Speed up decoding column id (#10188)

* codec: speed up decoding column id by removing a redundant decoding of varint

* add a comment for skipped flag

tablecodec/tablecodec.go

@@ -372,25 +372,23 @@ func CutRowNew(data []byte, colIDs map[int64]int) ([][]byte, error) {
 		cnt int
 		b   []byte
 		err error
+		cid int64
 	)
 	row := make([][]byte, len(colIDs))
 	for len(data) > 0 && cnt < len(colIDs) {
 		// Get col id.
-		b, data, err = codec.CutOne(data)
-		if err != nil {
-			return nil, errors.Trace(err)
-		}
-		_, cid, err := codec.DecodeOne(b)
+		data, cid, err = codec.CutColumnID(data)
 		if err != nil {
 			return nil, errors.Trace(err)
 		}
+
 		// Get col value.
 		b, data, err = codec.CutOne(data)
 		if err != nil {
 			return nil, errors.Trace(err)
 		}
-		id := cid.GetInt64()
-		offset, ok := colIDs[id]
+
+		offset, ok := colIDs[cid]
 		if ok {
 			row[offset] = b
 			cnt++

util/codec/codec.go

@@ -429,6 +429,17 @@ func CutOne(b []byte) (data []byte, remain []byte, err error) {
 	return b[:l], b[l:], nil
 }
 
+// CutColumnID cuts the column ID from b.
+// It will return the remains as byte slice and column ID
+func CutColumnID(b []byte) (remain []byte, n int64, err error) {
+	if len(b) < 1 {
+		return nil, 0, errors.New("invalid encoded key")
+	}
+	// skip the flag
+	b = b[1:]
+	return DecodeVarint(b)
+}
+
 // SetRawValues set raw datum values from a row data.
 func SetRawValues(data []byte, values []types.Datum) error {
 	for i := 0; i < len(values); i++ {