It would be useful if EXPLAIN supported ALTER TABLE and/or other DDL statements.

For example, something like this:

EXPLAIN ALTER TABLE tab ADD COLUMN c varchar(50), ALGORITHM=INPLACE;

And probably also something like this:

EXPLAIN EXTENDED ALTER TABLE tab ADD COLUMN c varchar(50), ALGORITHM=INPLACE;

The output should probably include:

• What algorithm will be used (i.e. COPY, INPLACE, NOCOPY, INSTANT).
• Whether the table will be rebuilt.
• Whether the table will be locked.
• If the table is locked, what kind of lock will be used.
• Whether the table's ROW_FORMAT will be changed.
• For InnoDB, whether the table will use the non-canonical format that is required to implement some instant alter operations.
• For InnoDB, whether the table will be in the system tablespace or in a file-per-table tablespace.

The more basic pieces of information could probably be output by EXPLAIN, and the more detailed pieces of information could probably be output by EXPLAIN EXTENDED.