The design of a database management system is affected by the CAP theorem. Relational databases and NoSQL databases are not rivals but complementary. One can utilize different database categories in a single software application. In certain scenarios, one can use the key-value store as a cache engine on top of the relational database to gain performance.

Relational and object-relational databases are the market-dominant databases. Relational databases are based on the concept of relation and have a very robust mathematical model. Object-relational databases such as PostgreSQL overcome the limitations of relational databases by introducing complex data types, inheritance, and rich extensions.

Relational databases are based on the relation, tuple, and attribute concepts. They ensure data validity and consistency by employing several techniques such as entity integrity, constraints, referential integrity, and data normalization.

The next chapter, provides first-hand experience in installing the PostgreSQL server and client tools on different platforms. The next chapter also introduces PostgreSQL capabilities, such as out-of-the-box replication support and its very rich data types.