MySQL foreign key question

Question

Does defining a foreign key also defines a index? I have mysql v5.1.46 & I am looking at the MySQL Administrator tool and its shows the foreign key as an index, so I wanted to confirm?

Answer 1

• If there already is a usable index (an index where the foreign key columns are listed as the first columns in the same order) then a new index is not created.
• If there is no usable index then creating a foreign key also creates an index.

This is covered in the documentation.

InnoDB requires indexes on foreign keys and referenced keys so that foreign key checks can be fast and not require a table scan. In the referencing table, there must be an index where the foreign key columns are listed as the first columns in the same order. Such an index is created on the referencing table automatically if it does not exist. (This is in contrast to some older versions, in which indexes had to be created explicitly or the creation of foreign key constraints would fail.) index_name, if given, is used as described previously.