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Right now distinct will pull data out of indexes, but will scan the entire thing.
Can skip whole regions where the key is the same

Correct way to implement is probably adding a method to a btree bucket like

getDisinctKeys( int keyOffset )

depends on

SERVER-1673 Allow distinct to use indexes

SERVER-11939 Implement advanceToNextUniqueKey access method on btree

duplicates

SERVER-2094 distinct cheat with indexes

is related to

SERVER-13271 remove surplus projection from distinct

related to

SERVER-12297 Distinct - nscanned for $in and $or greater than nscannedObjects

SERVER-9507 Optimize $sort+$group+$first pipeline to avoid full index scan

(1 related to)