问题描述

mysql 5.6innodb compac的vachar长度标记位1还是2byte的问题：

For each non-NULL variable-length field, the record header contains the length of the column in one or two bytes. Two bytes will only be needed if part of the column is stored externally in overflow pages or the maximum length exceeds 255 bytes and the actual length exceeds 127 bytes. For an externally stored column, the 2-byte length indicates the length of the internally stored part plus the 20-byte pointer to the externally stored part. The internal part is 768 bytes, so the length is 768+20. The 20-byte pointer stores the true length of the column.

5.6文档上有如上的一句话，文档上说当最大长度超过255字节和实际存储超过127字节的时候，使用2Byte标识，是什么意思呢？那如果最大长度超过255,实际存储小于127byte呢？