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Add log_statement_sample_rate parameter

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This allows to set a lower log_min_duration_statement value without
incurring excessive log traffic (which reduces performance).  This can
be useful to analyze workloads with lots of short queries.

Author: Adrien Nayrat
Reviewed-by: David Rowley, Vik Fearing
Discussion: https://postgr.es/m/c30ee535-ee1e-db9f-fa97-146b9f62caed@anayrat.info
This commit is contained in:
Alvaro Herrera
2018-11-29 18:42:53 -03:00
parent 826eff57c4
commit 88bdbd3f74
5 changed files with 61 additions and 12 deletions
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16
src/backend/tcop/postgres.c
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@@ -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 +2241,17 @@ 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.
*/
if (exceeded)
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);

15
src/backend/utils/misc/guc.c
View File

@@ -486,6 +486,7 @@ 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;
@@ -2642,7 +2643,8 @@ static struct config_int ConfigureNamesInt[] =
{"log_min_duration_statement", PGC_SUSET, LOGGING_WHEN,
gettext_noop("Sets the minimum execution time above which "
"statements will be logged."),
gettext_noop("Zero prints all queries. -1 turns this feature off."),
gettext_noop("Zero prints all queries, subject to log_statement_sample_rate. "
"-1 turns this feature off."),
GUC_UNIT_MS
},
&log_min_duration_statement,
@@ -3319,6 +3321,17 @@ static struct config_real ConfigureNamesReal[] =
NULL, NULL, NULL
},
{
{"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_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

11
src/backend/utils/misc/postgresql.conf.sample
View File

@@ -483,11 +483,14 @@
# fatal
# panic (effectively off)
#log_min_duration_statement = -1 # -1 is disabled, 0 logs all statements
# and their durations, > 0 logs only
# statements running at least this number
# of milliseconds
#log_min_duration_statement = -1 # logs statements and their durations
# according to log_statement_sample_rate. -1 is disabled,
# 0 logs all statement, > 0 logs only statements running at
# least this number of milliseconds.
#log_statement_sample_rate = 1 # Fraction of logged statements over
# log_min_duration_statement. 1.0 logs all statements,
# 0 never logs.
# - What to Log -