Allow sampling of statements depending on duration

tvondra · tvondra · commit 6e3e6cc0e884 · 2019-11-06T19:11:07.000+01:00
This allows logging a sample of statements, without incurring excessive log traffic (which may impact performance). This can be useful when analyzing workloads with lots of short queries. The sampling is configured using two new GUC parameters: * log_min_duration_sample - minimum required statement duration * log_statement_sample_rate - sample rate (0.0 - 1.0) Only statements with duration exceeding log_min_duration_sample are considered for sampling. To enable sampling, both those GUCs have to be set correctly. The existing log_min_duration_statement GUC has a higher priority, i.e. statements with duration exceeding log_min_duration_statement will be always logged, irrespectedly of how the sampling is configured. This means only configurations log_min_duration_sample < log_min_duration_statement do actually sample the statements, instead of logging everything. Author: Adrien Nayrat Reviewed-by: David Rowley, Vik Fearing, Tomas Vondra Discussion: https://postgr.es/m/bbe0a1a8-a8f7-3be2-155a-888e661cc06c@anayrat.info
diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
@@ -5950,6 +5950,12 @@ local0.*    /var/log/postgresql
          Only superusers can change this setting.
         </para>
 
+        <para>
+         This overrides <xref linkend="guc-log-min-duration-sample"/>,
+         meaning that queries with duration exceeding this setting are not
+         subject to sampling and are always logged.
+        </para>
+
         <para>
          For clients using extended query protocol, durations of the Parse,
          Bind, and Execute steps are logged independently.
@@ -5972,6 +5978,85 @@ local0.*    /var/log/postgresql
        </listitem>
       </varlistentry>
 
+     <varlistentry id="guc-log-min-duration-sample" xreflabel="log_min_duration_sample">
+      <term><varname>log_min_duration_sample</varname> (<type>integer</type>)
+      <indexterm>
+       <primary><varname>log_min_duration_sample</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+       <listitem>
+        <para>
+         Allows to sample the logging of the duration of each completed
+         statement if the statement ran for at least the specified amount of
+         time. If this value is specified without units, it is taken as milliseconds.
+         Setting this to zero samples all statement durations.
+         Minus-one (the default) disables sampling statement durations.
+         For example, if you set it to <literal>250ms</literal>
+         then all SQL statements that run 250ms or longer will be considered
+         for sampling, with sample rate is controlled by <xref linkend="guc-log-statement-sample-rate"/>. 
+         Enabling this parameter can be helpful when the traffic too high to
+         sample all queries.
+         Only superusers can change this setting.
+        </para>
+
+        <para>
+         This option has lower priority than <xref linkend="guc-log-min-duration-statement"/>,
+         meaning that statements with durations exceeding <xref linkend="guc-log-min-duration-statement"/>
+         are not subject to sampling and are always logged.
+        </para>
+
+        <para>
+         For clients using extended query protocol, durations of the Parse,
+         Bind, and Execute steps are logged independently.
+        </para>
+
+       <note>
+        <para>
+         When using this option together with
+         <xref linkend="guc-log-statement"/>,
+         the text of statements that are logged because of
+         <varname>log_statement</varname> will not be repeated in the
+         duration log message.
+         If you are not using <application>syslog</application>, it is recommended
+         that you log the PID or session ID using
+         <xref linkend="guc-log-line-prefix"/>
+         so that you can link the statement message to the later
+         duration message using the process ID or session ID.
+        </para>
+       </note>
+       </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 statements with duration exceeding
+         <xref linkend="guc-log-min-duration-sample"/> to be logged.
+         This is a statistical parameter, for example <literal>0.5</literal>
+         means there is statistically one in two chances to log the statement.
+         The default is <literal>1.0</literal>, meaning log all such
+         statements.
+         Setting this to zero disables sampling logging, same as setting
+         <varname>log_min_duration_sample</varname> to
+         <literal>-1</literal>.
+         <varname>log_statement_sample_rate</varname> is helpful when the
+         traffic is too high to log all queries.
+         Only superusers can change this setting.
+        </para>
+       <note>
+        <para>
+         Like all statement-logging options, this option can add significant
+         overhead.
+        </para>
+       </note>
+       </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-log-transaction-sample-rate" xreflabel="log_transaction_sample_rate">
       <term><varname>log_transaction_sample_rate</varname> (<type>real</type>)
       <indexterm>
diff --git a/src/backend/tcop/postgres.c b/src/backend/tcop/postgres.c
@@ -2235,33 +2235,51 @@ check_log_statement(List *stmt_list)
 int
 check_log_duration(char *msec_str, bool was_logged)
 {
-	if (log_duration || log_min_duration_statement >= 0 || xact_is_sampled)
+	if (log_duration || log_min_duration_sample >= 0 ||
+		log_min_duration_statement >= 0 || xact_is_sampled)
 	{
 		long		secs;
 		int			usecs;
 		int			msecs;
-		bool		exceeded;
+		bool		exceeded_duration;
+		bool		exceeded_sample_duration;
+		bool		in_sample = false;
 
 		TimestampDifference(GetCurrentStatementStartTimestamp(),
 							GetCurrentTimestamp(),
 							&secs, &usecs);
 		msecs = usecs / 1000;
 
 		/*
-		 * This odd-looking test for log_min_duration_statement being exceeded
-		 * is designed to avoid integer overflow with very long durations:
-		 * don't compute secs * 1000 until we've verified it will fit in int.
+		 * This odd-looking test for log_min_duration_* being exceeded is
+		 * designed to avoid integer overflow with very long durations: don't
+		 * compute secs * 1000 until we've verified it will fit in int.
 		 */
-		exceeded = (log_min_duration_statement == 0 ||
-					(log_min_duration_statement > 0 &&
-					 (secs > log_min_duration_statement / 1000 ||
-					  secs * 1000 + msecs >= log_min_duration_statement)));
+		exceeded_duration = (log_min_duration_statement == 0 ||
+							 (log_min_duration_statement > 0 &&
+							  (secs > log_min_duration_statement / 1000 ||
+							   secs * 1000 + msecs >= log_min_duration_statement)));
 
-		if (exceeded || log_duration || xact_is_sampled)
+		exceeded_sample_duration = (log_min_duration_sample == 0 ||
+									(log_min_duration_sample > 0 &&
+									 (secs > log_min_duration_sample / 1000 ||
+									  secs * 1000 + msecs >= log_min_duration_sample)));
+
+		/*
+		 * 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_sample_duration)
+			in_sample = log_statement_sample_rate != 0 &&
+				(log_statement_sample_rate == 1 ||
+				 random() <= log_statement_sample_rate * MAX_RANDOM_VALUE);
+
+		if (exceeded_duration || in_sample || log_duration || xact_is_sampled)
 		{
 			snprintf(msec_str, 32, "%ld.%03d",
 					 secs * 1000 + msecs, usecs % 1000);
-			if ((exceeded || xact_is_sampled) && !was_logged)
+			if ((exceeded_duration || in_sample || xact_is_sampled) && !was_logged)
 				return 2;
 			else
 				return 1;
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
@@ -509,8 +509,10 @@ bool		session_auth_is_superuser;
 int			log_min_error_statement = ERROR;
 int			log_min_messages = WARNING;
 int			client_min_messages = NOTICE;
+int			log_min_duration_sample = -1;
 int			log_min_duration_statement = -1;
 int			log_temp_files = -1;
+double		log_statement_sample_rate = 1.0;
 double		log_xact_sample_rate = 0;
 int			trace_recovery_messages = LOG;
 
@@ -2700,10 +2702,23 @@ static struct config_int ConfigureNamesInt[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"log_min_duration_sample", PGC_SUSET, LOGGING_WHEN,
+			gettext_noop("Sets the minimum execution time above which "
+						 "a sample of statements will be logged."
+						 " Sampling is determined by log_statement_sample_rate."),
+			gettext_noop("Zero log a sample of all queries. -1 turns this feature off."),
+			GUC_UNIT_MS
+		},
+		&log_min_duration_sample,
+		-1, -1, INT_MAX,
+		NULL, NULL, NULL
+	},
+
 	{
 		{"log_min_duration_statement", PGC_SUSET, LOGGING_WHEN,
 			gettext_noop("Sets the minimum execution time above which "
-						 "statements will be logged."),
+						 "all statements will be logged."),
 			gettext_noop("Zero prints all queries. -1 turns this feature off."),
 			GUC_UNIT_MS
 		},
@@ -3430,6 +3445,16 @@ static struct config_real ConfigureNamesReal[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"log_statement_sample_rate", PGC_SUSET, LOGGING_WHEN,
+			gettext_noop("Fraction of statements exceeding log_min_duration_sample to be logged."),
+			gettext_noop("Use a value between 0.0 (never log) and 1.0 (always log).")
+		},
+		&log_statement_sample_rate,
+		1.0, 0.0, 1.0,
+		NULL, NULL, NULL
+	},
+
 	{
 		{"log_transaction_sample_rate", PGC_SUSET, LOGGING_WHEN,
 			gettext_noop("Set the fraction of transactions to log for new transactions."),
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
@@ -493,6 +493,17 @@
 					# statements running at least this number
 					# of milliseconds
 
+#log_min_duration_sample = -1	# -1 is disabled, 0 logs a sample of statements
+					# and their durations, > 0 logs only a sample of
+					# statements running at least this number
+					# of milliseconds
+					# Sample fraction is determined by log_statement_sample_rate
+
+#log_statement_sample_rate = 1.0   # Fraction of logged statements exceeding
+                   # log_min_duration_sample to be logged.
+                   # 1.0 logs all such statements, 0.0 never logs.
+
+
 #log_transaction_sample_rate = 0.0	# Fraction of transactions whose statements
 					# are logged regardless of their duration. 1.0 logs all
 					# statements from all transactions, 0.0 never logs.
diff --git a/src/include/utils/guc.h b/src/include/utils/guc.h
@@ -251,8 +251,10 @@ extern bool session_auth_is_superuser;
 extern int	log_min_error_statement;
 extern PGDLLIMPORT int log_min_messages;
 extern PGDLLIMPORT int client_min_messages;
+extern int	log_min_duration_sample;
 extern int	log_min_duration_statement;
 extern int	log_temp_files;
+extern double log_statement_sample_rate;
 extern double log_xact_sample_rate;
 
 extern int	temp_file_limit;
