Interaction with Index Selection

Because max() requires an index on a field, and forces the query to use this index, you may prefer the $lt operator for the query if possible. Consider the following example:

db.collection.find( { _id: 7 } ).max( { age: 25 } )

The query uses the index on the age field, even if the index on _id may be better.

Index Bounds

If you use $max with $min to specify a range:

• the index bounds specified in $min and $max must both refer to the keys of the same index.

• the bound specified by $max must be greater than the bound specified by $min.

  Changed in version 3.6.6.

$max without $min

The min and max operators indicate that the system should avoid normal query planning. Instead they construct an index scan where the index bounds are explicitly specified by the values given in min and max.

Specify Exclusive Upper Bound

Consider the following operations on a collection named collection that has an index { age: 1 }:

db.collection.find( { <query> } ).max( { age: 100 } )

This operation limits the query to those documents where the field age is less than 100 and forces a query plan which scans the { age: 1 } index from MinKey to 100.

Index Selection

You can explicitly specify the corresponding index with hint(). Otherwise, MongoDB selects the index using the fields in the $max and $min bounds; however, if multiple indexes exist on same fields with different sort orders, the selection of the index may be ambiguous.

Consider a collection named collection that has the following two indexes:

{ age: 1, type: -1 }
{ age: 1, type: 1 }

Without explicitly using hint(), MongoDB may select either index for the following operation:

db.collection.find().max( { age: 50, type: 'B' } )

Use with $min

Use $max alone or in conjunction with $min to limit results to a specific range for the same index, as in the following example:

db.collection.find().min( { age: 20 } ).max( { age: 25 } )