Fix parallel index and index-only scans to fall back to serial.

Parallel executor nodes can't assume that parallel execution will
happen in every case where the plan calls for it, because it might
not work out that way.  However, parallel index scan and parallel
index-only scan failed to do the right thing here.  Repair.

Amit Kapila, per a report from me.

Discussion: http://postgr.es/m/CAA4eK1Kq5qb_u2AOoda5XBB91vVWz90w=LgtRLgsssriS8pVTw@mail.gmail.com

src/backend/executor/nodeIndexonlyscan.c

@@ -78,6 +78,38 @@ IndexOnlyNext(IndexOnlyScanState *node)
 	econtext = node->ss.ps.ps_ExprContext;
 	slot = node->ss.ss_ScanTupleSlot;
 
+	if (scandesc == NULL)
+	{
+		/*
+		 * We reach here if the index only scan is not parallel, or if we're
+		 * executing a index only scan that was intended to be parallel
+		 * serially.
+		 */
+		scandesc = index_beginscan(node->ss.ss_currentRelation,
+								   node->ioss_RelationDesc,
+								   estate->es_snapshot,
+								   node->ioss_NumScanKeys,
+								   node->ioss_NumOrderByKeys);
+
+		node->ioss_ScanDesc = scandesc;
+
+
+		/* Set it up for index-only scan */
+		node->ioss_ScanDesc->xs_want_itup = true;
+		node->ioss_VMBuffer = InvalidBuffer;
+
+		/*
+		 * If no run-time keys to calculate or they are ready, go ahead and
+		 * pass the scankeys to the index AM.
+		 */
+		if (node->ioss_NumRuntimeKeys == 0 || node->ioss_RuntimeKeysReady)
+			index_rescan(scandesc,
+						 node->ioss_ScanKeys,
+						 node->ioss_NumScanKeys,
+						 node->ioss_OrderByKeys,
+						 node->ioss_NumOrderByKeys);
+	}
+
 	/*
 	 * OK, now that we have what we need, fetch the next tuple.
 	 */
@@ -571,34 +603,6 @@ ExecInitIndexOnlyScan(IndexOnlyScan *node, EState *estate, int eflags)
 		indexstate->ioss_RuntimeContext = NULL;
 	}
 
-	/*
-	 * Initialize scan descriptor.
-	 */
-	if (!node->scan.plan.parallel_aware)
-	{
-		indexstate->ioss_ScanDesc = index_beginscan(currentRelation,
-											   indexstate->ioss_RelationDesc,
-													estate->es_snapshot,
-												indexstate->ioss_NumScanKeys,
-											indexstate->ioss_NumOrderByKeys);
-
-
-		/* Set it up for index-only scan */
-		indexstate->ioss_ScanDesc->xs_want_itup = true;
-		indexstate->ioss_VMBuffer = InvalidBuffer;
-
-		/*
-		 * If no run-time keys to calculate, go ahead and pass the scankeys to
-		 * the index AM.
-		 */
-		if (indexstate->ioss_NumRuntimeKeys == 0)
-			index_rescan(indexstate->ioss_ScanDesc,
-						 indexstate->ioss_ScanKeys,
-						 indexstate->ioss_NumScanKeys,
-						 indexstate->ioss_OrderByKeys,
-						 indexstate->ioss_NumOrderByKeys);
-	}
-
 	/*
 	 * all done.
 	 */
@@ -657,10 +661,10 @@ ExecIndexOnlyScanInitializeDSM(IndexOnlyScanState *node,
 	node->ioss_VMBuffer = InvalidBuffer;
 
 	/*
-	 * If no run-time keys to calculate, go ahead and pass the scankeys to
-	 * the index AM.
+	 * If no run-time keys to calculate or they are ready, go ahead and pass
+	 * the scankeys to the index AM.
 	 */
-	if (node->ioss_NumRuntimeKeys == 0)
+	if (node->ioss_NumRuntimeKeys == 0 || node->ioss_RuntimeKeysReady)
 		index_rescan(node->ioss_ScanDesc,
 					 node->ioss_ScanKeys, node->ioss_NumScanKeys,
 					 node->ioss_OrderByKeys, node->ioss_NumOrderByKeys);
@@ -687,10 +691,10 @@ ExecIndexOnlyScanInitializeWorker(IndexOnlyScanState *node, shm_toc *toc)
 	node->ioss_ScanDesc->xs_want_itup = true;
 
 	/*
-	 * If no run-time keys to calculate, go ahead and pass the scankeys to the
-	 * index AM.
+	 * If no run-time keys to calculate or they are ready, go ahead and pass
+	 * the scankeys to the index AM.
 	 */
-	if (node->ioss_NumRuntimeKeys == 0)
+	if (node->ioss_NumRuntimeKeys == 0 || node->ioss_RuntimeKeysReady)
 		index_rescan(node->ioss_ScanDesc,
 					 node->ioss_ScanKeys, node->ioss_NumScanKeys,
 					 node->ioss_OrderByKeys, node->ioss_NumOrderByKeys);

src/backend/executor/nodeIndexscan.c

@@ -102,6 +102,30 @@ IndexNext(IndexScanState *node)
 	econtext = node->ss.ps.ps_ExprContext;
 	slot = node->ss.ss_ScanTupleSlot;
 
+	if (scandesc == NULL)
+	{
+		/*
+		 * We reach here if the index scan is not parallel, or if we're
+		 * executing a index scan that was intended to be parallel serially.
+		 */
+		scandesc = index_beginscan(node->ss.ss_currentRelation,
+								   node->iss_RelationDesc,
+								   estate->es_snapshot,
+								   node->iss_NumScanKeys,
+								   node->iss_NumOrderByKeys);
+
+		node->iss_ScanDesc = scandesc;
+
+		/*
+		 * If no run-time keys to calculate or they are ready, go ahead and
+		 * pass the scankeys to the index AM.
+		 */
+		if (node->iss_NumRuntimeKeys == 0 || node->iss_RuntimeKeysReady)
+			index_rescan(scandesc,
+						 node->iss_ScanKeys, node->iss_NumScanKeys,
+						 node->iss_OrderByKeys, node->iss_NumOrderByKeys);
+	}
+
 	/*
 	 * ok, now that we have what we need, fetch the next tuple.
 	 */
@@ -154,6 +178,7 @@ IndexNext(IndexScanState *node)
 static TupleTableSlot *
 IndexNextWithReorder(IndexScanState *node)
 {
+	EState	   *estate;
 	ExprContext *econtext;
 	IndexScanDesc scandesc;
 	HeapTuple	tuple;
@@ -164,6 +189,8 @@ IndexNextWithReorder(IndexScanState *node)
 	bool	   *lastfetched_nulls;
 	int			cmp;
 
+	estate = node->ss.ps.state;
+
 	/*
 	 * Only forward scan is supported with reordering.  Note: we can get away
 	 * with just Asserting here because the system will not try to run the
@@ -174,12 +201,36 @@ IndexNextWithReorder(IndexScanState *node)
 	 * explicitly.
 	 */
 	Assert(!ScanDirectionIsBackward(((IndexScan *) node->ss.ps.plan)->indexorderdir));
-	Assert(ScanDirectionIsForward(node->ss.ps.state->es_direction));
+	Assert(ScanDirectionIsForward(estate->es_direction));
 
 	scandesc = node->iss_ScanDesc;
 	econtext = node->ss.ps.ps_ExprContext;
 	slot = node->ss.ss_ScanTupleSlot;
 
+	if (scandesc == NULL)
+	{
+		/*
+		 * We reach here if the index scan is not parallel, or if we're
+		 * executing a index scan that was intended to be parallel serially.
+		 */
+		scandesc = index_beginscan(node->ss.ss_currentRelation,
+								   node->iss_RelationDesc,
+								   estate->es_snapshot,
+								   node->iss_NumScanKeys,
+								   node->iss_NumOrderByKeys);
+
+		node->iss_ScanDesc = scandesc;
+
+		/*
+		 * If no run-time keys to calculate or they are ready, go ahead and
+		 * pass the scankeys to the index AM.
+		 */
+		if (node->iss_NumRuntimeKeys == 0 || node->iss_RuntimeKeysReady)
+			index_rescan(scandesc,
+						 node->iss_ScanKeys, node->iss_NumScanKeys,
+						 node->iss_OrderByKeys, node->iss_NumOrderByKeys);
+	}
+
 	for (;;)
 	{
 		/*
@@ -1038,31 +1089,6 @@ ExecInitIndexScan(IndexScan *node, EState *estate, int eflags)
 		indexstate->iss_RuntimeContext = NULL;
 	}
 
-	/*
-	 * for parallel-aware node, we initialize the scan descriptor after
-	 * initializing the shared memory for parallel execution.
-	 */
-	if (!node->scan.plan.parallel_aware)
-	{
-		/*
-		 * Initialize scan descriptor.
-		 */
-		indexstate->iss_ScanDesc = index_beginscan(currentRelation,
-												indexstate->iss_RelationDesc,
-												   estate->es_snapshot,
-												 indexstate->iss_NumScanKeys,
-											 indexstate->iss_NumOrderByKeys);
-
-		/*
-		 * If no run-time keys to calculate, go ahead and pass the scankeys to
-		 * the index AM.
-		 */
-		if (indexstate->iss_NumRuntimeKeys == 0)
-			index_rescan(indexstate->iss_ScanDesc,
-					   indexstate->iss_ScanKeys, indexstate->iss_NumScanKeys,
-				indexstate->iss_OrderByKeys, indexstate->iss_NumOrderByKeys);
-	}
-
 	/*
 	 * all done.
 	 */
@@ -1674,10 +1700,10 @@ ExecIndexScanInitializeDSM(IndexScanState *node,
 								 piscan);
 
 	/*
-	 * If no run-time keys to calculate, go ahead and pass the scankeys to the
-	 * index AM.
+	 * If no run-time keys to calculate or they are ready, go ahead and pass
+	 * the scankeys to the index AM.
 	 */
-	if (node->iss_NumRuntimeKeys == 0)
+	if (node->iss_NumRuntimeKeys == 0 || node->iss_RuntimeKeysReady)
 		index_rescan(node->iss_ScanDesc,
 					 node->iss_ScanKeys, node->iss_NumScanKeys,
 					 node->iss_OrderByKeys, node->iss_NumOrderByKeys);
@@ -1703,10 +1729,10 @@ ExecIndexScanInitializeWorker(IndexScanState *node, shm_toc *toc)
 								 piscan);
 
 	/*
-	 * If no run-time keys to calculate, go ahead and pass the scankeys to the
-	 * index AM.
+	 * If no run-time keys to calculate or they are ready, go ahead and pass
+	 * the scankeys to the index AM.
 	 */
-	if (node->iss_NumRuntimeKeys == 0)
+	if (node->iss_NumRuntimeKeys == 0 || node->iss_RuntimeKeysReady)
 		index_rescan(node->iss_ScanDesc,
 					 node->iss_ScanKeys, node->iss_NumScanKeys,
 					 node->iss_OrderByKeys, node->iss_NumOrderByKeys);