Table Codec – 编写tinysql的解码器

1. 编码规则

整体上的规则是一个信息前缀+信息ID

设置的前缀有

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var (
	tablePrefix     = []byte{'t'}
	recordPrefixSep = []byte("_r")
	indexPrefixSep  = []byte("_i")
)

相关信息和前缀的长度

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const (
	idLen     = 8
	prefixLen = 1 + idLen /*tableID*/ + 2
	// RecordRowKeyLen is public for calculating average row size.
	RecordRowKeyLen       = prefixLen + idLen /*handle*/
	tablePrefixLength     = 1
	recordPrefixSepLength = 2
)
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t[table ID]_r[rowID]
得到的是这一行的数据 
t[table ID]_i[indexID]_indexColval 
得到的是行的id
t[table ID]_i[indexID]_ColVal_rowID 
唯一编码

DecodeRecordKey

对含有tableID的

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func DecodeRecordKey(key kv.Key) (tableID int64, handle int64, err error) {
	/* Project 1-2: your code here
	 * DecodeRecordKey decodes the key and gets the tableID, handle.
	 * Decode is actually the reverse of encoding, so you can refer to EncodeRowKeyWithHandle.
	 *
	 * Parameters
	 *   key: the key needs to be decoded. It may be invalid.
	 *
	 * Return value
	 *   tableID: decoded value.
	 *   handle:  decoded value.
	 *   err:     error during decoding, otherwise nil.
	 *
	 * DecodeRecordKey may need to follow the steps:
	 *   1. check if the key is valid.
	 *   2. get the table prefix and decode.
	 *   3. get the record prefix and decode.
	 *   4. create handle.
	 *   5. return decoded value.
	 *
	 * Some hints:
	 *   1. you may need codec.DecodeInt to decode string to int
	 *   2. const `prefixLen`, `tablePrefixLength` and `recordPrefixSepLength` are useful.
	 *   3. errInvalidRecordKey.GenWithStack is a useful function to generate invalid record key errors.
	 *   4. if an error occurs, return 0 as tableID, nil as handle and the error.
	 *   5. understanding the coding rules is a prerequisite for implementing this function,
	 *      you can learn it in the projection 1-2 course documentation.
	 */
	// t[tableID]_i
	// 后缀:_i
	// len: RecordRowKeyLen
	// 1. 查看合法性
	error_return := func() (tableID int64, handle int64, err error) {
		return 0, 0, errInvalidRecordKey.GenWithStack("Invaild Record Key- %q", key)

	}

	if len(key) < RecordRowKeyLen || key[0] != tablePrefix[0] {
		return error_return()
	}

	// 减去t
	key = key[tablePrefixLength:]

	// 2. 进行解码
	key, tableID, err = codec.DecodeInt(key)

	// 判断解码是否成功
	if err != nil {
		return 0, 0, errors.Trace(err)
	}

	// 判断前缀部位recordPrefixsep
	if !key.HasPrefix(recordPrefixSep) {
		return error_return()
	}
	key = key[recordPrefixSepLength:]
	key, handle, err = codec.DecodeInt(key)

	if err != nil {
		return 0, 0, errors.Trace(err)
	}
	return tableddddID, handle, nil 
}

DecodeIndexKeyPrefix

和上面的是相似的处理方式，去除前缀，解码

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func DecodeIndexKeyPrefix(key kv.Key) (tableID int64, indexID int64, indexValues []byte, err error) {
	/* Project 1-2: your code here
	 * DecodeIndexKeyPrefix decodes the key and gets the tableID, indexID, indexValues.
	 * Decode is actually the reverse of encoding, so you can refer to EncodeIndexSeekKey.
	 *
	 * Parameters
	 *   key: the key needs to be decoded. It may be invalid.
	 *
	 * Return value
	 *   tableID:     decoded value.
	 *   indexID:     decoded value.
	 *   indexValues: decoded value.
	 *   err:         error during decoding, otherwise nil.
	 *
	 * DecodeRecordKey may need to follow the steps:
	 *   1. check if the key is valid.
	 *   2. decode tableID.
	 *   3. decode indexID.
	 *   4. decode indexValues.
	 *   5. return decoded value.
	 *
	 * Some hints:
	 *   1. you may need codec.DecodeInt to decode string to int
	 *   2. const `prefixLen`, `idLen` and others are useful.
	 *   3. errInvalidRecordKey.GenWithStack is a useful function to generate invalid record key errors.
	 *   4. if an error occurs, return 0 as tableID, 0 as indexID, nil as indexValues and the error.
	 *   5. understanding the coding rules is a prerequisite for implementing this function,
	 *      you can learn it in the projection 1-2 course documentation.
	 */
	// 错误返回
	error_return := func() (table int64, indexID int64, indexValues []byte, err error) {
		return 0, 0, nil, errInvalidIndexKey.GenWithStack("invalid index key - %q", key)
	}

	// 判断长度和开头的格式
	if len(key) < RecordRowKeyLen || key[0] != 't' {
		return error_return()
	}

	//去除开头的t
	key = key[tablePrefixLength:]

	// 进行解码得到tableID
	key, tableID, err = codec.DecodeInt(key)
	if err != nil {
		return 0, 0, nil, errors.Trace(err)
	}

	// 判断是否存储index前缀
	if !key.HasPrefix(indexPrefixSep) {
		return error_return()
	}

	// 去除前缀
	key = key[recordPrefixSepLength:]

	// 进行解码
	key, indexID, err = codec.DecodeInt(key)

	// 是否解码发生错误
	if err != nil {
		return 0, 0, nil, errors.Trace(err)
	}
	return tableID, indexID, key, nil

}