log_min_duration_sample.patch

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 886632ff43..f69b04db05 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -5855,6 +5855,49 @@ 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 duration
+         (default in milliseconds). Setting this to zero sample all statement
+         durations. Minus-one (the default) disables sampling statement durations.
+         Sampling is controlled by <xref linkend="guc-log-statement-sample-rate"/>.
+         For example, if you set it to <literal>250ms</literal>
+         then all SQL statements that run 250ms or longer will be
+         sampled to logs. Enabling this parameter can be helpful when the traffic
+         is too high to log all queries.
+         Only superusers can change this setting.
+        </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-min-duration-statement" xreflabel="log_min_duration_statement">
       <term><varname>log_min_duration_statement</varname> (<type>integer</type>)
       <indexterm>
@@ -5894,7 +5937,37 @@ local0.*    /var/log/postgresql
         </para>
        </note>
        </listitem>
-      </varlistentry>
+     </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 that exceed
+         <xref linkend="guc-log-min-duration-sample"/> to be logged.
+         This is a statistical parameter, for example <literal>0.5</literal>
+         means there is statiscally 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 sample 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>)
diff --git a/src/backend/tcop/postgres.c b/src/backend/tcop/postgres.c
index e8d8e6f828..6cdc4f3349 100644
--- a/src/backend/tcop/postgres.c
+++ b/src/backend/tcop/postgres.c
@@ -2230,12 +2230,15 @@ 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(),
@@ -2243,20 +2246,35 @@ check_log_duration(char *msec_str, bool was_logged)
 		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
index 31a5ef0474..e84c8cc4cf 100644
--- 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
index 0fc23e3a61..be02a76d9d 100644
--- 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
index 6791e0cbc2..9aa3d02596 100644
--- 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;