How much used diskspace will increase when doing Mongo Indexing?
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Let say, I have 1GB of data in format: {_id:ObjectId(), expiration:<Date>, value:<String>}.  let say around 1000000 records.

If I do an indexing on a non-existing field, e.g. "key", (  db.mytest.ensureIndex({key:1})  ), then in general, how many diskspace will it increase?

of I do indexing on the expiration date, how many will be increased?  