If you are reading this, you are probably most familiar with MS SQL Server out of all the databases out there.
We - might - be able to find a way to run the Linux version of MS SQL Server using FreeBSD's Linux Compatibility or in a Bhyve Jail, but until that is as easy as "pkg install ms-sql-server", we are probably better off just learning Postgresql.
I also suspect that when it comes to performance, Postgresql will outperform MS SQL in an apples to apples real world comparision and that it will be easier to maintain and administer on FreeBSD.
I welcome anyone who wants to put the effort into making MS SQL Sever for Linux run on FreeBSD, but personally, I'm not going to invest much time in figuring that out... at least not yet.