Earlier quoted context omitted.
No - it can only use one for a given query. But a compound index like the ones I describe in the post can index many fields.
Yes of course, but it's kinda slow if you have to maintain one compound index for each possible query you're going to run, that's why I was asking.
Nope, you can use the compound index 'a,b,c' to query over the following fields:
a
a.b
a,b,c