Optimizing Object IDs

The _id field in a MongoDB document is very important and is always indexed for normal collections. This page lists some recommendations. Note that it is common to use the BSON ObjectID datatype for _id's, but the values of an _id field can be of any type.

Use the collections 'natural primary key' in the _id field.

_id's can be any type, so if your objects have a natural unique identifier, consider using that in _id to both save space and avoid an additional index.

When possible, use _id values that are roughly in ascending order.

If the _id's are in a somewhat well defined order, on inserts the entire b-tree for the _id index need not be loaded. BSON ObjectIds have this property.

Store Binary GUIDs as BinData, rather than as hex encoded strings

BSON includes a binary data datatype for storing byte arrays. Using this will make the id values, and their respective keys in the _id index, twice as small.

Note that unlike the BSON Object ID type (see above), most UUIDs do not have a rough ascending order, which creates additional caching needs for their index.

> // mongo shell bindata info:
> help misc
        b = new BinData(subtype,base64str)  create a BSON BinData value
        b.subtype()                         the BinData subtype (0..255)
        b.length()                          length of the BinData data in bytes
        b.hex()                             the data as a hex encoded string
        b.base64()                          the data as a base 64 encoded string
        b.toString()

Extract insertion times from _id rather than having a separate timestamp field.

The BSON ObjectId format provides documents with a creation timestamp (one second granularity) for free. Almost all drivers implement methods for extracting these timestamps; see the relevant api docs for details. In the shell:

> // mongo shell ObjectId methods
> help misc
        o = new ObjectId()      create a new ObjectId
        o.getTimestamp()        return timestamp derived from first 32 bits of the OID
        o.isObjectId()
        o.toString()
        o.equals(otherid)

Sort by _id to sort by insertion time

BSON ObjectId's begin with a timestamp. Thus sorting by _id, when using the ObjectID type, results in sorting by time. Note: granularity of the timestamp portion of the ObjectID is to one second only.

> // get 10 newest items
> db.mycollection.find().sort({id:-1}).limit(10); 

See Also

• Object IDs