Scan operation for getting a list of hash keys in DynamoDB table?

I want to know whether I have to use a dynamodb "Scan" operation for getting a list of all hash key values in a dynamodb table or is there an another "less-expensive" approach to do that. I have tried with a "Query" operation, but it was unsuccessful in my case, since I have to define the table hash key to use this operation. I just want to get a list of all hash key values in the table.