Result Ordering

Unless you specify the sort() method or use the $near operator, MongoDB does not guarantee the order of query results.

Ascending/Descending Sort

Specify in the sort parameter the field or fields to sort by and a value of 1 or -1 to specify an ascending or descending sort respectively.

The following sample document specifies a descending sort by the age field and then an ascending sort by the posts field:

{ age : -1, posts: 1 }

When comparing values of different BSON types, MongoDB uses the following comparison order, from lowest to highest:

1. MinKey (internal type)
2. Null
3. Numbers (ints, longs, doubles, decimals)
4. Symbol, String
5. Object
6. Array
7. BinData
8. ObjectId
9. Boolean
10. Date
11. Timestamp
12. Regular Expression
13. MaxKey (internal type)

For details on the comparison/sort order for specific types, see Comparison/Sort Order.

Metadata Sort

Specify in the sort parameter a new field name for the computed metadata and specify the $meta expression as its value.

The following sample document specifies a descending sort by the "textScore" metadata:

{ score: { $meta: "textScore" } }

The specified metadata determines the sort order. For example, the "textScore" metadata sorts in descending order. See $meta for details.

Restrictions

When unable to obtain the sort order from an index, MongoDB will sort the results in memory, which requires that the result set being sorted is less than 32 megabytes.

When the sort operation consumes more than 32 megabytes, MongoDB returns an error. To avoid this error, either create an index supporting the sort operation (see Sort and Index Use) or use sort() in conjunction with limit() (see Limit Results).

Sort and Index Use

The sort can sometimes be satisfied by scanning an index in order. If the query plan uses an index to provide the requested sort order, MongoDB does not perform an in-memory sorting of the result set. For more information, see Use Indexes to Sort Query Results.

Limit Results

You can use sort() in conjunction with limit() to return the first (in terms of the sort order) k documents, where k is the specified limit.

If MongoDB cannot obtain the sort order via an index scan, then MongoDB uses a top-k sort algorithm. This algorithm buffers the first k results (or last, depending on the sort order) seen so far by the underlying index or collection access. If at any point the memory footprint of these k results exceeds 32 megabytes, the query will fail.

Interaction with Projection

When a set of results are both sorted and projected, the MongoDB query engine will always apply the sorting first.

Note

You cannot specify a $natural sort on a view.

Index Use

Queries that include a sort by $natural order do not use indexes to fulfill the query predicate with the following exception: If the query predicate is an equality condition on the _id field { _id: <value> }, then the query with the sort by $natural order can use the _id index.

MMAPv1

Typically, the natural order reflects insertion order with the following exception for the MMAPv1 storage engine. For the MMAPv1 storage engine, the natural order does not reflect insertion order if the documents relocate because of document growth or remove operations free up space which are then taken up by newly inserted documents.

Consider to following example which uses the MMAPv1 storage engine.

The following sequence of operations inserts documents into the trees collection:

db.trees.insert( { _id: 1, common_name: "oak", genus: "quercus" } )
db.trees.insert( { _id: 2, common_name: "chestnut", genus: "castanea" } )
db.trees.insert( { _id: 3, common_name: "maple", genus: "aceraceae" } )
db.trees.insert( { _id: 4, common_name: "birch"