关于数据类型

txwdhs

今天修改一个varchar（20000）的字段为varchar（60000），发现数据类型自动变为了mediumtext，请问mediumtext和varchar有什么区别么？我这样修改了会不会有错？
因为之前varchar（20000）满足不了业务需求，所以需要增加长度

txwdhs

请问大家没有对这个有了解的么

horizonhyg

Every table (regardless of storage engine) has a maximum row size of 65,535 bytes. Storage engines may place additional constraints on this limit, reducing the effective maximum row size.

The maximum row size constrains the number (and possibly size) of columns because the total length of all columns cannot exceed this size. For example, utf8 characters require up to three bytes per character, so for a CHAR(255) CHARACTER SET utf8 column, the server must allocate 255 × 3 = 765 bytes per value. Consequently, a table cannot contain more than 65,535 / 765 = 85 such columns.

Storage for variable-length columns includes length bytes, which are assessed against the row size. For example, a VARCHAR(255) CHARACTER SET utf8 column takes two bytes to store the length of the value, so each value can take up to 767 bytes.

BLOB and TEXT columns count from one to four plus eight bytes each toward the row-size limit because their contents are stored separately from the rest of the row.

超过行的长度了，TEXT是另外在别的地方存储的，不会占用行长度。