Add Secondary Indexes to Time Series Collections向时间序列集合添加二级索引

Last Point Queries on Time Series Collections时间序列集合的最后一点查询

In time series data, a last point query returns the data point with the latest timestamp for a given field. For time series collections, a last point query fetches the latest measurement for each unique metadata value. 在时间序列数据中，最后一个点查询返回给定字段的最新时间戳的数据点。对于时间序列集合，最后一点查询会获取每个唯一元数据值的最新测量值。For example, you may want to get the latest temperature reading from all sensors. Improve performance on last point queries by creating any of the following indexes:例如，您可能希望从所有传感器获取最新的温度读数。通过创建以下任何索引来提高最后一点查询的性能：

{ "metadata.sensorId": 1,  "timestamp": 1 }
{ "metadata.sensorId": 1,  "timestamp": -1 }
{ "metadata.sensorId": -1, "timestamp": 1 }
{ "metadata.sensorId": -1, "timestamp": -1 }

The following command creates a compound secondary index on metaField (ascending) and timeField (descending):以下命令在metaField（升序）和timeField（降序）上创建复合二级索引：

db.sensorData.createIndex( { "metadata.sensorId": 1,  "timestamp": -1 } )

The following last point query example uses the descending timeField compound secondary index created above:以下最后一个点查询示例使用上面创建的降序timeField复合二级索引：

db.sensorData.aggregate( [
   {
      $sort: { "metadata.sensorId": 1, "timestamp": -1 }
   },
   {
      $group: {
         _id: "$metadata.sensorId",
         ts: { $first: "$timestamp" },
         temperatureF: { $first: "$currentConditions.tempF" }
      }
   }
] )

To confirm that the last point query used the secondary index, run the operation again using .explain( "executionStats" ):要确认最后一个点查询使用了辅助索引，请使用.explain( "executionStats" )再次运行该操作：

db.getCollection( 'sensorData' ).explain( "executionStats" ).aggregate( [
   {
      $sort: { "metadata.sensorId": 1, "timestamp": -1 }
   },
   {
      $group: {
         _id: "$metadata.sensorId",
         ts: { $first: "$timestamp" },
         temperatureF: { $first: "$currentConditions.tempF" }
      }
   }
] )

The winningPlan.queryPlan.inputStage.stage is DISTINCT_SCAN, which indicates that the index was used. For more information on the explain plan output, see Explain Results.winningPlan.queryPlan.inputStage.stage阶段为DISTINCT_SCAN，表示使用了索引。有关解释计划输出的更多信息，请参阅解释结果。

Specify Index Hints for Time Series Collections指定时间序列集合的索引提示

Index hints cause MongoDB to use a specific index for a query. Some operations on time series collections can only take advantage of an index if that index is specified in a hint.索引提示导致MongoDB对查询使用特定的索引。只有在提示中指定了索引的情况下，对时间序列集合的某些操作才能利用该索引。

For example, the following query causes MongoDB to use the timestamp_1_metadata.sensorId_1 index:例如，以下查询导致MongoDB使用timestamp_1_metadata.sensorId_1索引：

db.sensorData.find( { "metadata.sensorId": 5578 } ).hint( "timestamp_1_metadata.sensorId_1" )

On a time series collection, you can specify hints using either the index name or the index key pattern. To get the names of the indexes on a collection, use the db.collection.getIndexes() method.在时间序列集合上，您可以使用索引名称或索引键模式指定提示。要获取集合上索引的名称，请使用db.collection.getIndexes()方法。

Create 2dsphere Indexes创建2dsphere索引

Starting in version 6.0 you can create 2dsphere indexes on the timeField, metaField, or measurement fields.从6.0版本开始，您可以在timeField、metaField或度量字段上创建2dsphere索引。

For example, the following operation creates a 2dsphere index on the location field:例如，以下操作在location字段上创建2dsphere索引：

db.sensorData.createIndex({ "metadata.location": "2dsphere" })

Additionally, the following operation creates a 2dsphere index on measurement fields:此外，以下操作在测量字段上创建2dsphere索引：

db.sensorData.createIndex({ "currentConditions.tempF": "2dsphere" })