Log all statements from a sample of transactions

This is useful to obtain a view of the different transaction types in an
application, regardless of the durations of the statements each runs.

Author: Adrien Nayrat
Reviewed-by: Masahiko Sawada, Hayato Kuroda, Andres Freund

doc/src/sgml/config.sgml

@@ -5871,6 +5871,32 @@ local0.*    /var/log/postgresql
        </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>
+       <primary><varname>log_transaction_sample_rate</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+       <listitem>
+        <para>
+         Set the fraction of transactions whose statements are all logged,
+         in addition to statements logged for other reasons.  It applies to
+         each new transaction regardless of its statements' durations.
+         The default is <literal>0</literal>, meaning not to log statements
+         from any additional transaction.  Setting this to <literal>1</literal>
+         logs all statements for all transactions.
+         <varname>log_transaction_sample_rate</varname> is helpful to track a
+         sample of transaction.
+        </para>
+       <note>
+        <para>
+         Like all statement-logging options, this option can add significant
+         overhead.
+        </para>
+       </note>
+       </listitem>
+     </varlistentry>
+
      </variablelist>
 
     <para>

src/backend/access/transam/xact.c

@@ -264,6 +264,9 @@ 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
@@ -1903,6 +1906,11 @@ StartTransaction(void)
 	s->state = TRANS_START;
 	s->fullTransactionId = InvalidFullTransactionId;	/* until assigned */
 
+	/* Determine if statements are logged in this transaction */
+	xact_is_sampled = log_xact_sample_rate != 0 &&
+		(log_xact_sample_rate == 1 ||
+		 random() <= log_xact_sample_rate * MAX_RANDOM_VALUE);
+
 	/*
 	 * initialize current transaction state fields
 	 *

src/backend/tcop/postgres.c

@@ -2194,6 +2194,8 @@ 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
@@ -2209,7 +2211,7 @@ 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)
 	{
 		long		secs;
 		int			usecs;
@@ -2243,11 +2245,11 @@ 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)
 		{
 			snprintf(msec_str, 32, "%ld.%03d",
 					 secs * 1000 + msecs, usecs % 1000);
-			if (exceeded && !was_logged)
+			if ((exceeded || xact_is_sampled) && !was_logged)
 				return 2;
 			else
 				return 1;

src/backend/utils/misc/guc.c

@@ -510,6 +510,7 @@ 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;
@@ -3386,6 +3387,18 @@ static struct config_real ConfigureNamesReal[] =
 		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 0.0 (never log) and 1.0 (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

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

@@ -495,6 +495,10 @@
 					# log_min_duration_statement. 1.0 logs all statements,
 					# 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.
+
 # - What to Log -
 
 #debug_print_parse = off

src/include/access/xact.h

@@ -55,6 +55,9 @@ 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

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;