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Fix contrib/auto_explain to not cause problems in parallel workers.

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A parallel worker process should not be making any decisions of its
own about whether to auto-explain.  If the parent session process
passed down flags asking for instrumentation data, do that, otherwise
not.  Trying to enable instrumentation anyway leads to bugs like the
"could not find key N in shm TOC" failure reported in bug #15821
from Christian Hofstaedtler.

We can implement this cheaply by piggybacking on the existing logic
for not doing anything when we've chosen not to sample a statement.

While at it, clean up some tin-eared coding related to the sampling
feature, including an off-by-one error that meant that asking for 1.0
sampling rate didn't actually result in sampling every statement.

Although the specific case reported here only manifested in >= v11,
I believe that related misbehaviors can be demonstrated in any version
that has parallel query; and the off-by-one error is certainly there
back to 9.6 where that feature was added.  So back-patch to 9.6.

Discussion: https://postgr.es/m/15821-5eb422e980594075@postgresql.org
This commit is contained in:
Tom Lane 2019-06-03 18:06:04 -04:00
parent eaf0292c3b
commit 2cd4e83570
1 changed files with 28 additions and 15 deletions
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43
contrib/auto_explain/auto_explain.c
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@ -14,6 +14,7 @@
#include <limits.h>
#include "access/parallel.h"
#include "commands/explain.h"
#include "executor/instrument.h"
#include "jit/jit.h"
@ -59,19 +60,20 @@ static const struct config_enum_entry loglevel_options[] = {
/* Current nesting depth of ExecutorRun calls */
static int nesting_level = 0;
/* Is the current top-level query to be sampled? */
static bool current_query_sampled = false;
#define auto_explain_enabled() \
(auto_explain_log_min_duration >= 0 && \
(nesting_level == 0 || auto_explain_log_nested_statements) && \
current_query_sampled)
/* Saved hook values in case of unload */
static ExecutorStart_hook_type prev_ExecutorStart = NULL;
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))
void _PG_init(void);
void _PG_fini(void);
@ -249,14 +251,25 @@ static void
explain_ExecutorStart(QueryDesc *queryDesc, int eflags)
{
/*
* For rate sampling, randomly choose top-level statement. Either all
* nested statements will be explained or none will.
* At the beginning of each top-level statement, decide whether we'll
* sample this statement. If nested-statement explaining is enabled,
* either all nested statements will be explained or none will.
*
* When in a parallel worker, we should do nothing, which we can implement
* cheaply by pretending we decided not to sample the current statement.
* If EXPLAIN is active in the parent session, data will be collected and
* reported back to the parent, and it's no business of ours to interfere.
*/
if (auto_explain_log_min_duration >= 0 && nesting_level == 0)
current_query_sampled = (random() < auto_explain_sample_rate *
MAX_RANDOM_VALUE);
if (nesting_level == 0)
{
if (auto_explain_log_min_duration >= 0 && !IsParallelWorker())
current_query_sampled = (random() < auto_explain_sample_rate *
((double) MAX_RANDOM_VALUE + 1));
else
current_query_sampled = false;
}
if (auto_explain_enabled() && current_query_sampled)
if (auto_explain_enabled())
{
/* Enable per-node instrumentation iff log_analyze is required. */
if (auto_explain_log_analyze && (eflags & EXEC_FLAG_EXPLAIN_ONLY) == 0)
@ -275,7 +288,7 @@ explain_ExecutorStart(QueryDesc *queryDesc, int eflags)
else
standard_ExecutorStart(queryDesc, eflags);
if (auto_explain_enabled() && current_query_sampled)
if (auto_explain_enabled())
{
/*
* Set up to track total elapsed time in ExecutorRun. Make sure the
@ -346,7 +359,7 @@ explain_ExecutorFinish(QueryDesc *queryDesc)
static void
explain_ExecutorEnd(QueryDesc *queryDesc)
{
if (queryDesc->totaltime && auto_explain_enabled() && current_query_sampled)
if (queryDesc->totaltime && auto_explain_enabled())
{
double msec;