.. _es-guide-reference-mapping-uid-field:

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Uid Field
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Each document indexed is associated with an id and a type, the internal **_uid** field is the unique identifier of a document within an index and is composed of the type and the id (meaning that different types can have the same id and still maintain uniqueness).


The **_uid** field is automatically used when **_type** is not indexed to perform type based filtering, and does not require the **_id** to be indexed.