Mysql 5.7 Online DDL varchar column size변경
https://dev.mysql.com/doc/refman/5.7/en/innodb-online-ddl-operations.html
MySQL :: MySQL 5.7 Reference Manual :: 14.13.1 Online DDL Operations
14.13.1 Online DDL Operations Online support details, syntax examples, and usage notes for DDL operations are provided under the following topics in this section. The following table provides an overview of online DDL support for index operations. An aste
dev.mysql.com
The number of length bytes required by a VARCHAR column must remain the same. For VARCHAR columns of 0 to 255 bytes in size, one length byte is required to encode the value. For VARCHAR columns of 256 bytes in size or more, two length bytes are required. As a result, in-place ALTER TABLE only supports increasing VARCHAR column size from 0 to 255 bytes, or from 256 bytes to a greater size. In-place ALTER TABLE does not support increasing the size of a VARCHAR column from less than 256 bytes to a size equal to or greater than 256 bytes. In this case, the number of required length bytes changes from 1 to 2, which is only supported by a table copy (ALGORITHM=COPY). For example, attempting to change VARCHAR column size for a single byte character set from VARCHAR(255) to VARCHAR(256) using in-place ALTER TABLE returns this error:
칼럼길이가 256 byte 미만에서만 ALGORITHM=INPLACE 가능
그러나 utf8mb4 타입은 문자 1개당 4byte를 사용하므로 VARCHAR(63) 까지만 가능
