A single index and joint index

Use joint index higher efficiency;
When you create (a, b, c) joint index, the equivalent of creating (a) a separate index, (a, b) joint index and (a, b, c) joint index
want to index in effect, it can only use a and a, b and a, b, c three combinations a; of course, we tested the above, a, c combination can be, but in fact only used the index a, c and did not use !
Indexed fields need to be in a condition where