PostgreSQL is a relational database, open source, and very powerful. It has over 20 years of development and has a proved and reliable architecture. It is commonly adopted in many applications because of the following advantages:
An open-source SQL standard compliant RDBMS
Strong community
Strong third-party support
Extensible
Objective
PostgreSQL is provided on Heroku via an add-on. This add-on has a free initial plan, only for small applications such as websites or blogs and then is available at a given series of paid plans, according to your need.
Each year a new version of PostgreSQL is released and Heroku adopts the newest version as standard in a very short time. Heroku keeps the current version and the last three versions. The versions currently available are:
9.4 (beta)
9.3 (default)
9.2
9.1
9.0 (deprecated, support ended December 3, 2014)
This means that every three years you need to upgrade the version of PostgreSQL used in your application because a new version of PostgreSQL is available and the last one is deprecated.