executor: fix firstrow/max/min(bit col) error

executor/aggfuncs/builder.go

@@ -176,6 +176,9 @@ func buildFirstRow(aggFuncDesc *aggregation.AggFuncDesc, ordinal int) AggFunc {
 	}
 
 	evalType, fieldType := aggFuncDesc.RetTp.EvalType(), aggFuncDesc.RetTp
+	if fieldType.Tp == mysql.TypeBit {
+		evalType = types.ETString
+	}
 	switch aggFuncDesc.Mode {
 	case aggregation.DedupMode:
 	default:
@@ -215,6 +218,9 @@ func buildMaxMin(aggFuncDesc *aggregation.AggFuncDesc, ordinal int, isMax bool)
 	}
 
 	evalType, fieldType := aggFuncDesc.RetTp.EvalType(), aggFuncDesc.RetTp
+	if fieldType.Tp == mysql.TypeBit {
+		evalType = types.ETString
+	}
 	switch aggFuncDesc.Mode {
 	case aggregation.DedupMode:
 	default:

executor/aggregate_test.go

@@ -309,6 +309,17 @@ func (s *testSuite) TestAggregation(c *C) {
 	result = tk.MustQuery("select t.id, count(95), sum(95), avg(95), bit_or(95), bit_and(95), bit_or(95), max(95), min(95), group_concat(95) from t left join s on t.id = s.id")
 	result.Check(testkit.Rows("1 1 95 95.0000 95 95 95 95 95 95"))
 	tk.MustExec("set @@tidb_hash_join_concurrency=5")
+
+	// test agg bit col
+	tk.MustExec("drop table t")
+	tk.MustExec("CREATE TABLE `t` (`a` bit(1) NOT NULL, PRIMARY KEY (`a`))")
+	tk.MustExec("insert into t value(1), (0)")
+	tk.MustQuery("select a from t group by 1")
+	// This result is compatible with MySQL, the readable result is shown in the next case.
+	result = tk.MustQuery("select max(a) from t group by a")
+	result.Check(testkit.Rows(string([]byte{0x0}), string([]byte{0x1})))
+	result = tk.MustQuery("select cast(a as signed) as idx, cast(max(a) as signed),  cast(min(a) as signed) from t group by 1 order by idx")
+	result.Check(testkit.Rows("0 0 0", "1 1 1"))
 }
 
 func (s *testSuite) TestStreamAggPushDown(c *C) {