distsql: fix a bug of ranger.Range to kv.KeyRange conversion (#40204)

close #40158

distsql/request_builder.go

@@ -782,20 +782,5 @@ func EncodeIndexKey(sc *stmtctx.StatementContext, ran *ranger.Range) ([]byte, []
 	if !ran.HighExclude {
 		high = kv.Key(high).PrefixNext()
 	}
-
-	var hasNull bool
-	for _, highVal := range ran.HighVal {
-		if highVal.IsNull() {
-			hasNull = true
-			break
-		}
-	}
-
-	// NOTE: this is a hard-code operation to avoid wrong results when accessing unique index with NULL;
-	// Please see https://github.com/pingcap/tidb/issues/29650 for more details
-	if hasNull {
-		// Append 0 to make unique-key range [null, null] to be a scan rather than point-get.
-		high = kv.Key(high).Next()
-	}
 	return low, high, nil
 }

executor/issuetest/executor_issue_test.go

@@ -1339,3 +1339,14 @@ PARTITION BY LIST COLUMNS(c_int)
 	tk.MustExec("delete t1, t2 from t1, t2 where t1.c_enum in ('blue');")
 	tk.MustExec("commit")
 }
+
+func TestIssue40158(t *testing.T) {
+	store := testkit.CreateMockStore(t)
+	tk := testkit.NewTestKit(t, store)
+
+	tk.MustExec("use test")
+	tk.MustExec("drop table if exists t1")
+	tk.MustExec("create table t1 (_id int PRIMARY KEY, c1 char, index (c1));")
+	tk.MustExec("insert into t1 values (1, null);")
+	tk.MustQuery("select * from t1 where c1 is null and _id < 1;").Check(testkit.Rows())
+}