fix a bug that nullable unique index can not find null values using i…

…ndex. (pingcap#7163)

distsql/request_builder.go

@@ -306,8 +306,22 @@ func encodeIndexKey(sc *stmtctx.StatementContext, ran *ranger.NewRange) ([]byte,
 	if err != nil {
 		return nil, nil, errors.Trace(err)
 	}
+
 	if !ran.HighExclude {
 		high = []byte(kv.Key(high).PrefixNext())
 	}
+
+	var hasNull bool
+	for _, highVal := range ran.HighVal {
+		if highVal.IsNull() {
+			hasNull = true
+			break
+		}
+	}
+
+	if hasNull {
+		// Append 0 to make unique-key range [null, null] to be a scan rather than point-get.
+		high = []byte(kv.Key(high).Next())
+	}
 	return low, high, nil
 }

executor/distsql_test.go

@@ -167,3 +167,28 @@ func (s *testSuite) TestBigIntPK(c *C) {
 	tk.MustExec("insert into t values(1, 1, 1), (9223372036854775807, 2, 2)")
 	tk.MustQuery("select * from t use index(idx) order by a").Check(testkit.Rows("1 1 1", "9223372036854775807 2 2"))
 }
+
+func (s *testSuite) TestUniqueKeyNullValueSelect(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec("use test")
+	tk.MustExec("drop table if exists t")
+	// test null in unique-key
+	tk.MustExec("create table t (id int default null, c varchar(20), unique id (id));")
+	tk.MustExec("insert t (c) values ('a'), ('b'), ('c');")
+	res := tk.MustQuery("select * from t where id is null;")
+	res.Check(testkit.Rows("<nil> a", "<nil> b", "<nil> c"))
+
+	// test null in mul unique-key
+	tk.MustExec("drop table t")
+	tk.MustExec("create table t (id int default null, b int default 1, c varchar(20), unique id_c(id, b));")
+	tk.MustExec("insert t (c) values ('a'), ('b'), ('c');")
+	res = tk.MustQuery("select * from t where id is null and b = 1;")
+	res.Check(testkit.Rows("<nil> 1 a", "<nil> 1 b", "<nil> 1 c"))
+
+	tk.MustExec("drop table t")
+	// test null in non-unique-key
+	tk.MustExec("create table t (id int default null, c varchar(20), key id (id));")
+	tk.MustExec("insert t (c) values ('a'), ('b'), ('c');")
+	res = tk.MustQuery("select * from t where id is null;")
+	res.Check(testkit.Rows("<nil> a", "<nil> b", "<nil> c"))
+}

util/ranger/types.go

@@ -94,6 +94,10 @@ func (ran *NewRange) IsPoint(sc *stmtctx.StatementContext) bool {
 		if cmp != 0 {
 			return false
 		}
+
+		if a.IsNull() {
+			return false
+		}
 	}
 	return !ran.LowExclude && !ran.HighExclude
 }