diff --git a/planner/core/expression_rewriter_test.go b/planner/core/expression_rewriter_test.go
index 35e8776ce3825..a2253bf75d154 100644
--- a/planner/core/expression_rewriter_test.go
+++ b/planner/core/expression_rewriter_test.go
@@ -224,6 +224,15 @@ func (s *testExpressionRewriterSuite) TestCompareSubquery(c *C) {
 		"<nil> 2",
 	))
 	tk.MustQuery("select * from t t1 where b = all (select a from t t2)").Check(testkit.Rows())
+
+	// for issue 20059
+	tk.MustExec("DROP TABLE IF EXISTS `t`")
+	tk.MustExec("CREATE TABLE `t` (  `a` int(11) DEFAULT NULL) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin;")
+	tk.MustExec("INSERT INTO `t` VALUES (1);")
+	tk.MustExec("DROP TABLE IF EXISTS `table_40_utf8_4`;")
+	tk.MustExec("CREATE TABLE `table_40_utf8_4` (`col_tinyint_key_unsigned` tinyint(4) DEFAULT NULL,  `col_bit64_key_signed` bit(64) DEFAULT NULL) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_bin;")
+	tk.MustExec("INSERT INTO `table_40_utf8_4` VALUES (31, -18);")
+	tk.MustQuery("select count(1) from table_40_utf8_4 where ( select count(1) from t where table_40_utf8_4.col_bit64_key_signed!=table_40_utf8_4.col_tinyint_key_unsigned)").Check(testkit.Rows("1"))
 }
 
 func (s *testExpressionRewriterSuite) TestCheckFullGroupBy(c *C) {
diff --git a/types/datum.go b/types/datum.go
index 0405ca6b3877d..2a45e28fe7ecf 100644
--- a/types/datum.go
+++ b/types/datum.go
@@ -1578,6 +1578,11 @@ func (d *Datum) ToDecimal(sc *stmtctx.StatementContext) (*MyDecimal, error) {
 
 // ToInt64 converts to a int64.
 func (d *Datum) ToInt64(sc *stmtctx.StatementContext) (int64, error) {
+	switch d.Kind() {
+	case KindMysqlBit:
+		uintVal, err := d.GetBinaryLiteral().ToInt(sc)
+		return int64(uintVal), err
+	}
 	return d.toSignedInteger(sc, mysql.TypeLonglong)
 }