In 2.6.0-rc1 I have an array "attr" which is indexed. Its contents are strings. In a query of type:

db.prod2.find({attr:{$all:["string1","string2","string3"]}})

The nscanned seems to be dependent on the order of values inside the $all list. Putting the more selective string first causes nscanned to be equal to number of documents with that string present.

Explain(true) shows only one index boundary is considered - the one that's listed first.

Note: this does not happen if the array elements are subdocuments and $elemMatch is used - then explain(true) shows that each of the elements in the $all list are tried and the one that's most selective is chosen regardless of the order they are given in.