> For the complete documentation index, see [llms.txt](https://xiaoxiami.gitbook.io/elasticsearch/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://xiaoxiami.gitbook.io/elasticsearch/ji-chu/mapping/322meta-fieldsff08-yuan-zi-duan-ff09.md).
# 3.2.2.Meta-Fields（元字段）
每个文档都有与之关联的元数据，比如 `_index`，`_type` 和 `_id` 元字段。当创建 mapping type（映射类型）时，可以自定义一些这些元字段的行为。
## Identity meta-fields（身份的元字段）
| [\_index](https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping-index-field.html) | 文档所属的索引。 |
| --------------------------------------------------------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------- |
| [\_uid](https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping-uid-field.html) | 复合字段包含了**\_type** 和**\_id**。 |
| [\_type](https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping-type-field.html) | 文档的[mapping type（映射类型 ）](https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping.html#mapping-type)。 |
| [\_id](https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping-id-field.html) | 文档 ID。 |
## Document source meta-fields（文档源的元字段）
| \_source | 原始的JSON 字符串表示文档的正文。 |
| -------- | ------------------------------------------------------------------------------------------------------------------------ |
| \_size | \_source 字段的大小（以字节为单位），由[ mapper-size](https://www.elastic.co/guide/en/elasticsearch/plugins/5.3/mapper-size.html) 插件提供。 |
## Indexing meta-fields（索引的元字段）
| \_all | 一个索引其他全部字段值的全能字段。 |
| -------------- | ----------------- |
| \_field\_names | 文档中包含非空值的所有字段。 |
## Routing meta-fields（路由的元字段）
| \_parent | 用于创建两个映射类型之间的父子关系。 |
| --------- | ------------------ |
| \_routing | 将文档路由到特定分片的自定义路由值。 |
## Other meta-field（其他的元字段）
| \_meta | 特定应用的元数据。 |
| ------ | --------- |
---
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