Greenplum (https://github.com/greenplum-db/gpdb) is fork of PostgreSQL. Greenplum team performs merge with PostgreSQL major/minor versions into Greenplum to keep Greenplum codebase up to speed.
This repository is used as development space for these merge iterations. Plus it act as archive for all past merge iterations frequently referenced by Greenplum Developers. This repository houses the iterations branches which track individual commits during PostgreSQL merge work to Greenplum. No production code directly ships from this repository. Once the merge work in complete, Pull Request is raised against https://github.com/greenplum-db/gpdb repository and code merged via merge commit to GPDB repository. Given this purpose, this repository doesn't house any "main" branch as each iteration branch have their own originating points.