[aidos]

Touché! An index signals the data should be searched in a certain way!

I don't think I explained my arbitrary scenario particularly well :-) I'm definitely not suggesting 2 queries.

If I have articles that could be category: math|science and source: website_a|website_b|... and I only ever query for source and category together then the other indexes aren't used.

It's a contrived example, but in my mind the existence of a foreign key doesn't imply an index is required.