Reversing clustering order at query time scans the entire partition

Cassandra allows you to sort your query results in reverse clustering order, but it comes at a cost. Cassandra has to read all partition rows/columns and sort them in-memory, making your queries slower and increasing load on the cluster.

The slowdown is proportional to the number of rows and columns you have, and it can become a serious problem if your partitions grow to thousands of rows and dozens of columns.

See more info on StackOverflow.