diff --git a/src/backend/optimizer/path/costsize.c b/src/backend/optimizer/path/costsize.c
index 7bf67a0529..480fd250e9 100644
--- a/src/backend/optimizer/path/costsize.c
+++ b/src/backend/optimizer/path/costsize.c
@@ -2941,8 +2941,13 @@ final_cost_mergejoin(PlannerInfo *root, MergePath *path,
 		if (rescannedtuples < 0)
 			rescannedtuples = 0;
 	}
-	/* We'll inflate various costs this much to account for rescanning */
-	rescanratio = 1.0 + (rescannedtuples / inner_path_rows);
+
+	/*
+	 * We'll inflate various costs this much to account for rescanning.  Note
+	 * that this is to be multiplied by something involving inner_rows, or
+	 * another number related to the portion of the inner rel we'll scan.
+	 */
+	rescanratio = 1.0 + (rescannedtuples / inner_rows);
 
 	/*
 	 * Decide whether we want to materialize the inner input to shield it from
@@ -2969,7 +2974,7 @@ final_cost_mergejoin(PlannerInfo *root, MergePath *path,
 	 * of the generated Material node.
 	 */
 	mat_inner_cost = inner_run_cost +
-		cpu_operator_cost * inner_path_rows * rescanratio;
+		cpu_operator_cost * inner_rows * rescanratio;
 
 	/*
 	 * If we don't need mark/restore at all, we don't need materialization.