Went searching and didn't find the answer to this specific noob question. My apologies if I missed it.

In a MySQL database I have a table with the following primary key

PRIMARY KEY id (invoice, item)

In my application I will also frequently be selecting on "item" by itself and less frequently on only "invoice". I'm assuming I would benefit from indexes on these columns.

MySQL does not complain when I define the following:

INDEX (invoice), INDEX (item), PRIMARY KEY id (invoice, item)

But I don't see any evidence (using DESCRIBE -- the only way I know how to look) that separate indexes have been established for these two columns.

So the question is, are the columns that make up a primary key automatically indexed individually? Also, is there a better way than DESCRIBE to explore the structure of my table?