What type of index is best to add to a user ID UUID field?

For a field with a UUID user identifier, it is usually better to use a B-tree index (a regular index), since UUID is a unique identifier, and a B-tree index is well-suited for searching by unique values.

If the database supports it, you can consider UUID-specific indexes or use hash indexes to speed up exact searches by UUID, but they are not suitable for range queries.

Example for PostgreSQL:

CREATE INDEX idx_user_uuid ON users USING btree (user_uuid);

If the UUID is stored in binary format (for example, uuid type in PostgreSQL), the index will be more compact and efficient.

Hash indexes in PostgreSQL can be used as follows:

CREATE INDEX idx_user_uuid_hash ON users USING hash (user_uuid);

But keep in mind that hash indexes do not support range queries and are less versatile. Therefore, B-tree is most often chosen.