问题
Is it possible to add a constraint like
ALTER TABLE `t1` ADD UNIQUE(`col1`, `col2`);
without creating an index? The index wouldn't be used for any queries so it would be a waste of space.
It wouldn't be a problem if inserts and updates would be way slower, because the table doesn't get updated very often.
回答1:
No, this is not possible. A UNIQUE constraint contains an index definition and I barely imagine how it might be implemented without creating an index (in DBMS terms).
You should realize that indexes are not just 'wizardy' - they are a real data structure, which takes space to be placed, special procedures to be handled e.t.c. A unique constraint, itself, means unique index values, not unique column values.
来源：https://stackoverflow.com/questions/19907766/mysql-unique-constraint-without-index