1 files changed, 15 insertions, 14 deletions

diff --git a/src/backend/executor/nodeGatherMerge.c b/src/backend/executor/nodeGatherMerge.c
index f50841699c4..0bd5da38b4a 100644
--- a/src/backend/executor/nodeGatherMerge.c
+++ b/src/backend/executor/nodeGatherMerge.c
@@ -187,12 +187,15 @@ ExecGatherMerge(PlanState *pstate)
 		{
 			ParallelContext *pcxt;
 
-			/* Initialize data structures for workers. */
+			/* Initialize, or re-initialize, shared state needed by workers. */
 			if (!node->pei)
 				node->pei = ExecInitParallelPlan(node->ps.lefttree,
 												 estate,
 												 gm->num_workers,
 												 node->tuples_needed);
+			else
+				ExecParallelReinitialize(node->ps.lefttree,
+										 node->pei);
 
 			/* Try to launch workers. */
 			pcxt = node->pei->pcxt;
@@ -321,7 +324,7 @@ ExecShutdownGatherMergeWorkers(GatherMergeState *node)
 /* ----------------------------------------------------------------
  *		ExecReScanGatherMerge
  *
- *		Re-initialize the workers and rescans a relation via them.
+ *		Prepare to re-scan the result of a GatherMerge.
  * ----------------------------------------------------------------
  */
 void
@@ -330,20 +333,13 @@ ExecReScanGatherMerge(GatherMergeState *node)
 	GatherMerge *gm = (GatherMerge *) node->ps.plan;
 	PlanState  *outerPlan = outerPlanState(node);
 
-	/*
-	 * Re-initialize the parallel workers to perform rescan of relation. We
-	 * want to gracefully shutdown all the workers so that they should be able
-	 * to propagate any error or other information to master backend before
-	 * dying.  Parallel context will be reused for rescan.
-	 */
+	/* Make sure any existing workers are gracefully shut down */
 	ExecShutdownGatherMergeWorkers(node);
 
+	/* Mark node so that shared state will be rebuilt at next call */
 	node->initialized = false;
 	node->gm_initialized = false;
 
-	if (node->pei)
-		ExecParallelReinitialize(node->pei);
-
 	/*
 	 * Set child node's chgParam to tell it that the next scan might deliver a
 	 * different set of rows within the leader process.  (The overall rowset
@@ -355,10 +351,15 @@ ExecReScanGatherMerge(GatherMergeState *node)
 		outerPlan->chgParam = bms_add_member(outerPlan->chgParam,
 											 gm->rescan_param);
 
-
 	/*
-	 * if chgParam of subnode is not null then plan will be re-scanned by
-	 * first ExecProcNode.
+	 * If chgParam of subnode is not null then plan will be re-scanned by
+	 * first ExecProcNode.  Note: because this does nothing if we have a
+	 * rescan_param, it's currently guaranteed that parallel-aware child nodes
+	 * will not see a ReScan call until after they get a ReInitializeDSM call.
+	 * That ordering might not be something to rely on, though.  A good rule
+	 * of thumb is that ReInitializeDSM should reset only shared state, ReScan
+	 * should reset only local state, and anything that depends on both of
+	 * those steps being finished must wait until the first ExecProcNode call.
 	 */
 	if (outerPlan->chgParam == NULL)
 		ExecReScan(outerPlan);