$sort (aggregation)

Limits限制

You can sort on a maximum of 32 keys.最多可以按32个键排序。

Sort Consistency排序一致性

MongoDB does not store documents in a collection in a particular order. MongoDB不按特定顺序存储集合中的文档。When sorting on a field which contains duplicate values, documents containing those values may be returned in any order.在对包含重复值的字段进行排序时，包含这些值的文档可以按任何顺序返回。

If consistent sort order is desired, include at least one field in your sort that contains unique values. 如果需要一致的排序顺序，请在排序中至少包含一个包含唯一值的字段。The easiest way to guarantee this is to include the _id field in your sort query.保证这一点的最简单方法是在排序查询中包含_id字段。

Consider the following restaurant collection:考虑以下restaurant集合：

db.restaurants.insertMany( [
   { "_id" : 1, "name" : "Central Park Cafe", "borough" : "Manhattan"},
   { "_id" : 2, "name" : "Rock A Feller Bar and Grill", "borough" : "Queens"},
   { "_id" : 3, "name" : "Empire State Pub", "borough" : "Brooklyn"},
   { "_id" : 4, "name" : "Stan's Pizzaria", "borough" : "Manhattan"},
   { "_id" : 5, "name" : "Jane's Deli", "borough" : "Brooklyn"},
] );

The following command uses the $sort stage to sort on the borough field:以下命令使用$sort阶段对borough字段进行排序：

db.restaurants.aggregate(
   [
     { $sort : { borough : 1 } }
   ]
)

In this example, sort order may be inconsistent, since the borough field contains duplicate values for both Manhattan and Brooklyn. 在本例中，排序顺序可能不一致，因为borough字段包含Manhattan和Brooklyn的重复值。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. 文档按borough的字母顺序返回，但在相同排序的多个执行中，具有重复borough值的文档的顺序可能不同。For example, here are the results from two different executions of the above command:例如，以下是上述命令的两次不同执行的结果：

{ "_id" : 3, "name" : "Empire State Pub", "borough" : "Brooklyn" }
{ "_id" : 5, "name" : "Jane's Deli", "borough" : "Brooklyn" }
{ "_id" : 1, "name" : "Central Park Cafe", "borough" : "Manhattan" }
{ "_id" : 4, "name" : "Stan's Pizzaria", "borough" : "Manhattan" }
{ "_id" : 2, "name" : "Rock A Feller Bar and Grill", "borough" : "Queens" }
{ "_id" : 5, "name" : "Jane's Deli", "borough" : "Brooklyn" }
{ "_id" : 3, "name" : "Empire State Pub", "borough" : "Brooklyn" }
{ "_id" : 4, "name" : "Stan's Pizzaria", "borough" : "Manhattan" }
{ "_id" : 1, "name" : "Central Park Cafe", "borough" : "Manhattan" }
{ "_id" : 2, "name" : "Rock A Feller Bar and Grill", "borough" : "Queens" }

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.虽然borough的值仍按字母顺序排序，但包含borough（即Manhattan和Brooklyn）重复值的文档的顺序并不相同。

To achieve a consistent sort, add a field which contains exclusively unique values to the sort. 要实现一致排序，请在排序中添加包含唯一值的字段。The following command uses the $sort stage to sort on both the borough field and the _id field:以下命令使用$sort阶段对borough字段和_id字段进行排序：

db.restaurants.aggregate(
   [
     { $sort : { borough : 1, _id: 1 } }
   ]
)

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.由于_id字段始终保证包含唯一的值，因此在同一排序的多个执行中，返回的排序顺序始终相同。

Ascending/Descending Sort升序/降序排序

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:对于要排序的一个或多个字段，将排序顺序设置为1或-1以分别指定升序或降序排序，如以下示例所示：

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.此操作将users集合中的文档按age字段降序排序，然后按posts字段中的值升序排序。

When comparing values of different BSON types, MongoDB uses the following comparison order, from lowest to highest:当比较不同BSON类型的值时，MongoDB使用以下比较顺序，从最低到最高：

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.有关特定类型的比较/排序顺序的详细信息，请参阅比较/排序次序。

Text Score Metadata Sort文本得分元数据排序

For a pipeline that includes a $text search, you can sort by descending relevance score using the { $meta: "textScore" } expression. 对于包含$text搜索的管道，可以使用{ $meta: "textScore" }表达式按相关性得分降序排序。In the { <sort-key> } document, set the { $meta: "textScore" } expression to an arbitrary field name. 在{<sort key>}文档中，将{ $meta: "textScore" }表达式设置为任意字段名。The field name is ignored by the query system. 查询系统将忽略字段名。For example:例如：

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

This operation uses the $text operator to match the documents, and then sorts first by the "textScore" metadata in descending order, and then by the posts field in descending order. 此操作使用$text运算符来匹配文档，然后首先按"textScore"元数据降序排序，然后按posts字段降序排序。The score field name in the sort document is ignored by the query system. 查询系统将忽略排序文档中的score字段名称。In this pipeline, the "textScore" metadata is not included in the projection and is not returned as part of the matching documents. 在这个管道中，"textScore"元数据不包括在投影中，也不作为匹配文档的一部分返回。See $meta for more information.有关详细信息，请参阅$meta。

$sort + $limit Memory Optimization内存优化

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 $sort. 当$sort在$limit之前，并且没有修改文档数量的中间阶段时，优化器可以将$limit合并到$sort中。This allows the $sort operation to only maintain the top n results as it progresses, where n is the specified limit, and ensures that MongoDB only needs to store n items in memory. 这允许$sort操作在运行过程中只维护前n个结果，其中n是指定的限制，并确保MongoDB只需要在内存中存储n个项。This optimization still applies when allowDiskUse is true and the n items exceed the aggregation memory limit.当allowDiskUse为true且n项超过聚合内存限制时，此优化仍然适用。

Optimizations are subject to change between releases.优化可能会在不同版本之间发生变化。

$sort and Memory Restrictions和内存限制

The $sort stage has a limit of 100 megabytes of RAM for in-memory sorts. $sort阶段的内存排序限制为100兆字节。By default, if the stage exceeds this limit, $sort produces an error. 默认情况下，如果阶段超过此限制，$sort将产生错误。To allow pipeline processing to take up more space, use the allowDiskUse option to enable aggregation pipeline stages to write data to temporary files.要允许管道处理占用更多空间，请使用allowDiskUse选项启用聚合管道阶段将数据写入临时文件。