Comment by cachemiss

Cassandra is not a columnar database, columnar in this sense is about the storage layout. Values for a column are laid next to each other in storage, thus allowing for optimizations in compression and computation, at the expense of reconstructing entire rows. Postgres is a row store, meaning all the columns for a row are stored next to each other in storage, which makes sense if you need all of the values for a row (or the vast majority).