TTL Indexes索引

Create a TTL Index创建TTL索引

To create a TTL index, use createIndex(). Specify an index field that is either a date type or an array that contains date type values. Use the expireAfterSeconds option to specify a TTL value in seconds.要创建TTL索引，请使用createIndex()。指定一个索引字段，该字段可以是日期类型，也可以是包含日期类型值的数组。使用expireAfterSeconds选项以秒为单位指定TTL值。

The TTL index expireAfterSeconds value must be within 0 and 2147483647 inclusive.TTL索引expireAfterSeconds值必须在0和2147483647之间（包括0和21474647）。

For example, to create a TTL index on the lastModifiedDate field of the eventlog collection with a TTL value of 3600 seconds, use the following operation in mongosh:例如，要在eventlog集合的lastModifiedDate字段上创建TTL值为3600秒的TTL索引，请在mongosh中使用以下操作：

db.eventlog.createIndex(
   { "lastModifiedDate": 1 },
   { expireAfterSeconds: 3600 }
)

Starting in MongoDB 7.0, you can create partial TTL indexes on time series collections. 从MongoDB 7.0开始，您可以在时间序列集合上创建部分TTL索引。These indexes use the collection timeField as the key field, and require a partial filter expression on the metaField.这些索引使用集合timeField作为键字段，并要求在metaField上使用部分筛选器表达式。

Time series collections include an optional expireAfterSeconds field. If you don't set expireAfterSeconds, a TTL index with a partialFilterExpression lets you set an expiration period for documents that match the filter. 时间序列集合包括一个可选的expireAfterSeconds字段。如果不设置expireAfterSeconds，则使用带有partialFilterExpression的TTL索引可以为与筛选器匹配的文档设置过期时间。If you do set expireAfterSeconds, a partial TTL index lets you set a different, shorter expiration period for matching documents. You can only create a partialFilterExpression on the metaField.如果您设置了expireAfterSeconds，部分TTL索引允许您为匹配的文档设置不同的、更短的过期时间。您只能在metaField上创建partialFilterExpression。

This weather data time series collection deletes documents after 24 hours:此天气数据时间序列集合将在24小时后删除文档：

db.createCollection(
   "weather24h",
   {
      timeseries: {
         timeField: "timestamp",
         metaField: "sensor",
         granularity: "hours"
      },
      expireAfterSeconds: 86400
   }
)

This TTL index deletes documents from the MongoDB NYC headquarters weather sensor after 1 hour, instead of 24 hours:此TTL索引在1小时后而不是24小时后从MongoDB NYC总部天气传感器中删除文档：

db.eventlog.createIndex(
  { "timestamp": 1 },
  { partialFilterExpression: { "sensor": { $eq: "40.761873, -73.984287" } } },
  { expireAfterSeconds: 3600 } )

Convert a non-TTL single-field Index into a TTL Index将非TTL单字段索引转换为TTL索引

Starting in MongoDB 5.1, you can add the expireAfterSeconds option to an existing single-field index. To change a non-TTL single-field index to a TTL index, use the collMod database command:从MongoDB 5.1开始，您可以将expireAfterSeconds选项添加到现有的单字段索引中。要将非TTL单字段索引更改为TTL索引，请使用collMod数据库命令：

db.runCommand({
  "collMod": <collName>,
  "index": {
    "keyPattern": <keyPattern>,
    "expireAfterSeconds": <number>
  }
})

The following example converts a non-TTL single-field index with the pattern { "lastModifiedDate": 1 } into a TTL index:以下示例将模式为{ "lastModifiedDate": 1 }的非TTL单字段索引转换为TTL索引：

db.runCommand({
  "collMod": "tickets",
  "index": {
    "keyPattern": { "lastModifiedDate": 1 },
    "expireAfterSeconds": 100
  }
})

Change the expireAfterSeconds value for a TTL Index更改TTL索引的expireAfterSeconds值

To change the expireAfterSeconds value for a TTL Index, use the collMod database command:要更改TTL索引的expireAfterSeconds值，请使用collMod数据库命令：

db.runCommand({
  "collMod": <collName>,
  "index": {
    "keyPattern": <keyPattern>,
    "expireAfterSeconds": <number>
  }
})

The following example changes the expireAfterSeconds value for an index with the pattern { "lastModifiedDate": 1 } on the tickets collection:以下示例更改tickets集合上具有模式{ "lastModifiedDate": 1 }的索引的expireAfterSeconds值：

db.runCommand({
  "collMod": "tickets",
  "index": {
    "keyPattern": { "lastModifiedDate": 1 },
    "expireAfterSeconds": 100
  }
})

Behavior行为

TTL and Schema validationTTL和模式验证

Even though TTL indexes do not require schema validation, using validation can help ensure consistent behavior by standardizing the existence and format of the date field used for expiration.尽管TTL索引不需要模式验证，但使用验证可以通过标准化用于过期的日期字段的存在和格式来帮助确保一致的行为。

For example, you can use schema validation to enforce the presence of a lastModifiedDate field and ensure its value adheres to a valid date format:例如，您可以使用模式验证来强制lastModifiedDate字段的存在，并确保其值遵循有效的日期格式：

db.createCollection(
   "eventlog",
   {
      validator: {
         $jsonSchema: {
            bsonType: "object",
            required: [ "lastModifiedDate" ],
            properties: {
               lastModifiedDate: {
                  bsonType: "date",
                  description: "Must be a valid date."
               }
            }
         }
      }
   }
)

This schema validation rule ensures that:此模式验证规则可确保：

• Every document in the eventlog collection includes the lastModifiedDate field.eventlog集合中的每个文档都包含lastModifiedDate字段。
• The lastModifiedDate field contains a valid date value.lastModifiedDate字段包含有效的日期值。

Restrictions限制

• TTL indexes are single-field indexes. TTL索引是单字段索引。Compound indexes do not support TTL and ignore the expireAfterSeconds option.复合索引不支持TTL，并忽略expireAfterSeconds选项。
• The _id field does not support TTL indexes._id字段不支持TTL索引。
• Starting in MongoDB 7.0, you can create a partial TTL index on a time series collection's metaField. 从MongoDB 7.0开始，您可以在时间序列集合的metaField上创建部分TTL索引。In earlier MongoDB versions, you can only create a TTL index for a time series collection's timeField.在早期的MongoDB版本中，您只能为时间序列集合的timeField创建TTL索引。
• You cannot use createIndex() to change the value of expireAfterSeconds of an existing index. 您不能使用createIndex()更改现有索引的expireAfterSeconds值。Instead, use the collMod database command. 请改用collMod数据库命令。For details, see Change the expireAfterSeconds value for a TTL Index.有关详细信息，请参阅更改TTL索引的expireAfterSeconds值。
• If a non-TTL single-field index already exists for a field, you cannot create a TTL index on the same field because you cannot create indexes that have the same key specification and differ only by the options. 如果字段已存在非TTL单字段索引，则无法在同一字段上创建TTL索引，因为无法创建具有相同键规范且仅在选项上不同的索引。To change a non-TTL single-field index to a TTL index, use the collMod database command.要将非TTL单字段索引更改为TTL索引，请使用collMod数据库命令。