Does a foreign key automatically create an index?

Question

I've been told that if I foreign key two tables, that SQL Server will create something akin to an index in the child table. I have a hard time believing this to be true, but can't find much out there related specifically to this.

My real reason for asking this is because we're experiencing some very slow response time in a delete statement against a table that has probably 15 related tables. I've asked our database guy and he says that if there is a foreign key on the fields, then it acts like an index. What is your experience with this? Should I add indexes on all foreign key fields or are they just unnecessary overhead?

Answer 1

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

But it is a known fact that it makes a lot of sense to index all the columns that are part of any foreign key relationship, because through a FK-relationship, you'll often need to lookup 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 a FK, but a 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?".

Answer 2

Wow, the answers are all over the map. So the Documentation says:

A FOREIGN KEY constraint is a candidate for an index because:

• Changes to PRIMARY KEY constraints are checked with FOREIGN KEY constraints in related tables.

• Foreign key columns are often used in join criteria when the data from related tables is combined in queries by matching the column(s) in the FOREIGN KEY constraint of one table with the primary or unique key column(s) in the other table. An index allows Microsoft® SQL Server™ 2000 to find related data in the foreign key table quickly. However, creating this index is not a requirement. Data from two related tables can be combined even if no PRIMARY KEY or FOREIGN KEY constraints are defined between the tables, but a foreign key relationship between two tables indicates that the two tables have been optimized to be combined in a query that uses the keys as its criteria.

So it seems pretty clear (although the documentation is a bit muddled) that it does not in fact create an index.