Is the foreign key automatically indexed in MySQL?
Foreign keys are not automatically indexed in MySQL*.  If you should index, the answer is always: &quot;It depends.&quot;
If you are going to primarily search based on that column, it's more than likely it should form the first part of that table's primary key and there would be no need for an additional index.
If the values of that column are not particularly selective, adding an index won't improve performance as a table scan will still be required.
So as always, test and validate.
*This depends on where you are expecting the foreign key to be created, on the table with the FK constraint or the table referenced by that constraint.  Most RDBMs follow the relational model, you can only create a FK reference to a primary or candidate key in another table (in practical terms an index would already exist).  MySQL does not have this restriction and will automatically create an index on the referenced column (or use an existing index if the column(s) referenced comprise(s) the start of that index).