diff --git a/src/backend/executor/execIndexing.c b/src/backend/executor/execIndexing.c
index 0cb0b8f111..6a8735edf7 100644
--- a/src/backend/executor/execIndexing.c
+++ b/src/backend/executor/execIndexing.c
@@ -699,13 +699,19 @@ check_exclusion_or_unique_constraint(Relation heap, Relation index,
 	}
 
 	/*
-	 * If any of the input values are NULL, the constraint check is assumed to
-	 * pass (i.e., we assume the operators are strict).
+	 * If any of the input values are NULL, and the index uses the default
+	 * nulls-are-distinct mode, the constraint check is assumed to pass (i.e.,
+	 * we assume the operators are strict).  Otherwise, we interpret the
+	 * constraint as specifying IS NULL for each column whose input value is
+	 * NULL.
 	 */
-	for (i = 0; i < indnkeyatts; i++)
+	if (!indexInfo->ii_NullsNotDistinct)
 	{
-		if (isnull[i])
-			return true;
+		for (i = 0; i < indnkeyatts; i++)
+		{
+			if (isnull[i])
+				return true;
+		}
 	}
 
 	/*
@@ -717,7 +723,7 @@ check_exclusion_or_unique_constraint(Relation heap, Relation index,
 	for (i = 0; i < indnkeyatts; i++)
 	{
 		ScanKeyEntryInitialize(&scankeys[i],
-							   0,
+							   isnull[i] ? SK_ISNULL | SK_SEARCHNULL : 0,
 							   i + 1,
 							   constr_strats[i],
 							   InvalidOid,
diff --git a/src/backend/executor/nodeModifyTable.c b/src/backend/executor/nodeModifyTable.c
index a49c3da5b6..deda321502 100644
--- a/src/backend/executor/nodeModifyTable.c
+++ b/src/backend/executor/nodeModifyTable.c
@@ -956,9 +956,11 @@ ExecInsert(ModifyTableContext *context,
 			 *
 			 * We loop back here if we find a conflict below, either during
 			 * the pre-check, or when we re-check after inserting the tuple
-			 * speculatively.
+			 * speculatively.  Better allow interrupts in case some bug makes
+			 * this an infinite loop.
 			 */
 	vlock:
+			CHECK_FOR_INTERRUPTS();
 			specConflict = false;
 			if (!ExecCheckIndexConstraints(resultRelInfo, slot, estate,
 										   &conflictTid, arbiterIndexes))
diff --git a/src/test/regress/expected/constraints.out b/src/test/regress/expected/constraints.out
index 36ccbb5f15..05b7244e4a 100644
--- a/src/test/regress/expected/constraints.out
+++ b/src/test/regress/expected/constraints.out
@@ -449,15 +449,16 @@ DROP TABLE UNIQUE_TBL;
 CREATE TABLE UNIQUE_TBL (i int UNIQUE NULLS NOT DISTINCT, t text);
 INSERT INTO UNIQUE_TBL VALUES (1, 'one');
 INSERT INTO UNIQUE_TBL VALUES (2, 'two');
-INSERT INTO UNIQUE_TBL VALUES (1, 'three');
+INSERT INTO UNIQUE_TBL VALUES (1, 'three');  -- fail
 ERROR:  duplicate key value violates unique constraint "unique_tbl_i_key"
 DETAIL:  Key (i)=(1) already exists.
 INSERT INTO UNIQUE_TBL VALUES (4, 'four');
 INSERT INTO UNIQUE_TBL VALUES (5, 'one');
 INSERT INTO UNIQUE_TBL (t) VALUES ('six');
-INSERT INTO UNIQUE_TBL (t) VALUES ('seven');
+INSERT INTO UNIQUE_TBL (t) VALUES ('seven');  -- fail
 ERROR:  duplicate key value violates unique constraint "unique_tbl_i_key"
 DETAIL:  Key (i)=(null) already exists.
+INSERT INTO UNIQUE_TBL (t) VALUES ('eight') ON CONFLICT DO NOTHING;  -- no-op
 SELECT * FROM UNIQUE_TBL;
  i |  t   
 ---+------
diff --git a/src/test/regress/sql/constraints.sql b/src/test/regress/sql/constraints.sql
index 34de0c969a..833819a32e 100644
--- a/src/test/regress/sql/constraints.sql
+++ b/src/test/regress/sql/constraints.sql
@@ -310,11 +310,12 @@ CREATE TABLE UNIQUE_TBL (i int UNIQUE NULLS NOT DISTINCT, t text);
 
 INSERT INTO UNIQUE_TBL VALUES (1, 'one');
 INSERT INTO UNIQUE_TBL VALUES (2, 'two');
-INSERT INTO UNIQUE_TBL VALUES (1, 'three');
+INSERT INTO UNIQUE_TBL VALUES (1, 'three');  -- fail
 INSERT INTO UNIQUE_TBL VALUES (4, 'four');
 INSERT INTO UNIQUE_TBL VALUES (5, 'one');
 INSERT INTO UNIQUE_TBL (t) VALUES ('six');
-INSERT INTO UNIQUE_TBL (t) VALUES ('seven');
+INSERT INTO UNIQUE_TBL (t) VALUES ('seven');  -- fail
+INSERT INTO UNIQUE_TBL (t) VALUES ('eight') ON CONFLICT DO NOTHING;  -- no-op
 
 SELECT * FROM UNIQUE_TBL;