Is a primary key an index?

Yes, a primary key in a database is usually an index.

A primary key is a unique identifier for a record in a table, which guarantees uniqueness and does not allow NULL values. To ensure fast search and uniqueness, the DBMS automatically creates an index on the field or set of fields declared as the primary key.

Thus, a primary key is always accompanied by an index, but not every index is a primary key. Indexes can be created to speed up queries on any columns, while a primary key is a special index with a uniqueness constraint and no NULLs.