How does MySQL determine if an INSERT is unique?

Question

I would like to know if there is an implicit SELECT being run prior to performing an INSERT on a table that has any column defined as UNIQUE. I cannot find anything about this i

Answer 1

This is the very purpose of a UNIQUE constraint:

A UNIQUE index creates a constraint such that all values in the index must be distinct. An error occurs if you try to add a new row [or update an existing row] with a key value that matches [another] existing row.

Earlier in the same manual page, it is stated that

A column list of the form (col1,col2,...) creates a multiple-column index. Index key values are formed by concatenating the values of the given columns.

How this constraint is implemented is not documented, but it must somehow equate to a preliminary SELECT with the values to be inserted/updated. The cost of such a check is often negligible, because, by definition, the fields are indexed (this overhead becomes relevant when dealing with bulk inserts).

The number of columns covered by the index is not meaningful in terms of performance (for example, compared to the number of rows in the table). It does impact the disk space occupied by the index, but this should really not matter in your design decisions.