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Commitba38967

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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 parent5b8c93c commitba38967

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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"utils/guc.h"
@@ -42,19 +43,20 @@ static const struct config_enum_entry format_options[] = {
4243
/* Current nesting depth of ExecutorRun calls */
4344
staticintnesting_level=0;
4445

46+
/* Is the current top-level query to be sampled? */
47+
staticboolcurrent_query_sampled= false;
48+
49+
#defineauto_explain_enabled() \
50+
(auto_explain_log_min_duration >= 0 && \
51+
(nesting_level == 0 || auto_explain_log_nested_statements) && \
52+
current_query_sampled)
53+
4554
/* Saved hook values in case of unload */
4655
staticExecutorStart_hook_typeprev_ExecutorStart=NULL;
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staticExecutorRun_hook_typeprev_ExecutorRun=NULL;
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staticExecutorFinish_hook_typeprev_ExecutorFinish=NULL;
4958
staticExecutorEnd_hook_typeprev_ExecutorEnd=NULL;
5059

51-
/* Is the current query sampled, per backend */
52-
staticboolcurrent_query_sampled= true;
53-
54-
#defineauto_explain_enabled() \
55-
(auto_explain_log_min_duration >= 0 && \
56-
(nesting_level == 0 || auto_explain_log_nested_statements))
57-
5860
void_PG_init(void);
5961
void_PG_fini(void);
6062

@@ -209,14 +211,25 @@ static void
209211
explain_ExecutorStart(QueryDesc*queryDesc,inteflags)
210212
{
211213
/*
212-
* For rate sampling, randomly choose top-level statement. Either all
213-
* nested statements will be explained or none will.
214+
* At the beginning of each top-level statement, decide whether we'll
215+
* sample this statement. If nested-statement explaining is enabled,
216+
* either all nested statements will be explained or none will.
217+
*
218+
* When in a parallel worker, we should do nothing, which we can implement
219+
* cheaply by pretending we decided not to sample the current statement.
220+
* If EXPLAIN is active in the parent session, data will be collected and
221+
* reported back to the parent, and it's no business of ours to interfere.
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*/
215-
if (auto_explain_log_min_duration >=0&&nesting_level==0)
216-
current_query_sampled= (random()<auto_explain_sample_rate*
217-
MAX_RANDOM_VALUE);
223+
if (nesting_level==0)
224+
{
225+
if (auto_explain_log_min_duration >=0&& !IsParallelWorker())
226+
current_query_sampled= (random()<auto_explain_sample_rate*
227+
((double)MAX_RANDOM_VALUE+1));
228+
else
229+
current_query_sampled= false;
230+
}
218231

219-
if (auto_explain_enabled()&&current_query_sampled)
232+
if (auto_explain_enabled())
220233
{
221234
/* Enable per-node instrumentation iff log_analyze is required. */
222235
if (auto_explain_log_analyze&& (eflags&EXEC_FLAG_EXPLAIN_ONLY)==0)
@@ -235,7 +248,7 @@ explain_ExecutorStart(QueryDesc *queryDesc, int eflags)
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else
236249
standard_ExecutorStart(queryDesc,eflags);
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238-
if (auto_explain_enabled()&&current_query_sampled)
251+
if (auto_explain_enabled())
239252
{
240253
/*
241254
* Set up to track total elapsed time in ExecutorRun. Make sure the
@@ -306,7 +319,7 @@ explain_ExecutorFinish(QueryDesc *queryDesc)
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staticvoid
307320
explain_ExecutorEnd(QueryDesc*queryDesc)
308321
{
309-
if (queryDesc->totaltime&&auto_explain_enabled()&&current_query_sampled)
322+
if (queryDesc->totaltime&&auto_explain_enabled())
310323
{
311324
doublemsec;
312325

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