From 7124c93fb54fdd3bcf98e29392fc66ddb81f5343 Mon Sep 17 00:00:00 2001
From: Amit Kapila <akapila@postgresql.org>
Date: Fri, 3 Aug 2018 10:07:56 +0530
Subject: [PATCH] Match the buffer usage tracking for leader and worker
 backends.

In the leader backend, we don't track the buffer usage for ExecutorStart
phase whereas in worker backend we track it for ExecutorStart phase as
well.  This leads to different value for buffer usage stats for the
parallel and non-parallel query.  Change the code so that worker backend
also starts tracking buffer usage after ExecutorStart.

Author: Amit Kapila and Robert Haas
Reviewed-by: Robert Haas and Andres Freund
Backpatch-through: 9.6 where this code was introduced
Discussion: https://postgr.es/m/86137f17-1dfb-42f9-7421-82fd786b04a1@anayrat.info
---
 src/backend/executor/execParallel.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/src/backend/executor/execParallel.c b/src/backend/executor/execParallel.c
index 6eed6db3355..b1a3d91a196 100644
--- a/src/backend/executor/execParallel.c
+++ b/src/backend/executor/execParallel.c
@@ -735,12 +735,19 @@ ParallelQueryMain(dsm_segment *seg, shm_toc *toc)
 		instrument_options = instrumentation->instrument_options;
 	queryDesc = ExecParallelGetQueryDesc(toc, receiver, instrument_options);
 
-	/* Prepare to track buffer usage during query execution. */
-	InstrStartParallelQuery();
-
 	/* Start up the executor, have it run the plan, and then shut it down. */
 	ExecutorStart(queryDesc, 0);
 	ExecParallelInitializeWorker(queryDesc->planstate, toc);
+
+	/*
+	 * Prepare to track buffer usage during query execution.
+	 *
+	 * We do this after starting up the executor to match what happens in the
+	 * leader, which also doesn't count buffer accesses that occur during
+	 * executor startup.
+	 */
+	InstrStartParallelQuery();
+
 	ExecutorRun(queryDesc, ForwardScanDirection, 0L);
 	ExecutorFinish(queryDesc);
 
-- 
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