diff --git a/expression/distsql_builtin.go b/expression/distsql_builtin.go
index b81719754f..3cc43212cb 100644
--- a/expression/distsql_builtin.go
+++ b/expression/distsql_builtin.go
@@ -684,10 +684,15 @@ func convertFloat(val []byte, f32 bool) (*Constant, error) {
 
 func convertDecimal(val []byte) (*Constant, error) {
 	_, dec, err := codec.DecodeDecimal(val)
+	var d types.Datum
+	precision, frac := dec.PrecisionAndFrac()
+	d.SetMysqlDecimal(dec)
+	d.SetLength(precision)
+	d.SetFrac(frac)
 	if err != nil {
 		return nil, errors.Errorf("invalid decimal % x", val)
 	}
-	return &Constant{Value: dec, RetType: types.NewFieldType(mysql.TypeNewDecimal)}, nil
+	return &Constant{Value: d, RetType: types.NewFieldType(mysql.TypeNewDecimal)}, nil
 }
 
 func convertDuration(val []byte) (*Constant, error) {
diff --git a/types/mydecimal.go b/types/mydecimal.go
index aec5395fe8..2219465afc 100644
--- a/types/mydecimal.go
+++ b/types/mydecimal.go
@@ -1235,7 +1235,8 @@ func (d *MyDecimal) FromBin(bin []byte, precision, frac int) (binSize int, err e
 		mask = 0
 	}
 	binSize = decimalBinSize(precision, frac)
-	dCopy := make([]byte, binSize)
+	dCopy := make([]byte, 40)
+	dCopy = dCopy[:binSize]
 	copy(dCopy, bin)
 	dCopy[0] ^= 0x80
 	bin = dCopy
diff --git a/util/codec/bench_test.go b/util/codec/bench_test.go
index 97e56b199c..c3fda693b6 100644
--- a/util/codec/bench_test.go
+++ b/util/codec/bench_test.go
@@ -60,3 +60,14 @@ func BenchmarkEncodeIntWithOutSize(b *testing.B) {
 		EncodeInt(nil, 10)
 	}
 }
+
+func BenchmarkDecodeDecimal(b *testing.B) {
+	dec := &types.MyDecimal{}
+	dec.FromFloat64(1211.1211113)
+	precision, frac := dec.PrecisionAndFrac()
+	raw := EncodeDecimal([]byte{}, dec, precision, frac)
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		DecodeDecimal(raw)
+	}
+}
diff --git a/util/codec/codec.go b/util/codec/codec.go
index 504d9cca2a..9dc92656ca 100644
--- a/util/codec/codec.go
+++ b/util/codec/codec.go
@@ -334,7 +334,12 @@ func DecodeOne(b []byte) (remain []byte, d types.Datum, err error) {
 		b, v, err = DecodeCompactBytes(b)
 		d.SetBytes(v)
 	case decimalFlag:
-		b, d, err = DecodeDecimal(b)
+		var dec *types.MyDecimal
+		b, dec, err = DecodeDecimal(b)
+		precision, frac := dec.PrecisionAndFrac()
+		d.SetMysqlDecimal(dec)
+		d.SetLength(precision)
+		d.SetFrac(frac)
 	case durationFlag:
 		var r int64
 		b, r, err = DecodeInt(b)
@@ -534,12 +539,12 @@ func DecodeOneToChunk(b []byte, chk *chunk.Chunk, colIdx int, ft *types.FieldTyp
 		}
 		chk.AppendBytes(colIdx, v)
 	case decimalFlag:
-		var d types.Datum
-		b, d, err = DecodeDecimal(b)
+		var dec *types.MyDecimal
+		b, dec, err = DecodeDecimal(b)
 		if err != nil {
 			return nil, errors.Trace(err)
 		}
-		chk.AppendMyDecimal(colIdx, d.GetMysqlDecimal())
+		chk.AppendMyDecimal(colIdx, dec)
 	case durationFlag:
 		var r int64
 		b, r, err = DecodeInt(b)
diff --git a/util/codec/decimal.go b/util/codec/decimal.go
index 2368e596a8..1ccdabae1d 100644
--- a/util/codec/decimal.go
+++ b/util/codec/decimal.go
@@ -35,10 +35,9 @@ func EncodeDecimal(b []byte, dec *types.MyDecimal, precision, frac int) []byte {
 }
 
 // DecodeDecimal decodes bytes to decimal.
-func DecodeDecimal(b []byte) ([]byte, types.Datum, error) {
-	var d types.Datum
+func DecodeDecimal(b []byte) ([]byte, *types.MyDecimal, error) {
 	if len(b) < 3 {
-		return b, d, errors.New("insufficient bytes to decode value")
+		return b, nil, errors.New("insufficient bytes to decode value")
 	}
 	precision := int(b[0])
 	frac := int(b[1])
@@ -47,10 +46,7 @@ func DecodeDecimal(b []byte) ([]byte, types.Datum, error) {
 	binSize, err := dec.FromBin(b, precision, frac)
 	b = b[binSize:]
 	if err != nil {
-		return b, d, errors.Trace(err)
+		return b, nil, errors.Trace(err)
 	}
-	d.SetLength(precision)
-	d.SetFrac(frac)
-	d.SetMysqlDecimal(dec)
-	return b, d, nil
+	return b, dec, nil
 }
diff --git a/util/codec/decimal_test.go b/util/codec/decimal_test.go
index 59eebf7650..64d27d4c1f 100644
--- a/util/codec/decimal_test.go
+++ b/util/codec/decimal_test.go
@@ -15,7 +15,6 @@ package codec
 
 import (
 	. "github.com/pingcap/check"
-	"github.com/pingcap/tidb/sessionctx/stmtctx"
 	"github.com/pingcap/tidb/types"
 	"github.com/pingcap/tidb/util/testleak"
 )
@@ -51,7 +50,7 @@ func (s *testDecimalSuite) TestDecimalCodec(c *C) {
 		b := EncodeDecimal([]byte{}, datum.GetMysqlDecimal(), datum.Length(), datum.Frac())
 		_, d, err := DecodeDecimal(b)
 		c.Assert(err, IsNil)
-		c.Assert(v.Compare(d.GetMysqlDecimal()), Equals, 0)
+		c.Assert(v.Compare(d), Equals, 0)
 	}
 }
 
@@ -72,11 +71,7 @@ func testFrac(c *C, v *types.MyDecimal) {
 	var d1 types.Datum
 	d1.SetMysqlDecimal(v)
 	b := EncodeDecimal([]byte{}, d1.GetMysqlDecimal(), d1.Length(), d1.Frac())
-	_, d2, err := DecodeDecimal(b)
+	_, dec, err := DecodeDecimal(b)
 	c.Assert(err, IsNil)
-	sc := new(stmtctx.StatementContext)
-	cmp, err := d1.CompareDatum(sc, &d2)
-	c.Assert(err, IsNil)
-	c.Assert(cmp, Equals, 0)
-	c.Assert(d1.GetMysqlDecimal().String(), Equals, d2.GetMysqlDecimal().String())
+	c.Assert(dec.String(), Equals, v.String())
 }