Try again to fix accumulation of parallel worker instrumentation.

When a Gather or Gather Merge node is started and stopped multiple
times, accumulate instrumentation data only once, at the end, instead
of after each execution, to avoid recording inflated totals.

Commit 778e78ae9fa51e58f41cbdc72b293291d02d8984, the previous attempt
at a fix, instead reset the state after every execution, which worked
for the general instrumentation data but had problems for the additional
instrumentation specific to Sort and Hash nodes.

Report by hubert depesz lubaczewski.  Analysis and fix by Amit Kapila,
following a design proposal from Thomas Munro, with a comment tweak
by me.

Discussion: http://postgr.es/m/20171127175631.GA405@depesz.com

src/backend/executor/execParallel.c

@@ -723,7 +723,7 @@ ExecParallelRetrieveInstrumentation(PlanState *planstate,
 
 /*
  * Finish parallel execution.  We wait for parallel workers to finish, and
- * accumulate their buffer usage and instrumentation.
+ * accumulate their buffer usage.
  */
 void
 ExecParallelFinish(ParallelExecutorInfo *pei)
@@ -769,23 +769,23 @@ ExecParallelFinish(ParallelExecutorInfo *pei)
 	for (i = 0; i < nworkers; i++)
 		InstrAccumParallelQuery(&pei->buffer_usage[i]);
 
-	/* Finally, accumulate instrumentation, if any. */
-	if (pei->instrumentation)
-		ExecParallelRetrieveInstrumentation(pei->planstate,
-											pei->instrumentation);
-
 	pei->finished = true;
 }
 
 /*
- * Clean up whatever ParallelExecutorInfo resources still exist after
- * ExecParallelFinish.  We separate these routines because someone might
- * want to examine the contents of the DSM after ExecParallelFinish and
- * before calling this routine.
+ * Accumulate instrumentation, and then clean up whatever ParallelExecutorInfo
+ * resources still exist after ExecParallelFinish.  We separate these
+ * routines because someone might want to examine the contents of the DSM
+ * after ExecParallelFinish and before calling this routine.
  */
 void
 ExecParallelCleanup(ParallelExecutorInfo *pei)
 {
+	/* Accumulate instrumentation, if any. */
+	if (pei->instrumentation)
+		ExecParallelRetrieveInstrumentation(pei->planstate,
+											pei->instrumentation);
+
 	if (pei->area != NULL)
 	{
 		dsa_detach(pei->area);

src/test/regress/expected/select_parallel.out

@@ -300,7 +300,28 @@ select count(*) from bmscantest where a>1;
  99999
 (1 row)
 
+-- test accumulation of stats for parallel nodes
 reset enable_seqscan;
+alter table tenk2 set (parallel_workers = 0);
+explain (analyze, timing off, summary off, costs off)
+   select count(*) from tenk1, tenk2 where tenk1.hundred > 1
+        and tenk2.thousand=0;
+                                QUERY PLAN                                
+--------------------------------------------------------------------------
+ Aggregate (actual rows=1 loops=1)
+   ->  Nested Loop (actual rows=98000 loops=1)
+         ->  Seq Scan on tenk2 (actual rows=10 loops=1)
+               Filter: (thousand = 0)
+               Rows Removed by Filter: 9990
+         ->  Gather (actual rows=9800 loops=10)
+               Workers Planned: 4
+               Workers Launched: 4
+               ->  Parallel Seq Scan on tenk1 (actual rows=1960 loops=50)
+                     Filter: (hundred > 1)
+                     Rows Removed by Filter: 40
+(11 rows)
+
+alter table tenk2 reset (parallel_workers);
 reset enable_indexscan;
 reset enable_hashjoin;
 reset enable_mergejoin;

src/test/regress/sql/select_parallel.sql

@@ -116,7 +116,14 @@ insert into bmscantest select r, 'fooooooooooooooooooooooooooooooooooooooooooooo
 create index i_bmtest ON bmscantest(a);
 select count(*) from bmscantest where a>1;
 
+-- test accumulation of stats for parallel nodes
 reset enable_seqscan;
+alter table tenk2 set (parallel_workers = 0);
+explain (analyze, timing off, summary off, costs off)
+   select count(*) from tenk1, tenk2 where tenk1.hundred > 1
+        and tenk2.thousand=0;
+alter table tenk2 reset (parallel_workers);
+
 reset enable_indexscan;
 reset enable_hashjoin;
 reset enable_mergejoin;