PostgreSQL Forks

A fork is an independent development of a software project based on another project. there are more than 20 PostgreSQL forks; PostgreSQL extensible APIs make PostgreSQL a great candidate for forking. Over the years, many groups forked PostgreSQL and contributed their findings to PostgreSQL.

The following is a list of the popular PostgreSQL forks:

Greenplum is built on the foundation of PostgreSQL. It utilizes the shared-nothing and massively parallel processing (MPP) architectures. It is used as a data warehouse and for analytical workloads. Greenplum started as proprietary software and open-sourced in 2015.

The EnterpriseDB Advanced Server is a proprietary DBMS that provides Oracle with the capability to cap the oracle fees.

Vertica is a column-oriented database system that was started by Michael Stonebraker in 2005 and was acquisitioned by HP in 2011. Vertica reused the SQL parser, semantic analyzer, and standard SQL rewrites from the PostgreSQL implementation.

Netezza, a popular data warehouse appliances solution, was started as a PostgreSQL fork.

Further reading here 

Better Handling of Massive Amounts of Data

The following table describes various hard limits of PostgreSQL. However, practical limits, such as performance limitations or available disk space may apply before absolute hard limits are reached