Time Series Collection Limitations时间序列集合限制

Unsupported Features不支持的功能

MongoDB does not support the following features with time series collections:MongoDB不支持时间序列集合的以下功能：

• MongoDB Search
• Change streams更改流
• Client-Side Field Level Encryption客户端字段级加密
• Database Triggers数据库触发器
• Schema validation rules架构验证规则
• reIndex
• renameCollection

Aggregation $merge聚合$merge

You cannot use the $merge aggregation stage to add data from another collection to a time series collection. 您不能使用$merge聚合阶段将其他集合中的数据添加到时间序列集合中。Use the $out aggregation stage to write documents to a time series collection.使用$out聚合阶段将文档写入时间序列集合。

You can use $merge to move data from a time series collection to another collection.您可以使用$merge将数据从时间序列集合移动到另一个集合。

distinct Command独特的指挥

Due to the unique data structure of time series collections, MongoDB can't efficiently index them for distinct values. 由于时间序列集合的独特数据结构，MongoDB无法有效地为它们索引不同的值。Avoid using the distinct command or db.collection.distinct() helper method on time series collections. 避免在时间序列集合上使用distinct命令或db.collection.distinct()辅助方法。Instead, use a $group aggregation to group documents by distinct values.相反，使用$group聚合按不同的值对文档进行分组。

For example, to query for distinct meta.type values on documents where meta.project = 10, instead of:例如，在meta.project = 10的文档上查询不同的meta.type值，而不是：

db.foo.distinct("meta.type", {"meta.project": 10})

Use:用途：

db.foo.createIndex({"meta.project":1, "meta.type":1})
db.foo.aggregate([{$match: {"meta.project": 10}},
                  {$group: {_id: "$meta.type"}}])

This works as follows:其工作原理如下：

1. Creating a compound index on meta.project and meta.type and supports the aggregation.在meta.project和meta.type上创建复合索引，并支持聚合。
2. The $match stage filters for documents where meta.project = 10.$match阶段筛选器用于筛选meta.project = 10的文档。
3. The $group stage uses meta.type as the group key to output one document per unique value.$group阶段使用meta.type作为组键，为每个唯一值输出一个文档。

Geospatial Queries地理空间查询

Time series collections only support the $geoNear aggregation stage for sorting geospatial data from queries against 2dsphere indexes. 时间序列集合仅支持$geoNear聚合阶段，用于对2dsphere索引查询中的地理空间数据进行排序。You can't use $near and $nearSphere operators on time series collections不能在时间序列集合上使用$near和$nearSphere运算符

Document Size文档大小

The maximum size for documents within a time series collection is 4 MB.时间序列集合中文档的最大大小为4 MB。

Extended Date Range扩展日期范围

If your time series collection contains documents with timeField timestamps before 1970-01-01T00:00:00.000Z or after 2038-01-19T03:14:07.000Z, create an index on the timeField to optimize queries.如果时间序列集合包含1970-01-01T00:00:00.000Z之前或2038-01-1T03:14:07.000Z之后具有timeField时间戳的文档，请在timeField上创建索引以优化查询。

Updates更新

Update commands must meet the following requirements:更新命令必须满足以下要求：

• You can only match on the metaField field value.您只能匹配metaField字段值。
• You can only modify the metaField field value.您只能修改metaField字段值。
• Your update document can only contain update operator expressions.更新文档只能包含更新运算符表达式。
• Your update command must not limit the number of documents to be updated. Set multi: true or use the updateMany() method.更新命令不得限制要更新的文档数量。设置multi:true或使用updateMany()方法。
• Your update command must not set upsert: true.更新命令不得设置upsert:true。

To automatically delete old data, set up automatic removal (TTL).要自动删除旧数据，请设置自动删除（TTL）。

Time Series Secondary Indexes时间序列二级指标

MongoDB partially supports the following indexes on time series collections:MongoDB部分支持时间序列集合上的以下索引：

• You can only create multikey indexes on the metaField.您只能在metaField上创建多键索引。
• You can only create 2d indexes on the metaField.您只能在metaField上创建2d索引。
• You can only create sparse indexes on the metaField.您只能在metaField上创建稀疏索引。

MongoDB doesn't support the following index types on time series collections:MongoDB不支持时间序列集合上的以下索引类型：

• Text indexes全文索引
• Unique indexes唯一索引

If there are secondary indexes on time series collections and you need to downgrade the feature compatibility version (FCV), you must first drop any secondary indexes that are incompatible with the downgraded FCV. 如果时间序列集合上有辅助索引，并且需要降级功能兼容性版本（FCV），则必须首先删除与降级的FCV不兼容的任何辅助索引。For more information, see setFeatureCompatibilityVersion.有关更多信息，请参阅setFeatureCompatibilityVersion。

Capped Collections封顶集合

You cannot create a time series collection as a capped collection.您不能将时间序列集合创建为封顶集合。

Modification of Collection Type集合类型的修改

You can only set the collection type when you create a collection:您只能在创建集合时设置集合类型：

• You cannot convert an existing collection into a time series collection.您无法将现有集合转换为时间序列集合。
• You cannot convert a time series collection into a different collection type.您无法将时间序列集合转换为其他集合类型。

To move data from an existing collection to a time series collection, migrate data into a time series collection.要将数据从现有集合移动到时间序列集合，请将数据迁移到时间序列集中。

Modification of timeField and metaField修改timeField和metaField

You can only set a collection's timeField and metaField parameters when you create the collection. You cannot modify these parameters later.创建集合时，只能设置集合的timeField和metaField参数。以后不能修改这些参数。

Granularity粒度

db.runCommand({
   collMod: "timeseries",
   timeseries: { bucketMaxSpanSeconds: 3600, bucketRoundingSeconds: 3600 }
})

Sharding分片

Time series collections are subject to several sharding limitations.时间序列集合受到几个分片限制。

Transactions事务

You cannot write to time series collections in transactions.您不能在事务中写入时间序列集合。

Views视图

Time series collections are writable non-materialized views. Limitations for views apply to time series collections.时间序列集合是可写的非物化视图。视图的限制适用于时间序列集合。

Snapshot Isolation快照隔离

Read operations on time series collections with read concern "snapshot" guarantee snapshot isolation only in the absence of concurrent drop or rename operations on collections in the read operation. 具有读取关注"snapshot"的时间序列集合上的读取操作仅在读取操作中没有对集合进行并发删除或重命名操作的情况下保证快照隔离。Re-creating a time series collection on the same namespace with different granularity setting does not yield full snapshot isolation.在具有不同粒度设置的同一命名空间上重新创建时间序列集合不会产生完全的快照隔离。