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Allow sampling of statements depending on duration

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This allows logging a sample of statements, without incurring excessive
log traffic (which may impact performance).  This can be useful when
analyzing workloads with lots of short queries.

The sampling is configured using two new GUC parameters:

 * log_min_duration_sample - minimum required statement duration

 * log_statement_sample_rate - sample rate (0.0 - 1.0)

Only statements with duration exceeding log_min_duration_sample are
considered for sampling. To enable sampling, both those GUCs have to
be set correctly.

The existing log_min_duration_statement GUC has a higher priority, i.e.
statements with duration exceeding log_min_duration_statement will be
always logged, irrespectedly of how the sampling is configured. This
means only configurations

  log_min_duration_sample < log_min_duration_statement

do actually sample the statements, instead of logging everything.

Author: Adrien Nayrat
Reviewed-by: David Rowley, Vik Fearing, Tomas Vondra
Discussion: https://postgr.es/m/bbe0a1a8-a8f7-3be2-155a-888e661cc06c@anayrat.info
This commit is contained in:
Tomas Vondra
2019-11-04 01:57:45 +01:00
parent 11d9ac28e5
commit 6e3e6cc0e8
5 changed files with 153 additions and 12 deletions
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27
src/backend/utils/misc/guc.c
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@ -509,8 +509,10 @@ bool session_auth_is_superuser;
int log_min_error_statement = ERROR;
int log_min_messages = WARNING;
int client_min_messages = NOTICE;
int log_min_duration_sample = -1;
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;
@ -2700,10 +2702,23 @@ static struct config_int ConfigureNamesInt[] =
NULL, NULL, NULL
},
{
{"log_min_duration_sample", PGC_SUSET, LOGGING_WHEN,
gettext_noop("Sets the minimum execution time above which "
"a sample of statements will be logged."
" Sampling is determined by log_statement_sample_rate."),
gettext_noop("Zero log a sample of all queries. -1 turns this feature off."),
GUC_UNIT_MS
},
&log_min_duration_sample,
-1, -1, INT_MAX,
NULL, NULL, NULL
},
{
{"log_min_duration_statement", PGC_SUSET, LOGGING_WHEN,
gettext_noop("Sets the minimum execution time above which "
"statements will be logged."),
"all statements will be logged."),
gettext_noop("Zero prints all queries. -1 turns this feature off."),
GUC_UNIT_MS
},
@ -3430,6 +3445,16 @@ static struct config_real ConfigureNamesReal[] =
NULL, NULL, NULL
},
{
{"log_statement_sample_rate", PGC_SUSET, LOGGING_WHEN,
gettext_noop("Fraction of statements exceeding log_min_duration_sample to be logged."),
gettext_noop("Use a value between 0.0 (never log) and 1.0 (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."),