ScanIndexForward is the correct way to get items in descending order by the range key of the table or index you are querying. From the AWS API Reference:
A value that specifies ascending (true) or descending (false)
traversal of the index. DynamoDB returns results reflecting the
requested order determined by the range key. If the data type is
Number, the results are returned in numeric order. For type String,
the results are returned in order of ASCII character code values. For
type Binary, DynamoDB treats each byte of the binary data as unsigned
when it compares binary values.
Based on the docs for Scan, I conclude that there is no way to Scan in reverse. However, I would say that you are not using DynamoDB correctly if you need to do that. When designing a schema for a database like DyanmoDB you should plan the schema based on your expected queries to ensure that almost all application queries have a good index. Scans are meant more for sys admin operations or for feeding into MapReduce or analytics. "A Scan operation always scans the entire table, then filters out values to provide the desired result, essentially adding the extra step of removing data from the result set." (Query and Scan Performance) That can lead to performance problems and other issues.
Using DynamoDB is fundamentally different from working with a traditional relational database and requires a big change in the way you think about using it. You need to decide whether DynamoDB's advantages of availability in storage and performance, reliability and availability are worth accepting its limitations.
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