func (db *DB) sizeFromTag(field *reflect.StructField) (size uint64, err error) {
	if s := field.Tag.Get(dbSizeTag); s != "" {
		size, err = strconv.ParseUint(s, 10, 64)
	}
	return size, err
}
func (db *DB) sizeFromTag(field *reflect.StructField) (size uint64, err error) {
	if s := field.Tag.Get(dbSizeTag); s != "" {
		size, err = strconv.ParseUint(s, 10, 64)
	}
	return size, err
}
// sizeFromTag returns a size from tag.
// If "size" tag specified to struct field, it will converted to uint64 and returns it.
// If it doesn't specify, it returns 0.
// If value of "size" tag cannot convert to uint64, it returns 0 and error.