Sparse indexes can have unexpected results
> db.cities.find( { admin1_code: { $exists: true } } ).count();
|00cc00|22816|
> db.cities.find( { admin1_code: { $exists: false } } ).count();
|f10000|0|
> db.cities.count();
22840
Let's look at %explain()%:
> db.cities.find( { admin1_code: { $exists: false } } ).explain();
{
*"cursor" : "BtreeCursor admin1_code_1",*
"nscanned" : 0,
"nscannedObjects" : 0,
"n" : 0,
...
A sparse index doesn't contain links to documents without the indexed field.