Book Image

Learning Apache Cassandra - Second Edition

Book Image

Learning Apache Cassandra - Second Edition

Overview of this book

Cassandra is a distributed database that stands out thanks to its robust feature set and intuitive interface, while providing high availability and scalability of a distributed data store. This book will introduce you to the rich feature set offered by Cassandra, and empower you to create and manage a highly scalable, performant and fault-tolerant database layer. The book starts by explaining the new features implemented in Cassandra 3.x and get you set up with Cassandra. Then you’ll walk through data modeling in Cassandra and the rich feature set available to design a flexible schema. Next you’ll learn to create tables with composite partition keys, collections and user-defined types and get to know different methods to avoid denormalization of data. You will then proceed to create user-defined functions and aggregates in Cassandra. Then, you will set up a multi node cluster and see how the dynamics of Cassandra change with it. Finally, you will implement some application-level optimizations using a Java client. By the end of this book, you'll be fully equipped to build powerful, scalable Cassandra database layers for your applications.
Table of Contents (14 chapters)

Reversing the order of rows


We're on our way to building great data access logic for displaying a user's timeline of status updates. There's one minor problem though: if I'm viewing alice's status updates, I'm probably interested in reading her most recent ones first. So far, we've always gotten the status updates in ascending order of id, meaning the oldest ones come first. Fortunately, we've got a couple of ways to change this, reversing the order of rows.

Reversing clustering order at query time

Using our existing user_status_updates table, we can instruct Cassandra to return results in reverse order of id:

SELECT "id", DATEOF("id"), "body"
FROM "user_status_updates"
WHERE "username" = 'alice'
ORDER BY "id" DESC;

This is the first time we've seen an ORDER BY in CQL, but it should be familiar to anyone who's worked with a SQL database: the DESC tells Cassandra that we want to order rows by descending values in the id column:

You might assume that the ORDER BY gives us a lot of flexibility in...