CODEWITHSUNDEEP
sql-server
Nick DeVore
Nick DeVore

Reputation: 10176

Does a foreign key automatically create an index?

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?

Upvotes: 497

Views: 195374

Answers (11)

marc_s
marc_s

Reputation: 755197

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?".

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 for foreign keys discovered by convention. Most helpful. Not sure how far back that feature exists, I'm using EF Core 7.

Upvotes: 443

Lorenzo Franco Ranucci
Lorenzo Franco Ranucci

Reputation: 81

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.

Source: https://dev.mysql.com/doc/refman/8.0/en/constraint-foreign-key.html

The same for MySQL 5.6 eh.

Upvotes: 6

Luke Puplett
Luke Puplett

Reputation: 45253

I notice that Entity Framework 6.1 pointed at MSSQL does automatically add indexes on foreign keys.

Upvotes: 2

Sandeep Kanuri
Sandeep Kanuri

Reputation: 151

Foreign keys do not create indexes. Only alternate key constraints(UNIQUE) and primary key constraints create indexes. This is true in Oracle and SQL Server.

Upvotes: 15

Gregor
Gregor

Reputation: 622

In PostgeSql you can check for indexes yourself if you hit \d tablename

You will see that btree indexes have been automatically created on columns with primary key and unique constraints, but not on columns with foreign keys.

I think that answers your question at least for postgres.

Upvotes: 11

Paul Sonier
Paul Sonier

Reputation: 39510

SQL Server autocreates indices for Primary Keys, but not for Foreign Keys. Create the index for the Foreign Keys. It's probably worth the overhead.

Upvotes: 7

Andomar
Andomar

Reputation: 238256

Say you have a big table called orders, and a small table called customers. There is a foreign key from an order to a customer. Now if you delete a customer, Sql Server must check that there are no orphan orders; if there are, it raises an error.

To check if there are any orders, Sql Server has to search the big orders table. Now if there is an index, the search will be fast; if there is not, the search will be slow.

So in this case, the slow delete could be explained by the absence of an index. Especially if Sql Server would have to search 15 big tables without an index.

P.S. If the foreign key has ON DELETE CASCADE, Sql Server still has to search the order table, but then to remove any orders that reference the deleted customer.

Upvotes: 8

shylent
shylent

Reputation: 10086

Strictly speaking, foreign keys have absolutely nothing to do with indexes, yes. But, as the speakers above me pointed out, it makes sense to create one to speed up the FK-lookups. In fact, in MySQL, if you don't specify an index in your FK declaration, the engine (InnoDB) creates it for you automatically.

Upvotes: 4

Yishai
Yishai

Reputation: 91931

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.

Upvotes: 57

Michael Borgwardt
Michael Borgwardt

Reputation: 346476

No, there is no implicit index on foreign key fields, otherwise why would Microsoft say "Creating an index on a foreign key is often useful". Your colleague may be confusing the foreign key field in the referring table with the primary key in the referred-to table - primary keys do create an implicit index.

Upvotes: 26

Gandalf
Gandalf

Reputation: 9855

Not to my knowledge. A foreign key only adds a constraint that the value in the child key also be represented somewhere in the parent column. It's not telling the database that the child key also needs to be indexed, only constrained.

Upvotes: 3

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