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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);