Is a foreign key an index?

Question

MySQL

Hello!
Is a Foreign Key an index in MySQL and PostreSQL?

Answer 1

Where it refers to - must be a unique index (preferably PK), without this FK will not be created.
Referenced from - MySQL automatically builds an index, but PostreSQL does not.
https://www.postgresql.org/docs/current/static/ddl...

Answer 2

Foreign Key uses an index, i.e. No, but you can't do it without an index.