DDL 中的 first和aflter 是走COPY算法不?

有效信息太少了，没法判断。
关于online ddl的文档可以先翻阅 https://dev.mysql.com/doc/refman/8.0/en/innodb-online-ddl.html

Reordering columns

To reorder columns, use FIRST or AFTER in CHANGE or MODIFY operations.

ALTER TABLE tbl_name MODIFY COLUMN col_name column_definition FIRST, ALGORITHM=INPLACE, LOCK=NONE;
Data is reorganized substantially, making it an expensive operation.