I know a little late to the races, but will add this anyways.

You'd want to create a Global Secondary Index. This will allow you to do queries based off of any item attribute.

Scans do not scale well, and as your table grows...it will greatly increase your back end's fetch time as it goes through every record.

GSI's create a secondary 'lookup table' which operates much faster. The link below will get you started.

https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/GSI.html