OPTIONS

$sort (aggregation)

$sort

The $sort pipeline operator sorts all input documents and returns them to the pipeline in sorted order. Consider the following prototype form:

db.<collection-name>.aggregate(
    { $sort : { <sort-key> } }
);

This sorts the documents in the collection named <collection-name>, according to the key and specification in the { <sort-key> } document.

Specify the sort in a document with a field or fields that you want to sort by and a value of 1 or -1 to specify an ascending or descending sort respectively, as in the following example:

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:

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

MongoDB treats some types as equivalent for comparison purposes. For instance, numeric types undergo conversion before comparison.

The comparison treats a non-existent field as it would an empty BSON Object. As such, a sort on the a field in documents { } and { a: null } would treat the documents as equivalent in sort order.

With arrays, a less-than comparison or an ascending sort compares the smallest element of arrays, and a greater-than comparison or a descending sort compares the largest element of the arrays. As such, when comparing a field whose value is a single-element array (e.g. [ 1 ]) with non-array fields (e.g. 2), the comparison is between 1 and 2. A comparison of an empty array (e.g. [ ]) treats the empty array as less than null or a missing field.

Important

The $sort cannot begin sorting documents until previous operators in the pipeline have returned all output.

$sort operator can take advantage of an index when placed at the beginning of the pipeline or placed before the following aggregation operators: $project, $unwind, and $group.

Changed in version 2.4: $sort and memory requirements:

  • When a $sort immediately precedes a $limit in the pipeline, the $sort operation only maintains the top n results as it progresses, where n is the specified limit, and MongoDB only needs to store the number of items specified by $limit in memory. Before MongoDB 2.4, $sort would sort all the results in memory, and then limit the results to n results.
  • Unless the $sort operator can use an index or immediately precedes a $limit, the $sort operation must fit within memory. Before MongoDB 2.4, unless the $sort operator can use an index, the $sort operation must fit within memory.

$sort produces an error if the operation consumes 10 percent or more of RAM.

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