Answer

char(24)

Details

The default unique identifier generated as the primary key (_id) for a MongoDB document is an ObjectId. This is a 12 byte binary value which is often represented as a 24 character hex string, and one of the standard field types supported by the MongoDB BSON specification.

The 12 bytes of an ObjectId are constructed using:

• a 4 byte value representing the seconds since the Unix epoch
• a 3 byte machine identifier
• a 2 byte process id
• a 3 byte counter (starting with a random value)