Mongodb – Choosing shard key with compound index field

Before you go further you need to answer a few questions

• how do you represent files within folders into the database
• how do you represent folders
• do you have relations between folders (parent -> child)
• how often do you expect to create folders and files
• how often do you update existing files into folders and what is the number of files you update

Based on your answers you can have a write optimized schema or a read optimized schema. Write optimized is a schema that contains many entries that are very small or you can use built in operators like $inc over a collection. Read optimized is generally a larger collection like the one you described, into your scenario you could have very easy something like this (assuming all folders are at the same level)

{ "userid" : "email or id",
  [ 
     { "folder1" : [ "file1", "file2"] },  
     { "folder2" : [ "file3", "file4"] },
  ]
}

But with this schema it gets quite complicated if you need to link a folder to a parent folder ... But is obvious that the userid is the shard key.

In order to deploy a shard key in your case (name, age) you need to pre-create an index on (name, age).

The shard key can be unique, but I don't see how a unique constraint fits here, for example isn't it quite likely to have two same records? (name:Mary age:26?).

You cannot enforce a unique constraint directly to the database without a compound index.

Index intersection is to combine two index on the run-time. For example, you have an index on name and one on age and a query {name='somename' and age='someage'}. Prior to 2.6, you needed a compound index on (name, age). On 2.6, the optimizer can use these individual indexes to execute the above query.