Indexing parallel arrays in Mongodb

问题

I am starting to use MongoDb C#, but have run into a slight issue.

So I have a document with 2 embedded collections(of distinct types). I want to search on fields of both of these collections however I have discovered that if I try to index the searchable fields on the 2 collections I get "cannot index parallel arrays". Reading the Mongodb documentations on multikey indexes I discovered that this is indeed a limitation.

My question is what is the normal work around regarding this issue? I cant really combine these collections since they are pretty distinct? What pattern should I follow?

public class Capture
{
        [BsonId]
        public Guid Id { get; set; }
        ...Some other fields
        public IList<CustomerInformation> CustomerInformations { get; set; }
        public IList<VehicleLicenseDisk> VehicleLicenseDisks { get; set; }
}

回答1:

Before talking about possible workarounds, I just want to highlight why MongoDB has chosen to enforce this restriction on indexing parallel arrays. When you index an array in MongoDB, it creates a multikey index with one key per array element. Therefore, if you create a compound index on two arrays, one with M distinct values and one with N distinct values, the index essentially has MN keys. This is very bad- it's nonlinear in the number of distinct array elements. Consider the amount of work it takes to maintain an index like this when you add or remove array elements.

OK, justification aside, to work around this restriction it will be helpful to use the current MongoDB version (2.6), which supports index intersection. One can create an index on CustomerInformations and VehicleLicenseDisks and then MongoDB can use both indices and intersect them to serve queries that have restrictions on both.

If you are, for whatever reason, stuck with MongoDB < 2.6, then your options are either to consider redesigning the schema or to depend on indexes that use at most one of the array fields.

回答2:

It will help if you think about MongoDB schema concerns in terms of MongoDB -- not in terms of programming language objects. Do the arrays really need to be arrays? Can they be replaced with concrete field names? In your case, why is CustomerInformation an array? What does Capture object really represent? You might have to split out, as an example, CustomerInformation into a separate collection where each record contains a link/reference back to the Capture document it belongs to. I dont know the details of what you are trying to model, but whatever it is, forget about object oriented programming and put on a MongoDB hat -- objects will come later.

来源：https://stackoverflow.com/questions/25059525/indexing-parallel-arrays-in-mongodb