MongoDB Compound Index to Optimize Update with Key and Range Condition

1. Somewhat. But not optimally.

2. Should be this really, so index on the "element" of the object in the _id key:

   db.sample.createIndex({ "_id.userId": 1, "timestamp": 1 })
   

3. Use the $set operator and stop overwiting your documents:

   db.sample.update(
       { 
           "_id.userId": "sample", 
           "firstTimestamp": { "$gt": 100 }
       },
       {
           "$set": { "otherfield": "cfg"  }
       }
   )
   

But really your data "should" look like this:

{
    "_id": "sample", 
    "firstTimestamp": 200,
    "otherfield2": "sam"
}

And update like:

    db.sample.update(
        { 
            "_id.userId": "sample", 
            "firstTimestamp": { "$gt": 100 }
        },
        {
            "$set": { 
                "fistTimetamp": 100,
                "otherfield2": "efg"
            }
        }
    )

Or if you insist that fields other than "_id" and "firstTimestamp" are going to change a lot, then rather do this:

{
    "_id": "sample", 
    "firstTimestamp": 200,
    "data": {
        "otherfield2": "sam"
    }
}

When if you just want to replace data then do:

    db.sample.update(
        { 
            "_id.userId": "sample", 
            "firstTimestamp": { "$gt": 100 }
        },
        {
            "$set": { 
                "fistTimetamp": 100,
                "data": {
                   "overwritingField": "efg"
                }
            }
        }
    )

Since "data" can be replaced as an entire object if you wish, or just update a single key:

    db.sample.update(
        { 
            "_id.userId": "sample", 
            "firstTimestamp": { "$gt": 100 }
        },
        {
            "$set": { 
                "fistTimetamp": 100,
                "data.newfield": "efg"
            }
        }
    )

In all cases, try to use the operators rather than replacing the whole object as it typically works out as more traffic and more load to the server.

But overall, what makes sense here is that the "userId" part "should" be the portion of the index that narrows down the results the most. So it definately goes before the timestamp, of which there should be a lot more possible values.

Compound primary keys are fine, but make sure you actually use them. A singular value would not make any sense and could just be assigned to _id. If you can just query on one field of they key as you are here, then you probably don't need a compound object as the primary key.

Your _id in the update suggests that you are getting exact matches for the _id therefore it is not a compound field with other keys. With this being the case, it should just a value in the _id itself.

Also a "range" is okay, but again consider that you are trying to match a single document ( well you don't mention "multi" anywhere ), so again questin why is it needed and either then go for an exact match or at "least" an upper limit.

The $set will "only" update the fields that you specifiy. I think you made a mistake in typing your question though, as the syntax for the "update" portion would not be valid. But use update operators anyway, as they send less traffic by sending a single field, or just the fields you intend to update.