$match (aggregation)¶

Pipeline Optimization管道优化¶

• Place the $match as early in the aggregation pipeline as possible. 尽可能早地将$match放入聚合管道中。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操作可以最大限度地减少管道中的处理量。
• If you place a $match at the very beginning of a pipeline, the query can take advantage of indexes like any other db.collection.find() or db.collection.findOne().如果在管道的最开始处放置$match，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. $match查询语法与读取操作查询语法相同；亦即，$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 $match queries as part of the aggregation pipeline.不能在用作聚合管道的一部分的$match查询中使用$where。
• You cannot use $near or $nearSphere in $match queries as part of the aggregation pipeline. 不能在作为聚合管道的一部分的$match查询中使用$near或$nearSphere。As an alternative, you can either:作为替代方案，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。

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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数数¶

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，然后将结果传递给$group管道运算符以计算文档计数：

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但小于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 }