log_xact-5.patch

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 6d42b7afe7..dbf5b81243 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -5819,6 +5819,32 @@ local0.*    /var/log/postgresql
        </listitem>
       </varlistentry>
 
+     <varlistentry id="guc-transaction-sample-rate" xreflabel="log_transaction_sample_rate">
+      <term><varname>log_transaction_sample_rate</varname> (<type>real</type>)
+      <indexterm>
+       <primary><varname>log_transaction_sample_rate</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+       <listitem>
+        <para>
+         Set the fraction of transactions whose statements are logged. It applies
+         to each new transaction regardless of their duration. The default is
+         <literal>0</literal>, meaning do not log statements from this transaction.
+         Setting this to <literal>1</literal> logs all statements for all transactions.
+         Logging can be disabled inside a transaction by setting this to 0.
+         <varname>log_transaction_sample_rate</varname> is helpful to track a
+         sample of transaction.
+        </para>
+       <note>
+        <para>
+         This option can add overhead when transactions are large (many short queries).
+         Or, when the workload is high and <varname>log_transaction_sample_rate</varname>
+         is closed to 1.
+        </para>
+       </note>
+       </listitem>
+     </varlistentry>
+
      </variablelist>
 
     <para>
diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c
index e93262975d..2a301b88ea 100644
--- a/src/backend/access/transam/xact.c
+++ b/src/backend/access/transam/xact.c
@@ -244,6 +244,9 @@ static char *prepareGID;
  */
 static bool forceSyncCommit = false;
 
+/* Flag for logging statements in a transaction. */
+bool        xact_is_sampled = false;
+
 /*
  * Private context for transaction-abort work --- we reserve space for this
  * at startup to ensure that AbortTransaction and AbortSubTransaction can work
@@ -1822,6 +1825,9 @@ StartTransaction(void)
 	s->state = TRANS_START;
 	s->transactionId = InvalidTransactionId;	/* until assigned */
 
+	/* Determine if statements are logged in this transaction */
+	xact_is_sampled = random() <= log_xact_sample_rate * MAX_RANDOM_VALUE;
+
 	/*
 	 * initialize current transaction state fields
 	 *
diff --git a/src/backend/tcop/postgres.c b/src/backend/tcop/postgres.c
index 8b4d94c9a1..76f7f8a912 100644
--- a/src/backend/tcop/postgres.c
+++ b/src/backend/tcop/postgres.c
@@ -2203,6 +2203,8 @@ check_log_statement(List *stmt_list)
  * check_log_duration
  *		Determine whether current command's duration should be logged.
  *		If log_statement_sample_rate < 1.0, log only a sample.
+ *		We also check if this statement in this transaction must be logged
+ *		(regardless of its duration).
  *
  * Returns:
  *		0 if no logging is needed
@@ -2218,7 +2220,8 @@ check_log_statement(List *stmt_list)
 int
 check_log_duration(char *msec_str, bool was_logged)
 {
-	if (log_duration || log_min_duration_statement >= 0)
+	if (log_duration || log_min_duration_statement >= 0 ||
+		(xact_is_sampled && log_xact_sample_rate > 0))
 	{
 		long		secs;
 		int			usecs;
@@ -2252,11 +2255,17 @@ check_log_duration(char *msec_str, bool was_logged)
 			(log_statement_sample_rate == 1 ||
 			 random() <= log_statement_sample_rate * MAX_RANDOM_VALUE);
 
-		if ((exceeded && in_sample) || log_duration)
+		if ((exceeded && in_sample) || log_duration ||
+			(xact_is_sampled && log_xact_sample_rate > 0))
 		{
 			snprintf(msec_str, 32, "%ld.%03d",
 					 secs * 1000 + msecs, usecs % 1000);
-			if (exceeded && !was_logged)
+
+			/*
+			 * Recheck if transaction is sampled to allow to disable logging
+			 * within a transaction.
+			*/
+			if ((exceeded || 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 156d147c85..cf77d3b2ee 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -509,6 +509,7 @@ int			client_min_messages = NOTICE;
 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;
 
 int			temp_file_limit = -1;
@@ -3362,13 +3363,25 @@ static struct config_real ConfigureNamesReal[] =
 		{"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) and 1 (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."),
+			gettext_noop("Logs all statements from a fraction of transactions. "
+						 "Use a value between 0 (never log) and 1 (log all "
+						 "statements for all transactions).")
+		},
+		&log_xact_sample_rate,
+		0.0, 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
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index bd6ea65d0c..2e7e679660 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -493,6 +493,10 @@
 					# log_min_duration_statement. 1.0 logs all statements,
 					# 0 never logs.
 
+#log_transaction_sample_rate = 0	# Fraction of transactions whose statements
+					# are logged regardless of their duration. 1.0 logs all
+					# statements from all transactions, 0 never logs.
+
 # - What to Log -
 
 #debug_print_parse = off
diff --git a/src/include/access/xact.h b/src/include/access/xact.h
index 426e77846f..ee3d97c3fe 100644
--- a/src/include/access/xact.h
+++ b/src/include/access/xact.h
@@ -54,6 +54,9 @@ extern PGDLLIMPORT int XactIsoLevel;
 extern bool DefaultXactReadOnly;
 extern bool XactReadOnly;
 
+/* flag for logging statements in this transaction */
+extern bool xact_is_sampled;
+
 /*
  * Xact is deferrable -- only meaningful (currently) for read only
  * SERIALIZABLE transactions
diff --git a/src/include/utils/guc.h b/src/include/utils/guc.h
index c07e7b945e..cb67fbd907 100644
--- a/src/include/utils/guc.h
+++ b/src/include/utils/guc.h
@@ -252,6 +252,7 @@ extern PGDLLIMPORT int client_min_messages;
 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;