Wide column stores, or column-oriented database systems, are storage systems that store data by columns rather than by rows. For example, consider the following simple table:

In an RBDMS, the tuples would be stored row-wise, so the data on the disk would be stored as follows:

John,Smith,42|Bill,Cox,23|Jeff,Dean,35 

In online-transaction-processing (OLTP) applications, the I/O pattern is mostly reading and writing all of the values for entire records. As a result, row-wise storage is optimal for OLTP databases.

In a columnar database however, all of the columns are stored together. So, the tuples would be stored as follows:

John,Bill,Jeff|Smith,Cox,Dean|42,23,35 

The advantage here is that if we want to read values such as FirstName, reading one disk block reads a lot more information in the row-oriented case. Another advantage, since each block holds the similar type of data, is that we can use efficient compression for the block, further reducing disk space and I/O.

Such databases are useful in analytics explanations. Examples include Amazon Redshift and Vertica. The details are outside the scope of this book.