In a sub-document, the keys may be common across most documents in the collection, or they may contain a set of random values. For example, using hostnames as the keys in the "hosts" sub-document.

{
    ...
    hosts: {
        server1a: { ... }
        server2a: { ... }
        server3a: { ... }
    }
    } 
    {
    ...
    hosts: {
        server1b: { ... }
        server2b: { ... }
        server3b: { ... }
    }
}

The schema analysis should recognize the above situation to avoid creating thousands of sub-fields in the schema.