On 11/19/18 2:52 PM, Dmitry Dolgov wrote:
>> On Mon, Nov 19, 2018 at 2:40 PM Tomas Vondra <[email protected]>
>> wrote:
>>
>> On 11/19/18 2:57 AM, Michael Paquier wrote:
>>> On Sun, Nov 18, 2018 at 12:18:33PM +0100, Dmitry Dolgov wrote:
>>>> Since it's hard to come up with a concise name that will mention sampling
>>>> rate
>>>> in the context of min_duration_statement, I think it's fine to name this
>>>> configuration "log_sample_rate", as long as it's dependency from
>>>> log_min_duration_statements is clearly explained in the documentation.
>>>
>>> log_sample_rate looks fine to me as a name.
>>
>> That seems far too short to me - the name should indicate it applies to
>> statement logging. I'd say log_statement_sample_rate is better.
>
> I agree, sounds reasonable.
>
Thanks for your comments. Here is the updated patch. I fixed a warning for
missing parentheses in this expression:
if ((exceeded && in_sample) || log_duration)
It passed make check_world and make docs
diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index d1dab35559..0e21b6a95f 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -5247,6 +5247,26 @@ 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>
+ Causes logging only a fraction of the statements when <xref
+ linkend="guc-log-min-duration-statement"/> is used. The default is
+ <literal>1</literal>, meaning log all statements longer than
+ <varname>log_min_duration_statement</varname>. Setting this to zero
+ disables logging, same as setting
+ <varname>log_min_duration_statement</varname> to
+ <literal>-1</literal>. Using <varname>log_statement_sample_rate</varname> is
+ helpful when the traffic is too high to log all queries.
+ </para>
+ </listitem>
+ </varlistentry>
+
</variablelist>
<para>
diff --git a/src/backend/tcop/postgres.c b/src/backend/tcop/postgres.c
index a3b9757565..6777eebde1 100644
--- a/src/backend/tcop/postgres.c
+++ b/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 +2241,16 @@ 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.
+ */
+ 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);
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 0ec3ff0fd6..452bdd7484 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -471,6 +471,7 @@ 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;
@@ -3272,6 +3273,17 @@ static struct config_real ConfigureNamesReal[] =
NULL, NULL, NULL
},
+ {
+ {"log_statement_sample_rate", PGC_SUSET, LOGGING_WHEN,
+ gettext_noop("Fraction of statements to log."),
+ gettext_noop("1.0 logs all statements."),
+ NULL
+ },
+ &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
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index 3038fe627b..798888829e 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -441,6 +441,8 @@
# statements running at least this number
# of milliseconds
+#log_statement_sample_rate = 1 # Fraction of logged statements. 1 means log
+ # all statements.
# - What to Log -
diff --git a/src/include/utils/guc.h b/src/include/utils/guc.h
index df2e556b02..4c5cabcc5f 100644
--- a/src/include/utils/guc.h
+++ b/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;
