Currently, if I create an index on an array field and specify an element number ("MyArray.0"), the system still indexes the whole array as normal.

Normally, this might increase the index size but at least it's still useful when querying MyArray.0 later.

However, it would be better to be able to index individual numbered elements in cases like compound indexes where I might need to check whether two arrays are non-empty, and I can't use a compound index for a query on MyArray.0 and MyOtherArray.0 at the same time due to the multi-array limitation.

For MongoDB folks with access to it, see support case 01158569 for a scenario where I needed this capability, and a discussion about it.