mongodb sparse indexes and shard key <p>I looked through the docs, and couldn't find a clear answer</p> <p>Say I have a sparse index on [a,b,c] </p> <ol> <li><p>Will documents with "a" "b" fields but not "c" be inserted to the index? </p></li> <li><p>Is having the shard key indexed obligatory in the latest mongodb version ? </p></li> <li><p>If so, is it possible to shard on [a] using the above compound sparse index? (say a,b will always exist)</p></li> </ol>