For queries with multiple predicates over a multikey field, you can't intersect the index bounds. Suppose there is a multikey index on field 'a'. The following query
db.t.find({a: {$lte: 0, $gte: 1}});
would have empty index bounds if 'a' were not a multikey field. Since it is multikey, however, we either use bounds
[-Infinity, 0]
or
[1, Infinity]
Which bounds to use is chosen arbitrarily. As an optimization, we could make sure that we always choose the smallest interval over which to perform the index scan. For instance, for the query
db.t.find({a: {$gt: 4, $gte: 5}});
we would choose
[5, Infinity]
rather than
(4, Infinity]
because the former is a subset of the latter.
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SERVER-14519 $and $ne performance degradation in 2.6
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SERVER-16042 Optimise $all/$and to select smallest subset as initial index bounds
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