MariaDB from 2009, based on MySQL from 1995.

PostgreSQL from 1996, based on Postgres95 from 1995, based on POSTGRES from 1989, based on INGRES from 1974(?).

I wonder if any lines of 1970's or at least 1980's code still survive in some corner of the PostgeSQL code base or if everything has been rewritten at least once by now? Must have started out in K&R C, if it was even C?

We don't have the complete version history of postgres, so that's not easy to know. There definitely are still lines from Postgres95 that haven't been changed since the initial import into our repository.

Somewhere there's a CVS repository with some history from before the import into the current repository, but unfortunately there's a few years missing between that repository and the initial import. I've not done the work to analyze whether any lines from that historical repo still survive.