diff --git a/src/backend/executor/nodeModifyTable.c b/src/backend/executor/nodeModifyTable.c
new file mode 100644
index 4c5647a..28ace59
--- a/src/backend/executor/nodeModifyTable.c
+++ b/src/backend/executor/nodeModifyTable.c
@@ -3467,7 +3467,27 @@ lmerge_matched:
 
 								/* Switch lists, if necessary */
 								if (!*matched)
+								{
 									actionStates = mergeActions[MERGE_WHEN_NOT_MATCHED_BY_SOURCE];
+
+									/*
+									 * If we have both NOT MATCHED BY SOURCE
+									 * and NOT MATCHED BY TARGET actions (a
+									 * full join between the source and target
+									 * relations), the single previously
+									 * matched tuple from the outer plan node
+									 * is treated as two not matched tuples,
+									 * in the same way as if they had not
+									 * matched to start with.  Therefore, we
+									 * must adjust the outer plan node's tuple
+									 * count, if we're instrumenting the
+									 * query.
+									 */
+									if (outerPlanState(mtstate)->instrument &&
+										mergeActions[MERGE_WHEN_NOT_MATCHED_BY_SOURCE] &&
+										mergeActions[MERGE_WHEN_NOT_MATCHED_BY_TARGET])
+										InstrUpdateTupleCount(outerPlanState(mtstate)->instrument, 1.0);
+								}
 							}
 
 							/*