Add log_statement_sample_rate parameter

This allows to set a lower log_min_duration_statement value without
incurring excessive log traffic (which reduces performance).  This can
be useful to analyze workloads with lots of short queries.

Author: Adrien Nayrat
Reviewed-by: David Rowley, Vik Fearing
Discussion: https://postgr.es/m/c30ee535-ee1e-db9f-fa97-146b9f62caed@anayrat.info

Author: alvherre

doc/src/sgml/config.sgml

@@ -5721,11 +5721,11 @@ local0.*    /var/log/postgresql
         <para>
          Causes the duration of each completed statement to be logged
          if the statement ran for at least the specified number of
-         milliseconds.  Setting this to zero prints all statement durations.
-         Minus-one (the default) disables logging statement durations.
-         For example, if you set it to <literal>250ms</literal>
-         then all SQL statements that run 250ms or longer will be
-         logged.  Enabling this parameter can be helpful in tracking down
+         milliseconds, modulated by <varname>log_statement_sample_rate</varname>.
+         Setting this to zero prints all statement durations. Minus-one (the default)
+         disables logging statement durations. For example, if you set it to
+         <literal>250ms</literal> then all SQL statements that run 250ms or longer
+         will be logged. Enabling this parameter can be helpful in tracking down
          unoptimized queries in your applications.
          Only superusers can change this setting.
         </para>
@@ -5752,6 +5752,26 @@ local0.*    /var/log/postgresql
        </listitem>
       </varlistentry>
 
+     <varlistentry id="guc-log-statement-sample-rate" xreflabel="log_statement_sample_rate">
+      <term><varname>log_statement_sample_rate</varname> (<type>real</type>)
+      <indexterm>
+       <primary><varname>log_statement_sample_rate</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+       <listitem>
+        <para>
+         Determines the fraction of the statements that exceed
+         <xref linkend="guc-log-min-duration-statement"/> which to log.
+         The default is <literal>1</literal>, meaning log to all such
+         statements.
+         Setting this to zero disables logging, same as setting
+         <varname>log_min_duration_statement</varname>
+         to minus-one. <varname>log_statement_sample_rate</varname>
+         is helpful when the traffic is too high to log all queries.
+        </para>
+       </listitem>
+      </varlistentry>
+
      </variablelist>
 
     <para>

src/backend/tcop/postgres.c

@@ -2201,7 +2201,8 @@ check_log_statement(List *stmt_list)
 
 /*
  * check_log_duration
- *		Determine whether current command's duration should be logged
+ *		Determine whether current command's duration should be logged.
+ *		If log_statement_sample_rate < 1.0, log only a sample.
  *
  * Returns:
  *		0 if no logging is needed
@@ -2223,6 +2224,7 @@ check_log_duration(char *msec_str, bool was_logged)
 		int			usecs;
 		int			msecs;
 		bool		exceeded;
+		bool		in_sample;
 
 		TimestampDifference(GetCurrentStatementStartTimestamp(),
 							GetCurrentTimestamp(),
@@ -2239,7 +2241,17 @@ check_log_duration(char *msec_str, bool was_logged)
 					 (secs > log_min_duration_statement / 1000 ||
 					  secs * 1000 + msecs >= log_min_duration_statement)));
 
-		if (exceeded || log_duration)
+		/*
+		 * Do not log if log_statement_sample_rate = 0. Log a sample if
+		 * log_statement_sample_rate <= 1 and avoid unecessary random() call
+		 * if log_statement_sample_rate = 1.
+		 */
+		if (exceeded)
+			in_sample = log_statement_sample_rate != 0 &&
+				(log_statement_sample_rate == 1 ||
+				 random() <= log_statement_sample_rate * MAX_RANDOM_VALUE);
+
+		if ((exceeded && in_sample) || log_duration)
 		{
 			snprintf(msec_str, 32, "%ld.%03d",
 					 secs * 1000 + msecs, usecs % 1000);

src/backend/utils/misc/guc.c

@@ -486,6 +486,7 @@ int			log_min_messages = WARNING;
 int			client_min_messages = NOTICE;
 int			log_min_duration_statement = -1;
 int			log_temp_files = -1;
+double		log_statement_sample_rate = 1.0;
 int			trace_recovery_messages = LOG;
 
 int			temp_file_limit = -1;
@@ -2642,7 +2643,8 @@ static struct config_int ConfigureNamesInt[] =
 		{"log_min_duration_statement", PGC_SUSET, LOGGING_WHEN,
 			gettext_noop("Sets the minimum execution time above which "
 						 "statements will be logged."),
-			gettext_noop("Zero prints all queries. -1 turns this feature off."),
+			gettext_noop("Zero prints all queries, subject to log_statement_sample_rate. "
+						 "-1 turns this feature off."),
 			GUC_UNIT_MS
 		},
 		&log_min_duration_statement,
@@ -3319,6 +3321,17 @@ static struct config_real ConfigureNamesReal[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"log_statement_sample_rate", PGC_SUSET, LOGGING_WHEN,
+			gettext_noop("Fraction of statements over log_min_duration_statement to log."),
+			gettext_noop("If you only want a sample, use a value between 0 (never "
+						 "log) and 1.0 (always log).")
+		},
+		&log_statement_sample_rate,
+		1.0, 0.0, 1.0,
+		NULL, NULL, NULL
+	},
+
 	/* End-of-list marker */
 	{
 		{NULL, 0, 0, NULL, NULL}, NULL, 0.0, 0.0, 0.0, NULL, NULL, NULL

src/backend/utils/misc/postgresql.conf.sample

@@ -483,11 +483,14 @@
 					#   fatal
 					#   panic (effectively off)
 
-#log_min_duration_statement = -1	# -1 is disabled, 0 logs all statements
-					# and their durations, > 0 logs only
-					# statements running at least this number
-					# of milliseconds
-
+#log_min_duration_statement = -1	# logs statements and their durations
+					# according to log_statement_sample_rate. -1 is disabled,
+					# 0 logs all statement, > 0 logs only statements running at
+					# least this number of milliseconds.
+
+#log_statement_sample_rate = 1	# Fraction of logged statements over
+					# log_min_duration_statement. 1.0 logs all statements,
+					# 0 never logs.
 
 # - What to Log -
 

src/include/utils/guc.h

@@ -251,6 +251,7 @@ extern PGDLLIMPORT int log_min_messages;
 extern PGDLLIMPORT int client_min_messages;
 extern int	log_min_duration_statement;
 extern int	log_temp_files;
+extern double log_statement_sample_rate;
 
 extern int	temp_file_limit;