What is the difference between Relational database management system (RDMS) and Columnar database management system (CDBMS)?

      A column database management system (CDBMS) is a database management system (DBMS) that re-orients the focus of data in a database from rows to columns. 

      In a relational database, data access is always by row Changing the focus from the row to the column can improve performance when large amounts of data are aggrigate in a few columns. Generally speaking, a row-orientated focus is preferable for online transaction processing (OLTP) systems and a column-oriented focus is preferable for online analytical processing (OLTP) systems. Column stores are not useful for systems with wildly varying quries or supporting ACID transactions.

Column stores are also useful when data has an expiration date because it is possible to set up a column so it will expire automatically after a certain date. Because the data stored in columns is typically similar, and the columns are stored adjacent to one another, some CDBMSes can compress data and help storge capcity to be used more efficiently. 

The concept of a column store is not new and variations of the idea have been implemented as part of relational databases in the past. NOSQL and relational column stores both focus on the column as the unit of storage, but NoSQL column stores permits columns to differ across column family rows, which is not permitted in a relational database.

A CDBMS may also be called a columner database management system, column-oriented database management system or column store database management system.