One of the beauties of FreeBSD is of course its support for ZFS. For systems with ZFS volumes and UFS root, it makes sense to keep database data on a ZFS volume. The steps to change the data directory used by PostgreSQL are below.

Find the data directory

psql -U postgres

Then show data_directory; to find out what your data directory is

• Stop the service with sudo service postgresql stop
• Copy the data across, using rsync to preserve permissions

sudo rsync -a /var/db/postgres/data11 /titanium/db/postgres

• Edit rc.conf to include the new directory

sudo vim /etc/rc.conf

Note that titanium is used as an example volume below.

postgresql_enable="YES"
postgresql_data="/titanium/db/postgres/data11"

• Update the home directory for the postgres user so it is the directory one above (i.e. containing) the data directory

sudo vipw /etc/passwd

postgres:*:770:770::0:0:PostgreSQL Daemon:/titanium/db/postgres:/bin/sh

• Start the service with sudo service postgresql stop

Versions used: FreeBSD 12.1-RELEASE-p4, PostgreSQL 11.7

Reference: https://forums.freebsd.org/threads/migrating-postgresql-data-directory.69108/