How do you use a very long composite index in Cosmos DB?

I am trying to get the RU of my GET operations down, as I have loads of records but 99 percent of the time only want to return the most recent record.

So I added a Composite Index, this includes all of the fields that I'm querying on.

 [
            {
                "path": "/resourceIdentity/subscription",
                "order": "ascending"
            },
            {
                "path": "/resourceIdentity/resourceGroup",
                "order": "ascending"
            },
                        {
                "path": "/resourceIdentity/extensionProviderNamespace",
                "order": "ascending"
            },
                        {
                "path": "/resourceIdentity/extensionResourceType",
                "order": "ascending"
            },
                        {
                "path": "/resourceIdentity/extensionResourceName",
                "order": "ascending"
            },
             {
                "path": "/resourceIdentity/resourceType",
                "order": "ascending"
            },
            {
                "path": "/resource/systemData/createdAt",
                "order": "descending"
            }
        ]

These fields are all included as normal range indexes too. However even after giving it time to finish updating, my queries still run with literally the exact same RU cost and speed.

Here's what my query looks like, I'm just not sure how to specify that I want to use this composite index.

SELECT top 1  * FROM root WHERE ((((((root["resourceIdentity"]["subscription"] = "de453a71-7d81-493d-bce6-224fec7223a9") 
AND (LOWER(root["resourceIdentity"]["resourceGroup"]) = "newyears_group")) 
AND (LOWER(root["resourceIdentity"]["extensionProviderNamespace"]) = "x.compute")) 
AND (LOWER(root["resourceIdentity"]["extensionResourceType"]) = "virtualmachines")) 
AND (LOWER(root["resourceIdentity"]["extensionResourceName"]) = "newyearsro1")) 
AND (LOWER(root["resourceIdentity"]["resourceType"]) = "xResource/reports"))
order by  root["resource"]["systemData"]["createdAt"] desc

Is this just not the right time to use a composite index? Trying to make this as fast as possible

From the documentation it states,

> If a query filters on one or more properties and has different
> properties in the ORDER BY clause, it may be helpful to add the
> properties in the filter to the ORDER BY clause.

So in order to utilize the composite indexes, you need to include all of them in your ORDER BY clause in the same order as you defined in your composite index.
So can you try keeping the composite indexes as is but change your query to :

SELECT top 1  * FROM root WHERE ((((((root["resourceIdentity"]["subscription"] = "de453a71-7d81-493d-bce6-224fec7223a9") 
AND (LOWER(root["resourceIdentity"]["resourceGroup"]) = "newyears_group")) 
AND (LOWER(root["resourceIdentity"]["extensionProviderNamespace"]) = "x.compute")) 
AND (LOWER(root["resourceIdentity"]["extensionResourceType"]) = "virtualmachines")) 
AND (LOWER(root["resourceIdentity"]["extensionResourceName"]) = "newyearsro1")) 
AND (LOWER(root["resourceIdentity"]["resourceType"]) = "xResource/reports"))
order by root["resourceIdentity"]["subscription"] asc, root["resourceIdentity"]["resourceGroup"] asc, root["resourceIdentity"]["extensionProviderNamespace"] asc , root["resourceIdentity"]["extensionResourceType"] asc,  root["resourceIdentity"]["extensionResourceName"] asc, root["resourceIdentity"]["resourceType"] asc, root["resource"]["systemData"]["createdAt"] desc  

This should have no impact in your query results but it should start using your composite indexes more effectively.