From 92f03fe76fe6be683a8b7497579158b8a82b2c25 Mon Sep 17 00:00:00 2001 From: Magnus Hagander Date: Fri, 11 Mar 2016 15:08:34 +0100 Subject: [PATCH] Allow setting sample ratio for auto_explain New configuration parameter auto_explain.sample_ratio makes it possible to log just a fraction of the queries meeting the configured threshold, to reduce the amount of logging. Author: Craig Ringer and Julien Rouhaud Review: Petr Jelinek --- contrib/auto_explain/auto_explain.c | 31 ++++++++++++++++++++++++++--- doc/src/sgml/auto-explain.sgml | 18 +++++++++++++++++ 2 files changed, 46 insertions(+), 3 deletions(-) diff --git a/contrib/auto_explain/auto_explain.c b/contrib/auto_explain/auto_explain.c index 0950e18f6d..76d1831567 100644 --- a/contrib/auto_explain/auto_explain.c +++ b/contrib/auto_explain/auto_explain.c @@ -29,6 +29,7 @@ static bool auto_explain_log_triggers = false; static bool auto_explain_log_timing = true; static int auto_explain_log_format = EXPLAIN_FORMAT_TEXT; static bool auto_explain_log_nested_statements = false; +static double auto_explain_sample_ratio = 1; static const struct config_enum_entry format_options[] = { {"text", EXPLAIN_FORMAT_TEXT, false}, @@ -47,6 +48,9 @@ static ExecutorRun_hook_type prev_ExecutorRun = NULL; static ExecutorFinish_hook_type prev_ExecutorFinish = NULL; static ExecutorEnd_hook_type prev_ExecutorEnd = NULL; +/* Is the current query sampled, per backend */ +static bool current_query_sampled = true; + #define auto_explain_enabled() \ (auto_explain_log_min_duration >= 0 && \ (nesting_level == 0 || auto_explain_log_nested_statements)) @@ -159,6 +163,19 @@ _PG_init(void) NULL, NULL); + DefineCustomRealVariable("auto_explain.sample_ratio", + "Fraction of queries to process.", + NULL, + &auto_explain_sample_ratio, + 1.0, + 0.0, + 1.0, + PGC_SUSET, + 0, + NULL, + NULL, + NULL); + EmitWarningsOnPlaceholders("auto_explain"); /* Install hooks. */ @@ -191,7 +208,15 @@ _PG_fini(void) static void explain_ExecutorStart(QueryDesc *queryDesc, int eflags) { - if (auto_explain_enabled()) + /* + * For ratio sampling, randomly choose top-level statement. Either + * all nested statements will be explained or none will. + */ + if (auto_explain_log_min_duration >= 0 && nesting_level == 0) + current_query_sampled = (random() < auto_explain_sample_ratio * + MAX_RANDOM_VALUE); + + if (auto_explain_enabled() && current_query_sampled) { /* Enable per-node instrumentation iff log_analyze is required. */ if (auto_explain_log_analyze && (eflags & EXEC_FLAG_EXPLAIN_ONLY) == 0) @@ -210,7 +235,7 @@ explain_ExecutorStart(QueryDesc *queryDesc, int eflags) else standard_ExecutorStart(queryDesc, eflags); - if (auto_explain_enabled()) + if (auto_explain_enabled() && current_query_sampled) { /* * Set up to track total elapsed time in ExecutorRun. Make sure the @@ -280,7 +305,7 @@ explain_ExecutorFinish(QueryDesc *queryDesc) static void explain_ExecutorEnd(QueryDesc *queryDesc) { - if (queryDesc->totaltime && auto_explain_enabled()) + if (queryDesc->totaltime && auto_explain_enabled() && current_query_sampled) { double msec; diff --git a/doc/src/sgml/auto-explain.sgml b/doc/src/sgml/auto-explain.sgml index d527208271..6f1bde0d17 100644 --- a/doc/src/sgml/auto-explain.sgml +++ b/doc/src/sgml/auto-explain.sgml @@ -203,6 +203,24 @@ LOAD 'auto_explain'; + + + + auto_explain.sample_ratio (real) + + auto_explain.sample_ratio configuration parameter + + + + + auto_explain.sample_ratio (floating point) + causes auto_explain to only explain a fraction of the statements in each + session. The default is 1, meaning explain all the queries. In case + of nested statements, either all will be explained or none. Only + superusers can change this setting. + + + -- 2.40.0