The equivalent in a relational database would be writing a query without a where statement, and then implementing the where in application code. AWS has a lot of unimpressive bits, but DynamoDB is not one of them, you just have to know how to use it properly (like any database!), and design your schema for the database (which isn't always easy). In general, using scan means you aren't using Dynamo correctly.

Caveat: I used to work at Amazon (not in AWS), but don't anymore.