Instrument index-only scans to count heap fetches performed.

Patch by me; review by Tom Lane, Jeff Davis, and Peter Geoghegan.

src/backend/commands/explain.c

@@ -1012,6 +1012,9 @@ ExplainNode(PlanState *planstate, List *ancestors,
 			if (plan->qual)
 				show_instrumentation_count("Rows Removed by Filter", 1,
 										   planstate, es);
+			if (es->analyze)
+				ExplainPropertyLong("Heap Fetches",
+					((IndexOnlyScanState *) planstate)->ioss_HeapFetches, es);
 			break;
 		case T_BitmapIndexScan:
 			show_scan_qual(((BitmapIndexScan *) plan)->indexqualorig,

src/backend/executor/nodeIndexonlyscan.c

@@ -90,6 +90,7 @@ IndexOnlyNext(IndexOnlyScanState *node)
 			/*
 			 * Rats, we have to visit the heap to check visibility.
 			 */
+			node->ioss_HeapFetches++;
 			tuple = index_fetch_heap(scandesc);
 			if (tuple == NULL)
 				continue;	/* no visible tuple, try next index entry */
@@ -346,6 +347,7 @@ ExecInitIndexOnlyScan(IndexOnlyScan *node, EState *estate, int eflags)
 	indexstate = makeNode(IndexOnlyScanState);
 	indexstate->ss.ps.plan = (Plan *) node;
 	indexstate->ss.ps.state = estate;
+	indexstate->ioss_HeapFetches = 0;
 
 	/*
 	 * Miscellaneous initialization

src/include/nodes/execnodes.h

@@ -1260,6 +1260,7 @@ typedef struct IndexScanState
  *		RelationDesc	   index relation descriptor
  *		ScanDesc		   index scan descriptor
  *		VMBuffer		   buffer in use for visibility map testing, if any
+ *		HeapFetches		   number of tuples we were forced to fetch from heap
  * ----------------
  */
 typedef struct IndexOnlyScanState
@@ -1277,6 +1278,7 @@ typedef struct IndexOnlyScanState
 	Relation	ioss_RelationDesc;
 	IndexScanDesc ioss_ScanDesc;
 	Buffer		ioss_VMBuffer;
+	long		ioss_HeapFetches;
 } IndexOnlyScanState;
 
 /* ----------------