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- $sort (aggregation)
$sort (aggregation)¶
Definition¶
-
$sort
¶ Sorts all input documents and returns them to the pipeline in sorted order.
The
$sort
stage has the following prototype form:$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>
.
Behavior¶
Limits¶
You can sort on a maximum of 32 keys.
Sort Consistency¶
MongoDB does not store documents in a collection in a particular order. When sorting on a field which contains duplicate values, documents containing those values may be returned in any order.
If consistent sort order is desired, include at least one field in your
sort that contains unique values. The easiest way to guarantee this is
to include the _id
field in your sort query.
Consider the following restaurant
collection:
The following command uses the $sort
stage to sort on
the borough
field:
In this example, sort order may be inconsistent, since the borough
field contains duplicate values for both Manhattan
and Brooklyn
.
Documents are returned in alphabetical order by borough
, but the
order of those documents with duplicate values for borough
might not
the be the same across multiple executions of the same sort. For
example, here are the results from two different executions of the
above command:
While the values for borough
are still sorted in alphabetical order,
the order of the documents containing duplicate values for borough
(i.e. Manhattan
and Brooklyn
) is not the same.
To achieve a consistent sort, add a field which contains exclusively
unique values to the sort. The following command uses the
$sort
stage to sort on both the borough
field and the
_id
field:
Since the _id
field is always guaranteed to contain exclusively
unique values, the returned sort order will always be the same across
multiple executions of the same sort.
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:
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:
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.
See also
$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.