diff --git a/src/backend/access/nbtree/nbtree.c b/src/backend/access/nbtree/nbtree.c
index c822b49a71..c4f22f1c69 100644
--- a/src/backend/access/nbtree/nbtree.c
+++ b/src/backend/access/nbtree/nbtree.c
@@ -933,6 +933,12 @@ btvacuumcleanup(IndexVacuumInfo *info, IndexBulkDeleteResult *stats)
 	 * double-counting some index tuples, so disbelieve any total that exceeds
 	 * the underlying heap's count ... if we know that accurately.  Otherwise
 	 * this might just make matters worse.
+	 *
+	 * Posting list tuples are another source of inaccuracy.  Cleanup-only
+	 * btvacuumscan calls assume that the number of index tuples can be used
+	 * as num_index_tuples, even though num_index_tuples is supposed to
+	 * represent the number of TIDs in the index.  This naive approach can
+	 * underestimate the number of tuples in the index.
 	 */
 	if (!info->estimated_count)
 	{
@@ -1394,11 +1400,18 @@ backtrack:
 		 * separate live tuples).  We don't delete when backtracking, though,
 		 * since that would require teaching _bt_pagedel() about backtracking
 		 * (doesn't seem worth adding more complexity to deal with that).
+		 *
+		 * We don't count the number of live TIDs during cleanup-only calls to
+		 * btvacuumscan (i.e. when callback is not set).  We count the number
+		 * of index tuples directly instead.  This avoids the expense of
+		 * directly examining all of the tuples on each page.
 		 */
 		if (minoff > maxoff)
 			attempt_pagedel = (blkno == scanblkno);
-		else
+		else if (callback)
 			stats->num_index_tuples += nhtidslive;
+		else
+			stats->num_index_tuples += maxoff - minoff + 1;
 
 		Assert(!attempt_pagedel || nhtidslive == 0);
 	}