Columnar database should store group of columns together. But Cassandra stores data row-wise. SS Table will hold multiple rows of data mapped to their corresponding partition key. So I feel like Cassandra is a row wise data store like MySQL but has other benefits like "wide rows" and every columns are not necessarily to be present for all the rows and of course it's in memory . Please correct me if I'm wrong.
If you go to the Apache Cassandra project on GitHub, and scroll down to the "Executive Summary," you will get your answer:
Cassandra is a partitioned row store. Rows are organized into tables with a required primary key.
Partitioning means that Cassandra can distribute your data across multiple machines in an application-transparent matter. Cassandra will automatically repartition as machines are added and removed from the cluster.
Row store means that like relational databases, Cassandra organizes data by rows and columns.
"So I feel like Cassandra is a row wise data store"
And that would be correct.