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5.3 Understanding Document IDs

This sections describes in detail how document IDs are generated and how to interpret them. X DevAPI relies on server-based document ID generation, which results in sequentially increasing document IDs across all clients. InnoDB uses the document ID as a primary key, resulting in efficient page splits and tree reorganizations.

This section describes the properties and format of the automatically generated document IDs.

Document ID Properties

The _id field of a document behaves in the same way as any other fields of the document during queries, except that its value cannot be changed once it has been inserted to the collection. The _id field is used as the primary key of the collection . It is possible to override the automatic generation of document IDs by manually including an ID in an inserted document.


X Plugin is not aware of the data inserted into the collection, including any manual document IDs you use. When using manual document IDs, you must ensure that they do not clash with any IDs that might ever be generated automatically by the server (see Document ID Generation for details), in order to avoid any errors due to primary key duplication.

Whenever an _id field value is not present in an inserted document, the server generates an _id value. The generated _id value used for a document is returned to the client as part of the Result (Result for Connector/J) object of the add() operation. If you are using X DevAPI on an InnoDB Cluster, the automatically generated _id must be unique within the whole cluster. By setting the mysqlx_document_id_unique_prefix to a unique value per cluster instance, you can ensure document IDs are unique across all the instances.

The _id field must be sequential (always incrementing) for optimal InnoDB insertion performance (at least within a single server). The sequential nature of _id values is maintained across server restarts.

In a multi-primary Group Replication or InnoDB Cluster environment, the generated _id values of a table are unique across instances to avoid primary key conflicts and minimize transaction certification.

Document ID Generation

This section describes how document IDs are formatted.

The format of automatically generated document ID is:

unique_prefix start_timestamp serial
4 bytes 8 bytes 16 bytes


  • unique_prefix is a value assigned by InnoDB Cluster to the instance, which is used to make the document ID unique across all instances from the same cluster. The range of unique_prefix is from 0 to 216-1, which is hex encoded. Default value is 0, if it is neither set by InnoDB Cluster nor by the mysqlx_document_id_unique_prefix system variable.

  • start_timestamp is the time stamp of the startup time of the server instance, which is hex encoded. In the unlikely event that the value of serial overflows, the start_timestamp is incremented by 1 and the serial value then restarts at 0.

  • serial is a per-instance automatically incremented integer serial number value, which is hex encoded and has a range of 0 to 264-1. The initial value of serial is set to the auto_increment_offset system variable, and the increment of the value is set by the auto_increment_increment system variable.

This document ID format ensures that:

  • The primary key value monotonically increments for inserts originating from a single server instance, although the interval between values is not uniform within a table.

  • When using multi-primary Group Replication or InnoDB Cluster, inserts to the same table from different instances do not have conflicting primary key values, as long as the instances have the auto_increment_offset and the auto_increment_increment system variables configured properly (see descriptions of the variables for details).