cursor.max()¶

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Definition
Behavior
Example

Definition¶

cursor.max()¶

Specifies the exclusive upper bound for a specific index in order to constrain the results of find(). max() provides a way to specify an upper bound on compound key indexes.

The max() method has the following parameter:

Parameter	Type	Description
`indexBounds`	document	The exclusive upper bound for the index keys.

The indexBounds parameter has the following prototype form:

{ field1: <max value>, field2: <max value2> ... fieldN:<max valueN> }

The fields correspond to all the keys of a particular index in order. You can explicitly specify the particular index with the hint() method. Otherwise, mongod selects the index using the fields in the indexBounds; however, if multiple indexes exist on same fields with different sort orders, the selection of the index may be ambiguous.

参见

min().

max() exists primarily to support the mongos (sharding) process, and is a shell wrapper around the query modifier $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.

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.products.find( { _id: 7 } ).max( { price: 1.39 } )

The query will use the index on the price 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.

Example¶

This example assumes a collection named products that holds the following documents:

{ "_id" : 6, "item" : "apple", "type" : "cortland", "price" : 1.29 }
{ "_id" : 2, "item" : "apple", "type" : "fuji", "price" : 1.99 }
{ "_id" : 1, "item" : "apple", "type" : "honey crisp", "price" : 1.99 }
{ "_id" : 3, "item" : "apple", "type" : "jonagold", "price" : 1.29 }
{ "_id" : 4, "item" : "apple", "type" : "jonathan", "price" : 1.29 }
{ "_id" : 5, "item" : "apple", "type" : "mcintosh", "price" : 1.29 }
{ "_id" : 7, "item" : "orange", "type" : "cara cara", "price" : 2.99 }
{ "_id" : 10, "item" : "orange", "type" : "navel", "price" : 1.39 }
{ "_id" : 9, "item" : "orange", "type" : "satsuma", "price" : 1.99 }
{ "_id" : 8, "item" : "orange", "type" : "valencia", "price" : 0.99 }

The collection has the following indexes:

{ "_id" : 1 }
{ "item" : 1, "type" : 1 }
{ "item" : 1, "type" : -1 }
{ "price" : 1 }

Using the ordering of { item: 1, type: 1 } index, max() limits the query to the documents that are below the bound of item equal to apple and type equal to jonagold:
```
db.products.find().max( { item: 'apple', type: 'jonagold' } ).hint( { item: 1, type: 1 } )
```
The query returns the following documents:
```
{ "_id" : 6, "item" : "apple", "type" : "cortland", "price" : 1.29 }
{ "_id" : 2, "item" : "apple", "type" : "fuji", "price" : 1.99 }
{ "_id" : 1, "item" : "apple", "type" : "honey crisp", "price" : 1.99 }
```
If the query did not explicitly specify the index with the hint() method, it is ambiguous as to whether mongod would select the { item: 1, type: 1 } index ordering or the { item: 1, type: -1 } index ordering.
Using the ordering of the index { price: 1 }, max() limits the query to the documents that are below the index key bound of price equal to 1.99 and min() limits the query to the documents that are at or above the index key bound of price equal to 1.39:
```
db.products.find().min( { price: 1.39 } ).max( { price: 1.99 } ).hint( { price: 1 } )
```
The query returns the following documents:
```
{ "_id" : 10, "item" : "orange", "type" : "navel", "price" : 1.39 }
```

← cursor.maxTimeMS() cursor.min() →