索引MongoDB最初不存在的字段

Question

I have a collection where in initially we have a following structure of a review document

{
_id:ObjectId(...),
comment:"hi, test comment",
crBy:ObjectId(...)
}

Now when then admin reviews this, a status field gets added to it updating the doc to

   {
    _id:ObjectId(...),
    comment:"hi, test comment",
    crBy:ObjectId(...),
    status:"approved"
    }

For some reasons I can't have status initially and it has to be put in when an admin reviews it and it comes with a value approved or disapproved

So now the query to get all reviews created by a user and are approved would be like .... db.reviews.find({"crBy":ObjectId(...),"status":"approved"})

I am optimizing the reads by creating an index including both crBy and status fields

My questions are:

1.Am i creating the index in the right way?

2. I have read that indexes should contain fields that don't change. So in my case status field gets inserted afterwards. How does it
impact my performance?

3. Considering my case, what do you suggest to be the best possible way to optimize reads in such a scenario where a field comes later
on?

Thanks in advance

Answer 1

There is nothing stopping you from creating an index on a key that does not exist. Given your query, your index should mirror it

db.collection.ensureIndex({crBy: 1, status: 1}, {background: 1})

I've added in the background option. This will ensure that while rebuilding this index, it will not block other operations. It will still have an effect on the performance of the update to status. The only reason it's suggested to make an index on something that doesnt change, is because of the overhead of rebuilding the index upon write, in this case, you'll be gaining read speed because of this index, so it is worth it.