You should go with option 1.
The main reason is that you say you are worried about performance - using the _id index which is always there and already unique will allow you to save having to maintain a second unique index.
For option 1, I'm worried about the insert performance do to having non sequential keys. I know this can kill traditional RDBMS systems and I've seen indications that this could be true in MongoDB as well.
Your other options do not avoid this problem, they just shift it from the _id index to the secondary unique index - but now you have two indexes, once that's right-balanced and the other one that's random access.
There is only one reason to question option 1 and that is if you plan to access the documents by just one or just the other UUID value. As long as you are always providing both values and (this part is very important) you always order them the same way in all your queries, then the _id index will be efficiently serving its full purpose.
As an elaboration on why you have to make sure you always order the two UUID values the same way, when comparing subdocuments { a:1, b:2 }
is not equal to { b:2, a:1 }
- you could have a collection where two documents had those values for _id. So if you store _id with field a first, then you must always keep that order in all of your documents and queries.
The other caution is that index on _id:1
will be usable for query:
db.collection.find({_id:{a:1,b:2}})
but it will not be usable for query
db.collection.find({"_id.a":1, "_id.b":2})