$max¶

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Definition
Behavior
Examples

Definition¶

$max¶

注解

Deprecated in the mongo Shell since v3.2: Starting in v3.2, the $max operator is deprecated in the mongo shell. In the mongo shell, use cursor.max() instead.

Specify a $max value to specify the exclusive upper bound for a specific index in order to constrain the results of find(). The $max specifies the upper bound for all keys of a specific index in order.

The mongo shell provides the max() wrapper method:

db.collection.find( { <query> } ).max( { field1: <max value>, ... fieldN: <max valueN> } )

You can also specify $max with either of the two forms:

db.collection.find( { <query> } )._addSpecial( "$max", { field1: <max value1>, ... fieldN: <max valueN> } )
db.collection.find( { $query: { <query> }, $max: { field1: <max value1>, ... fieldN: <max valueN> } } )

Behavior¶

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.

`$max` without `$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.

警告

If one of the two boundaries is not specified, the query plan will be an index scan that is unbounded on one side. This may degrade performance compared to a query containing neither operator, or one that uses both operators to more tightly constrain the index scan.

Examples¶

The following examples use the mongo shell wrappers.

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 } )

← $maxTimeMS $min →