Establishing Relationships - Learning Apache Cassandra - page 107

Using secondary indexes to avoid
denormalization
So far, we've exclusively used primary key columns to look up rows—either the full
primary key when we're looking for a specific row, or just the partition key when retrieving
multiple rows in a single partition. We know that these kinds of lookups are very efficient,
because Cassandra can satisfy the query by accessing the single region of storage that holds
the partition's data in order.
This is the motivation for the denormalized follow structure we've built in this chapter:
whether we want to answer the question, "Who does alice follow?", or the question,
"Who follows alice ?", we can construct a query that only needs to access a single parti-
tion. However, we're accepting additional complexity in the form of storing two versions of
the same information, in user_inbound_follows and
user_outbound_follows .
As it happens, Cassandra does provide us with a way to answer both questions in a reason-
ably efficient way using a single table, with a single representation of each follow relation-
ship. By adding a secondary index , we can enable lookup of rows using columns other
than the primary key.