I know that a comma is necessary before a conjunction joining two independent clauses, like in the following examples:
John is tall, and Mary is fast.
The food is excellent, but the service is awful.
But what happens when there is a subordinate clause formed in turn by two independent clauses connected by a coordinating conjunction? Should there be a comma before the conjunction? I am referring to sentences like these:
She says that John is tall and Mary is fast.
I hate those restaurants where the food is excellent but the service is awful.
I am leaving the country so that the police cannot find me and I don't go to jail.
I tend to think there should be no comma in this kind of sentences, but I would appreciate if someone could confirm it. Assuming I am right, is that true independently of the length of the coordinate clauses?