Aggregation Pipeline and Sharded Collections聚合管道和分片集合
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The aggregation pipeline supports operations on sharded collections. 聚合管道支持对分片集合的操作。This section describes behaviors specific to the aggregation pipeline and sharded collections.本节描述特定于聚合管道和分片集合的行为。
Behavior行为
If the pipeline starts with an exact 如果管道以分片键上的完全$match on a shard key, and the pipeline does not contain $out or $lookup stages, the entire pipeline runs on the matching shard only.$match开始，并且管道不包含$out或$lookup阶段，则整个管道仅在匹配的分片上运行。
When aggregation operations run on multiple shards, the results are routed to the 当聚合操作在多个分片上运行时，结果会被路由到要合并的mongos to be merged, except in the following cases:mongos，但以下情况除外：
If the pipeline includes the如果管道包括$outor$lookupstages, the merge runs on the primary shard.$out或$lookup阶段，则合并将在主分片上运行。If the pipeline includes a sorting or grouping stage, and the allowDiskUse setting is enabled, the merge runs on a randomly-selected shard.如果管道包括排序或分组阶段，并且启用了allowDiskUse设置，则合并将在随机选择的分片上运行。
Optimization优化
When splitting the aggregation pipeline into two parts, the pipeline is split to ensure that the shards perform as many stages as possible with consideration for optimization.将聚合管道拆分为两部分时，会对管道进行拆分，以确保分片在考虑优化的情况下执行尽可能多的阶段。
To see how the pipeline was split, include the 要查看管道是如何拆分的，请在explain option in the db.collection.aggregate() method.db.collection.aggregate()方法中包含explain选项。
Optimizations are subject to change between releases.优化可能会在不同版本之间发生变化。