$match (aggregation)

Pipeline Optimization管道优化

• Place the $match as early in the aggregation pipeline as possible. 尽可能早地在聚合管道中放置$match。Because $match limits the total number of documents in the aggregation pipeline, earlier $match operations minimize the amount of processing down the pipe.因为$match限制了聚合管道中的文档总数，所以早期的$match操作将管道中的处理量最小化。
• If you place a $match at the very beginning of a pipeline, the query can take advantage of indexes like any other db.collection.find() or db.collection.findOne().如果将$match放在管道的最开始，查询可以像任何其他db.collection.find()或db.collection.findOne()一样利用索引。

Restrictions限制

• The $match query syntax is identical to the read operation query syntax; i.e. $match does not accept raw aggregation expressions. $match查询语法与读取操作查询语法相同；即，$match不接受原始聚合表达式。To include aggregation expression in $match, use a $expr query expression:要在$match中包含聚合表达式，请使用$expr查询表达式：

  { $match: { $expr: { <aggregation expression> } } }

• You cannot use $where in $match queries as part of the aggregation pipeline.不能将$where在$match查询中用作聚合管道的一部分。

• You cannot use $near or $nearSphere in $match queries as part of the aggregation pipeline. 不能在$match查询中使用$near或$nearSphere作为聚合管道的一部分。As an alternative, you can either:或者，您可以：

  • Use $geoNear stage instead of the $match stage.使用$geoNear阶段而不是$match阶段。
  • Use $geoWithin query operator with $center or $centerSphere in the $match stage.在$match阶段中，将$geoWithin查询运算符与$center或$centerSphere一起使用。

• To use $text in the $match stage, the $match stage has to be the first stage of the pipeline.要在$match阶段中使用$text，$match阶段必须是管道的第一阶段。

  Views do not support text search.视图不支持文本搜索。

Equality Match相等匹配

The following operation uses $match to perform a simple equality match:以下操作使用$match执行简单的相等匹配：

db.articles.aggregate(
    [ { $match : { author : "dave" } } ]
);

The $match selects the documents where the author field equals dave, and the aggregation returns the following:$match选择author字段等于dave的文档，聚合返回以下内容：

{ "_id" : ObjectId("512bc95fe835e68f199c8686"), "author" : "dave", "score" : 80, "views" : 100 }
{ "_id" : ObjectId("512bc962e835e68f199c8687"), "author" : "dave", "score" : 85, "views" : 521 }

Perform a Count计数

The following example selects documents to process using the $match pipeline operator and then pipes the results to the $group pipeline operator to compute a count of the documents:以下示例使用$match管道运算符选择要处理的文档，然后将结果管道传输到$group管道运算符，以计算文档的计数：

db.articles.aggregate( [
  { $match: { $or: [ { score: { $gt: 70, $lt: 90 } }, { views: { $gte: 1000 } } ] } },
  { $group: { _id: null, count: { $sum: 1 } } }
] );

In the aggregation pipeline, $match selects the documents where either the score is greater than 70 and less than 90 or the views is greater than or equal to 1000. 在聚合管道中，$match选择score大于70且小于90或views大于或等于1000的文档。These documents are then piped to the $group to perform a count. 然后，这些文档通过管道传输到$group以执行计数。The aggregation returns the following:聚合返回以下内容：

{ "_id" : null, "count" : 5 }

Filter Data on Atlas Using Atlas Search使用图集搜索筛选图集上的数据

For your $search queries against data on your Atlas cluster, you can use the Atlas Searchcompound operator filter option to match or filter documents. 对于针对Atlas集群上的数据的$search查询，可以使用Atlas Search复合运算符筛选选项匹配或筛选文档。Running $match after $search is less performant than running $search with the compound operator filter option. 在$search之后运行$match比使用复合运算符filter选项运行$search性能差。To learn more about the filter option, see compound.要了解有关filter选项的更多信息，请参阅复合。

Aggregation Tutorials聚合教程

• Aggregation with the Zip Code Data Set与邮政编码数据集的聚合
• Aggregation with User Preference Data与用户偏好数据的聚合