Two corrections:

1. Fossil repositories are explicitly not relational, they are however stored in SQLite databases. The data model for everything SCM-relevant (that also includes all content like tickets, wiki, forum) is stored as artifacts in a blob table (+ delta), which references other artifacts by hash value, and that provides the referential integrity. That, and the code that handles it. There are relations (via auxiliary tables) to speed up queries, but these tables are transient, get updated by inserting new artifacts, and can be regenerated from the artifacts.

(Users and their metadata, and configuration is not part of this scheme, so these tables might be viewed as relational tables. They are local-only; and not synced.)

See https://fossil-scm.org/home/doc/trunk/www/fossil-is-not-rela... and https://fossil-scm.org/home/doc/tip/www/theory1.wiki for more details.

2. There are no other databases like PostgreSQL to choose from.