There is one exception to this rule, and that is when a secondary index is in use. A secondary index should be a familiar concept to anyone who has worked with a relational database; in the relational world, they're often just referred to as indexes. Put simply, secondary indexes allow us to perform a reasonably efficient lookup of rows using columns other than the partition key.
In our case, we'd like to look up rows in user_follows by providing a value for the follower_username row but omitting a value for the followed_username partition key. To do this, we can use the CREATE INDEX statement:
CREATE INDEX ON "user_follows" ("follower_username");
The syntax here is pretty straightforward: we simply provide Cassandra ...
