Description
Simplified greatly, DBMS systems are challenged with storing tabular data made up of rows and columns. Further, the set of columns is generally fixed by program design, while the number of rows is variable through use of the program. In one simple example, DBMS data might contain the following table.
EmpId Lastname Firstname Salary
1 Smith Joe 40000
2 Jones Mary 50000
3 Johnson Cathy 44000
This simple table includes an employee identifier (EmpId), name fields (Lastname and Firstname) and a salary (Salary).
Modern computer systems simplify the details of underlying storage down to a one-dimensional storage space, using a single numerical address to refer to a location. This is true for random access memory, hard disk drives, and other computer storage media. DBMS systems therefore must orient their data by flattening it onto a one-dimensional address space. Combine the common expectation for a fixed number of columns and a variable number of rows with the mapping from two dimensions to one, and we arrive at a column or row oriented classification.
A row-oriented implementation of a DBMS would store every attribute of a given row in sequence, with the last entry of one row followed by the first entry of the next.
1,Smith,Joe,40000;2,Jones,Mary,50000;3,Johnson,Cathy,44000;
A column-oriented implementation of a DBMS would store every attribute of a given column in sequence, with the column values for the same column stored in sequence, with the end of one column followed by the beginning of the next.
1,2,3;Smith,Jones,Johnson;Joe,Mary,Cathy;40000,50000,44000;
It is important to recognize that modern DBMS systems are not so simply column-oriented or row-oriented as in the simple example above, since partitioning, indexing, caching, materializing views, OLAP cubes, and transactional systems such as write ahead logging or multiversion concurrency control all dramatically affect physical organization of DBMS data. That said, OLTP focused RDBMS systems are more row-oriented, while OLAP focused systems are a balance of row-oriented and column-oriented.