title: Dynamodb with unique secondary/third index
body: <p>I'm getting ready to rebuild a database that has 3 different tables all containing the same data. the difference is the HashKey for each, UserId, UserName, Email. I'm trying to combine them all into one table as I think the redundancy is bad as well as slow. What I plan on doing is have UserId as the HashKey, and having UserName and Email as secondary indexes. I have not found a way to have dynamoDb force uniqueness on the secondary indexes, so plan on using conditional writes that check for uniqueness in those before writing to the database. With SQL this would be very easy, is there a better way of doing this in DynamoDb? I need to be able to look up a user based off of either of the three UserId, UserName and Email. I'd like to keep this to one table and not use another table that references the Email to UserId or UserName to UserId.</p>