When you have a logical table with a bunch of rows and columns, there are multiple ways in which they can be stored physically on a disk.
You can store the contents of entire rows together so that all of the columns of a given row would be stored together. This works really well if the access pattern accesses a lot of the columns for a given set of rows. MySQL uses such a row-oriented storage model.
On the other hand, you could store the contents of entire columns together. In this scheme, all of the values from all of the rows for a given column can be stored together. This is really optimized for analytic use cases where you might need to scan through the entire table for a small set of columns. Storing data as column vectors allows for better compression (since there is less entropy between values within a column than there is between the values across a column). Also, these column vectors can be retrieved from a disk and processed quickly in a vectorized fashion through the SIMD capabilities of modern processors. SIMD processing on column vectors can approach throughputs of a billion data points/sec on a personal laptop.
Hybrid schemes are possible as well. Rather than storing an entire column vector together, it is possible to first break up all of the rows in a table into distinct row groups, and then, within a row group, you could store all of the column vectors together. Parquet and ORC use such a data placement strategy.
Another variant is that data is stored row-wise, but the rows are divided into row groups such that a row group is assigned to a shard. Within a row group, groups of columns that are often queried together, called column families, are then stored physically together on the disk. This storage model is used by HBase and is discussed in more detail in Chapter 6, HBase.
