Let me recap to make sure I understand.  If I'm essentially correct, no reply needed.

When you create a primary key on a table that  creates (or is) a unique index on the field(s)

When you reference that primary key in another table as a foreign key, a non-unique index is automatically created along with the foreign key constraints

Finally,  compound indexes will out-perform single indexes when the compound indexes match the selectivity requirements of the queries

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