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To improve query performance for time series collections, add one or more secondary indexes to support common time series query patterns. 要提高时间序列集合的查询性能，请添加一个或多个辅助索引以支持常见的时间序列查询模式。Specifically, we recommend that you create one or more compound indexes on the fields specified as the 具体来说，我们建议您在指定为timeField and the metaField. timeField和metaField的字段上创建一个或多个复合索引。If the field value for the 如果metaField field is a document, you can create secondary indexes on fields inside that document.metaField字段的字段值是文档，则可以在该文档内的字段上创建二级索引。
Not all index types are supported. 并非所有索引类型都受支持。For a list of unsupported index types, see Limitations for Secondary Indexes on Time Series Collections.有关不受支持的索引类型的列表，请参阅时间序列集合上辅助索引的限制。
For example, this command creates a compound index on the 例如，此命令在metadata.sensorId and timestamp fields:metadata.sensorId和timestamp字段上创建复合索引：
db.weather24h.createIndex({ "metadata.sensorId": 1, "timestamp": 1 })
To force MongoDB to use a specific index when querying a time series collection, use 要强制MongoDB在查询时间序列集合时使用特定索引，请使用带有索引名称的cursor.hint() with an index name:cursor.hint()：
db.weather24h.find({ "metadata.sensorId": 1235 }).hint("metadata.sensorId_1_timestamp_1")
On a time series collection, you can only specify hints using the index name, not the index key pattern. 在时间序列集合中，只能使用索引名而不是索引键模式指定提示。To get the names of the indexes on a collection, use the 要获取集合上索引的名称，请使用db.collection.getIndexes() method.db.collection.getIndexes()方法。