diff --git a/src/backend/optimizer/prep/prepjointree.c b/src/backend/optimizer/prep/prepjointree.c
index 4195a0a84fe..01239c0cb04 100644
--- a/src/backend/optimizer/prep/prepjointree.c
+++ b/src/backend/optimizer/prep/prepjointree.c
@@ -158,6 +158,9 @@ transform_MERGE_to_join(Query *parse)
 	int			joinrti;
 	List	   *vars;
 	RangeTblRef *rtr;
+	FromExpr   *target;
+	Node	   *source;
+	int			sourcerti;
 
 	if (parse->commandType != CMD_MERGE)
 		return;
@@ -226,13 +229,36 @@ transform_MERGE_to_join(Query *parse)
 	 * parse->jointree->quals are restrictions on the target relation (if the
 	 * target relation is an auto-updatable view).
 	 */
+	/* target rel, with any quals */
 	rtr = makeNode(RangeTblRef);
 	rtr->rtindex = parse->mergeTargetRelation;
+	target = makeFromExpr(list_make1(rtr), parse->jointree->quals);
+
+	/* source rel (expect exactly one -- see transformMergeStmt()) */
+	Assert(list_length(parse->jointree->fromlist) == 1);
+	source = linitial(parse->jointree->fromlist);
+
+	/*
+	 * index of source rel (expect either a RangeTblRef or a JoinExpr -- see
+	 * transformFromClauseItem()).
+	 */
+	if (IsA(source, RangeTblRef))
+		sourcerti = ((RangeTblRef *) source)->rtindex;
+	else if (IsA(source, JoinExpr))
+		sourcerti = ((JoinExpr *) source)->rtindex;
+	else
+	{
+		elog(ERROR, "unrecognized source node type: %d",
+			 (int) nodeTag(source));
+		sourcerti = 0;			/* keep compiler quiet */
+	}
+
+	/* Join the source and target */
 	joinexpr = makeNode(JoinExpr);
 	joinexpr->jointype = jointype;
 	joinexpr->isNatural = false;
-	joinexpr->larg = (Node *) makeFromExpr(list_make1(rtr), parse->jointree->quals);
-	joinexpr->rarg = linitial(parse->jointree->fromlist);	/* source rel */
+	joinexpr->larg = (Node *) target;
+	joinexpr->rarg = source;
 	joinexpr->usingClause = NIL;
 	joinexpr->join_using_alias = NULL;
 	joinexpr->quals = parse->mergeJoinCondition;
@@ -261,9 +287,39 @@ transform_MERGE_to_join(Query *parse)
 	 * use the join condition to distinguish between MATCHED and NOT MATCHED
 	 * BY SOURCE cases.  Otherwise, it's no longer needed, and we set it to
 	 * NULL, saving cycles during planning and execution.
+	 *
+	 * We need to be careful though: the executor evaluates this condition
+	 * using the output of the join subplan node, which nulls the output from
+	 * the source relation when the join condition doesn't match.  That risks
+	 * producing incorrect results when rechecking using a "non-strict" join
+	 * condition, such as "src.col IS NOT DISTINCT FROM tgt.col".  To guard
+	 * against that, we add an additional "src IS NOT NULL" check to the join
+	 * condition, so that it does the right thing when performing a recheck
+	 * based on the output of the join subplan.
 	 */
-	if (!have_action[MERGE_WHEN_NOT_MATCHED_BY_SOURCE])
-		parse->mergeJoinCondition = NULL;
+	if (have_action[MERGE_WHEN_NOT_MATCHED_BY_SOURCE])
+	{
+		Var		   *var;
+		NullTest   *ntest;
+
+		/* source wholerow Var (nullable by the new join) */
+		var = makeWholeRowVar(rt_fetch(sourcerti, parse->rtable),
+							  sourcerti, 0, false);
+		var->varnullingrels = bms_make_singleton(joinrti);
+
+		/* "src IS NOT NULL" check */
+		ntest = makeNode(NullTest);
+		ntest->arg = (Expr *) var;
+		ntest->nulltesttype = IS_NOT_NULL;
+		ntest->argisrow = false;
+		ntest->location = -1;
+
+		/* combine it with the original join condition */
+		parse->mergeJoinCondition =
+			(Node *) make_and_qual((Node *) ntest, parse->mergeJoinCondition);
+	}
+	else
+		parse->mergeJoinCondition = NULL;	/* join condition not needed */
 }
 
 /*
diff --git a/src/test/regress/expected/merge.out b/src/test/regress/expected/merge.out
index 3d33259e8ff..0e59bae1a7f 100644
--- a/src/test/regress/expected/merge.out
+++ b/src/test/regress/expected/merge.out
@@ -2689,6 +2689,28 @@ DETAIL:  drop cascades to table measurement_y2006m02
 drop cascades to table measurement_y2006m03
 drop cascades to table measurement_y2007m01
 DROP FUNCTION measurement_insert_trigger();
+--
+-- test non-strict join clause
+--
+CREATE TABLE src (a int, b text);
+INSERT INTO src VALUES (1, 'src row');
+CREATE TABLE tgt (a int, b text);
+INSERT INTO tgt VALUES (NULL, 'tgt row');
+MERGE INTO tgt USING src ON tgt.a IS NOT DISTINCT FROM src.a
+  WHEN MATCHED THEN UPDATE SET a = src.a, b = src.b
+  WHEN NOT MATCHED BY SOURCE THEN DELETE
+  RETURNING merge_action(), src.*, tgt.*;
+ merge_action | a | b | a |    b    
+--------------+---+---+---+---------
+ DELETE       |   |   |   | tgt row
+(1 row)
+
+SELECT * FROM tgt;
+ a | b 
+---+---
+(0 rows)
+
+DROP TABLE src, tgt;
 -- prepare
 RESET SESSION AUTHORIZATION;
 -- try a system catalog
diff --git a/src/test/regress/sql/merge.sql b/src/test/regress/sql/merge.sql
index 92163ec9fe0..2a7753c65b0 100644
--- a/src/test/regress/sql/merge.sql
+++ b/src/test/regress/sql/merge.sql
@@ -1710,6 +1710,24 @@ SELECT * FROM new_measurement ORDER BY city_id, logdate;
 DROP TABLE measurement, new_measurement CASCADE;
 DROP FUNCTION measurement_insert_trigger();
 
+--
+-- test non-strict join clause
+--
+CREATE TABLE src (a int, b text);
+INSERT INTO src VALUES (1, 'src row');
+
+CREATE TABLE tgt (a int, b text);
+INSERT INTO tgt VALUES (NULL, 'tgt row');
+
+MERGE INTO tgt USING src ON tgt.a IS NOT DISTINCT FROM src.a
+  WHEN MATCHED THEN UPDATE SET a = src.a, b = src.b
+  WHEN NOT MATCHED BY SOURCE THEN DELETE
+  RETURNING merge_action(), src.*, tgt.*;
+
+SELECT * FROM tgt;
+
+DROP TABLE src, tgt;
+
 -- prepare
 
 RESET SESSION AUTHORIZATION;