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Commit57e85fa

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Fix contrib/auto_explain to not cause problems in parallel workers.
A parallel worker process should not be making any decisions of itsown about whether to auto-explain. If the parent session processpassed down flags asking for instrumentation data, do that, otherwisenot. Trying to enable instrumentation anyway leads to bugs like the"could not find key N in shm TOC" failure reported in bug #15821from Christian Hofstaedtler.We can implement this cheaply by piggybacking on the existing logicfor not doing anything when we've chosen not to sample a statement.While at it, clean up some tin-eared coding related to the samplingfeature, including an off-by-one error that meant that asking for 1.0sampling 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 versionthat has parallel query; and the off-by-one error is certainly thereback to 9.6 where that feature was added. So back-patch to 9.6.Discussion:https://postgr.es/m/15821-5eb422e980594075@postgresql.org
1 parent601084e commit57e85fa

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‎contrib/auto_explain/auto_explain.c

Lines changed: 28 additions & 15 deletions
Original file line numberDiff line numberDiff line change
@@ -14,6 +14,7 @@
1414

1515
#include<limits.h>
1616

17+
#include"access/parallel.h"
1718
#include"commands/explain.h"
1819
#include"executor/instrument.h"
1920
#include"jit/jit.h"
@@ -43,19 +44,20 @@ static const struct config_enum_entry format_options[] = {
4344
/* Current nesting depth of ExecutorRun calls */
4445
staticintnesting_level=0;
4546

47+
/* Is the current top-level query to be sampled? */
48+
staticboolcurrent_query_sampled= false;
49+
50+
#defineauto_explain_enabled() \
51+
(auto_explain_log_min_duration >= 0 && \
52+
(nesting_level == 0 || auto_explain_log_nested_statements) && \
53+
current_query_sampled)
54+
4655
/* Saved hook values in case of unload */
4756
staticExecutorStart_hook_typeprev_ExecutorStart=NULL;
4857
staticExecutorRun_hook_typeprev_ExecutorRun=NULL;
4958
staticExecutorFinish_hook_typeprev_ExecutorFinish=NULL;
5059
staticExecutorEnd_hook_typeprev_ExecutorEnd=NULL;
5160

52-
/* Is the current query sampled, per backend */
53-
staticboolcurrent_query_sampled= true;
54-
55-
#defineauto_explain_enabled() \
56-
(auto_explain_log_min_duration >= 0 && \
57-
(nesting_level == 0 || auto_explain_log_nested_statements))
58-
5961
void_PG_init(void);
6062
void_PG_fini(void);
6163

@@ -210,14 +212,25 @@ static void
210212
explain_ExecutorStart(QueryDesc*queryDesc,inteflags)
211213
{
212214
/*
213-
* For rate sampling, randomly choose top-level statement. Either all
214-
* nested statements will be explained or none will.
215+
* At the beginning of each top-level statement, decide whether we'll
216+
* sample this statement. If nested-statement explaining is enabled,
217+
* either all nested statements will be explained or none will.
218+
*
219+
* When in a parallel worker, we should do nothing, which we can implement
220+
* cheaply by pretending we decided not to sample the current statement.
221+
* If EXPLAIN is active in the parent session, data will be collected and
222+
* reported back to the parent, and it's no business of ours to interfere.
215223
*/
216-
if (auto_explain_log_min_duration >=0&&nesting_level==0)
217-
current_query_sampled= (random()<auto_explain_sample_rate*
218-
MAX_RANDOM_VALUE);
224+
if (nesting_level==0)
225+
{
226+
if (auto_explain_log_min_duration >=0&& !IsParallelWorker())
227+
current_query_sampled= (random()<auto_explain_sample_rate*
228+
((double)MAX_RANDOM_VALUE+1));
229+
else
230+
current_query_sampled= false;
231+
}
219232

220-
if (auto_explain_enabled()&&current_query_sampled)
233+
if (auto_explain_enabled())
221234
{
222235
/* Enable per-node instrumentation iff log_analyze is required. */
223236
if (auto_explain_log_analyze&& (eflags&EXEC_FLAG_EXPLAIN_ONLY)==0)
@@ -236,7 +249,7 @@ explain_ExecutorStart(QueryDesc *queryDesc, int eflags)
236249
else
237250
standard_ExecutorStart(queryDesc,eflags);
238251

239-
if (auto_explain_enabled()&&current_query_sampled)
252+
if (auto_explain_enabled())
240253
{
241254
/*
242255
* Set up to track total elapsed time in ExecutorRun. Make sure the
@@ -307,7 +320,7 @@ explain_ExecutorFinish(QueryDesc *queryDesc)
307320
staticvoid
308321
explain_ExecutorEnd(QueryDesc*queryDesc)
309322
{
310-
if (queryDesc->totaltime&&auto_explain_enabled()&&current_query_sampled)
323+
if (queryDesc->totaltime&&auto_explain_enabled())
311324
{
312325
doublemsec;
313326

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