To limit the Items returned rather than returning all Items with a particular HASH key.
There are two different ways we can handle this
The ideal way is to build the element we want to query into the RANGE key. This allows us to use Key Expressions to query our data, allowing DynamoDB to quickly find the Items that satisfy our Query.
A second way to handle this is with filtering based on non-key attributes. This is less efficient than Key Expressions but can still be helpful in the right situations. Filter expressions are used to apply server-side filters on Item attributes before they are returned to the client making the call. Filtering is Applied after DynamoDB Query is completed . If you retrieve 100KB of data in Query step but filter it down to 1KB of data, you will consume the Read Capacity Units for 100KB of data
Moral is - Filtering and projection expressions aren't a magic bullet - they won't make it easy to quickly query your data in additional ways. However, they can save network transfer time by limiting the number and size of items transferred back to your network. They can also simplify application complexity by pre-filtering your results rather than requiring application-side filtering.
From dynamodbguide
dynamodbguide
