func (it *dbIter) Next(fn kv.FnKeyCmp) (kv.Iterator, error) {
	encKey := codec.EncodeBytes(nil, it.startKey)
	// max key
	encEndKey := codec.EncodeBytes(nil, []byte{0xff, 0xff})
	var retErr error
	var engineIter engine.Iterator
	for {
		engineIter, retErr = it.s.db.Seek(encKey)
		if retErr != nil {
			return nil, errors.Trace(retErr)
		}
		// Check if overflow
		if !engineIter.Next() {
			it.valid = false
			break
		}

		metaKey := engineIter.Key()
		// Check if meet the end of table.
		if bytes.Compare(metaKey, encEndKey) >= 0 {
			it.valid = false
			break
		}
		// Get real key from metaKey
		key, _, err := MvccDecode(metaKey)
		if err != nil {
			// It's not a valid metaKey, maybe overflow (other data).
			it.valid = false
			break
		}
		// Get kv pair.
		val, err := it.s.MvccGet(key, it.exceptedVersion)
		if err != nil && !errors2.ErrorEqual(err, kv.ErrNotExist) {
			// Get this version error
			it.valid = false
			retErr = err
			break
		}
		if val != nil {
			it.k = key
			it.v = val
			it.startKey = key.Next()
			break
		}
		// Release the iterator, and update key
		engineIter.Release()
		// Current key's all versions are deleted, just go next key.
		encKey = codec.EncodeBytes(nil, key.Next())
	}
	engineIter.Release()
	return it, errors.Trace(retErr)
}

func (it *dbIter) Next() (kv.Iterator, error) {
	encKey := codec.EncodeBytes(nil, it.startKey)
	var retErr error
	var engineIter engine.Iterator
	for {
		var err error
		engineIter, err = it.s.internalSeek(encKey)
		if err != nil {
			it.valid = false
			retErr = err
			break
		}

		metaKey := engineIter.Key()
		// Get real key from metaKey
		key, _, err := MvccDecode(metaKey)
		if err != nil {
			// It's not a valid metaKey, maybe overflow (other data).
			it.valid = false
			break
		}
		// Get kv pair.
		val, err := it.s.MvccGet(key, it.exceptedVersion)
		if err != nil && !errors2.ErrorEqual(err, kv.ErrNotExist) {
			// Get this version error
			it.valid = false
			retErr = err
			break
		}
		if val != nil {
			it.k = bytes.CloneBytes(key)
			it.v = bytes.CloneBytes(val)
			it.startKey = key.Next()
			break
		}
		// Current key's all versions are deleted, just go next key.
		encKey = codec.EncodeBytes(nil, key.Next())
	}
	return it, errors.Trace(retErr)
}

func newDBIter(it engine.Iterator) *dbIter {
	return &dbIter{
		Iterator: it,
		valid:    it.Next(),
	}
}