Question

The documentation says:
If you have a compound index on multiple fields, you can use it to
query on the beginning subset of fields. So if you have an index on
a,b,c
you can use it query on
a
a,b
a,b,c

My question is, if I have a compound index like
a,b,c
can I query on
a,c
and sort on c?

Answer 1

It looks like this question was answered on the Google Groups:

Not efficiently, no. Take a look at the explain() from queries to see
what happens.