections establishing)
pgbench -c8 -j2 -t 30000 -P5 bench  6.96s user 12.32s system 78% cpu 24.448 total

wc -l logfile

Same test with pg_sampletolog.statement_sample_rate = 0.1

wc -l logfile

We got less than 10% lines of first test because pg_sampletolog don't log BEGIN or COMMIT orders.

Bonus if you have pg_stat_statements installed, pg_sampletolog will report queryid:

LOG:  duration: 0.032 ms  statement: /* queryid = 5892081150081336634 */ UPDATE pgbench_tellers SET tbalance = tbalance + -3337 WHERE tid = 326;

It is useful to get query's parameters from a query you identified in pg_stat_statements' view.


No supported:

  • PREPARE statements are not supported.
  • Multiple statements in a single query will give this output:
select pg_sleep(1)\; select pg_sleep(1);
(1 row)

2019-06-07 23:07:24.556 CEST [26519] LOG:  duration: 1005.733 ms  statement: select pg_sleep(1); select pg_sleep(1);
2019-06-07 23:07:25.558 CEST [26519] LOG:  duration: 2006.789 ms  statement: select pg_sleep(1); select pg_sleep(1);


Tests are in sql/pg_sampletolog.sql and results in expected/pg_sampletolog.out. Use make installcheck to run them, you should see:

/tmp/pgbuild/lib/postgresql/pgxs/src/makefiles/../../src/test/regress/pg_regress --inputdir=./ --bindir='/tmp/pgbuild/bin'    --dbname=contrib_regression pg_sampletolog
(using postmaster on Unix socket, port 5432)
============== dropping database "contrib_regression" ==============
============== creating database "contrib_regression" ==============
============== running regression test queries        ==============
test pg_sampletolog               ... ok

 All 1 tests passed. 


  • Log statements by their queryid




  • Gilles Darold
  • Julien Rouhaud


pg_sampletolog is free software distributed under the PostgreSQL license.

Copyright (c) 2019, Adrien NAYRAT