Skip setting up shared instrumentation for Hash node if not needed.

We don't need to set up the shared space for hash join instrumentation data
if instrumentation hasn't been requested.  Let's follow the example of the
similar Sort node code and save a few cycles by skipping that when we can.

This reverts commit d59ff4ab and instead allows us to use the safer choice
of passing noError = false to shm_toc_lookup in ExecHashInitializeWorker,
since if we reach that call there should be a TOC entry to be found.

Thomas Munro

Discussion: https://postgr.es/m/E1ehkoZ-0005uW-43%40gemulon.postgresql.org

src/backend/executor/nodeHash.c

@@ -2549,6 +2549,10 @@ ExecHashEstimate(HashState *node, ParallelContext *pcxt)
 {
 	size_t		size;
 
+	/* don't need this if not instrumenting or no workers */
+	if (!node->ps.instrument || pcxt->nworkers == 0)
+		return;
+
 	size = mul_size(pcxt->nworkers, sizeof(HashInstrumentation));
 	size = add_size(size, offsetof(SharedHashInfo, hinstrument));
 	shm_toc_estimate_chunk(&pcxt->estimator, size);
@@ -2564,6 +2568,10 @@ ExecHashInitializeDSM(HashState *node, ParallelContext *pcxt)
 {
 	size_t		size;
 
+	/* don't need this if not instrumenting or no workers */
+	if (!node->ps.instrument || pcxt->nworkers == 0)
+		return;
+
 	size = offsetof(SharedHashInfo, hinstrument) +
 		pcxt->nworkers * sizeof(HashInstrumentation);
 	node->shared_info = (SharedHashInfo *) shm_toc_allocate(pcxt->toc, size);
@@ -2582,13 +2590,13 @@ ExecHashInitializeWorker(HashState *node, ParallelWorkerContext *pwcxt)
 {
 	SharedHashInfo *shared_info;
 
-	/* might not be there ... */
+	/* don't need this if not instrumenting */
+	if (!node->ps.instrument)
+		return;
+
 	shared_info = (SharedHashInfo *)
-		shm_toc_lookup(pwcxt->toc, node->ps.plan->plan_node_id, true);
-	if (shared_info)
+		shm_toc_lookup(pwcxt->toc, node->ps.plan->plan_node_id, false);
 	node->hinstrument = &shared_info->hinstrument[ParallelWorkerNumber];
-	else
-		node->hinstrument = NULL;
 }
 
 /*
@@ -2614,6 +2622,9 @@ ExecHashRetrieveInstrumentation(HashState *node)
 	SharedHashInfo *shared_info = node->shared_info;
 	size_t		size;
 
+	if (shared_info == NULL)
+		return;
+
 	/* Replace node->shared_info with a copy in backend-local memory. */
 	size = offsetof(SharedHashInfo, hinstrument) +
 		shared_info->num_workers * sizeof(HashInstrumentation);