$sort (aggregation)¶

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Definition
Examples
$sort Operator and Memory
$sort Operator and Performance

Definition¶

$sort¶

Sorts all input documents and returns them to the pipeline in sorted order.

The $sort stage has the following prototype form:

{ $sort: { <field1>: <sort order>, <field2>: <sort order> ... } }

$sort takes a document that specifies the field(s) to sort by and the respective sort order. <sort order> can have one of the following values:

Value	Description
`1`	Sort ascending.
`-1`	Sort descending.
`{ $meta: "textScore" }`	Sort by the computed `textScore` metadata in descending order. See Metadata Sort for an example.

If sorting on multiple fields, sort order is evaluated from left to right. For example, in the form above, documents are first sorted by <field1>. Then documents with the same <field1> values are further sorted by <field2>.

Examples¶

Ascending/Descending Sort¶

For the field or fields to sort by, set the sort order to 1 or -1 to specify an ascending or descending sort respectively, as in the following example:

copy

db.users.aggregate(
   [
     { $sort : { age : -1, posts: 1 } }
   ]
)

This operation sorts the documents in the users collection, in descending order according by the age field and then in ascending order according to the value in the posts field.

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

MinKey (internal type)
Null
Numbers (ints, longs, doubles, decimals)
Symbol, String
Object
Array
BinData
ObjectId
Boolean
Date
Timestamp
Regular Expression
MaxKey (internal type)

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

Metadata Sort¶

Specify in the { <sort-key> } document, a new field name for the computed metadata and specify the $meta expression as its value, as in the following example:

copy

db.users.aggregate(
   [
     { $match: { $text: { $search: "operating" } } },
     { $sort: { score: { $meta: "textScore" }, posts: -1 } }
   ]
)

This operation uses the $text operator to match the documents, and then sorts first by the "textScore" metadata and then by descending order of the posts field. The specified metadata determines the sort order. For example, the "textScore" metadata sorts in descending order. See $meta for more information on metadata.

`$sort` Operator and Memory¶

`$sort` + `$limit` Memory Optimization¶

When a $sort precedes a $limit and there are no intervening stages that modify the number of documents, the optimizer can coalesce the $limit into the $sort. This allows the $sort operation to only maintain the top n results as it progresses, where n is the specified limit, and ensures that MongoDB only needs to store n items in memory. This optimization still applies when allowDiskUse is true and the n items exceed the aggregation memory limit.

Optimizations are subject to change between releases.

`$sort` and Memory Restrictions¶

The $sort stage has a limit of 100 megabytes of RAM for in-memory sorts. By default, if the stage exceeds this limit, $sort produces an error. To allow pipeline processing to take up more space, use the allowDiskUse option to enable aggregation pipeline stages to write data to temporary files.

`$sort` Operator and Performance¶

The $sort operator can take advantage of an index if it’s used in the first stage of a pipeline or if it’s only preceeded by a $match stage.

When you use the $sort on a sharded cluster, each shard sorts its result documents using an index where available. Then the mongos or one of the shards performs a streamed merge sort.