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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.
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Under the hood a unique constraint is implemented the same way as a unique index - an index is needed to efficiently fulfill the requirement to enforce the constraint.
PRIMARY KEY or UNIQUE constraint When you create a PRIMARY KEY constraint, a unique clustered index on the column or columns is automatically created if a clustered index on the table does not already exist and you do not specify a unique nonclustered index.
To answer to question in bold: Yes, making a field unique does index it like s primary key.
If we use a UNIQUE constraint in the table, MySQL automatically creates a UNIQUE index behind the scenes. The following statement explains how to create a unique constraint when we create a table.
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.
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