> I'm guessing now that we're just talking about a regular index on a
> column that happens to have a foreign key reference to another column.
> Is that the case?
>
That's true, but just as the query planner can use an index for WHERE
conditions, it can use an index for a join condition. So if you've
got an index on your foreign key, the planner has the opportunity to
use the index any join operations across that foreign key.
