[SERVER-5063] $in on first compound key element and limit efficiency - MongoDB

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If you have an index on ( a , b ) and do a query like find( { a :

{ $in : [ ... }

} ).sort(

{ b : 1 }

).limit(10)
an easy optimization is limiting the results for each $in element to the limit.

is depended on by

SERVER-3310 Query optimizer should efficiently handle $in and sort with compound index.

related to

SERVER-5481 query on undefined field matches empty array

SERVER-5777 Add support for a range constraint on the first sort field to in/sort/limit optimization

SERVER-5450 refactor and optimize FieldRangeVectorIterator

SERVER-6268 report both winning plan nscanned and total nscanned in explain output