Does a foreign key automatically create an index?

It depends. On MySQL an index is created if you don't create it on your own:
>MySQL requires that foreign key columns be indexed; if you create a table with a foreign key constraint but no index on a given column, an index is created.

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The same for MySQL 5.6 eh.

A foreign key is a constraint, a relationship between two tables - that has nothing to do with an index per se.

However, it makes a lot of sense to index all the columns that are part of any foreign key relationship. An FK-relationship will often need to look up a relating table and extract certain rows based on a single value or a range of values.

So it makes good sense to index any columns involved in an FK, but an FK per se is not an index.

Check out Kimberly Tripp's excellent article ["When did SQL Server stop putting indexes on Foreign Key columns?"][1].

**Updated 5/31/2023**
For those reading this now, I (the original poster) wanted to update Marc's excellent answer to point out that Entity Framework Core [now auto generates indexes][2] for foreign keys discovered by convention. Most helpful. Not sure how far back that feature exists, I'm using EF Core 7.


[1]:

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[2]:

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