Steps to reproduce:

1. Change DBHost to 
   DBHost=/var/run/postgresql
2. Change postgresql.conf to listen_addresses = '' (no TCP, only unix socket)
3. Restart zabbix

Result:
See the result in below screenshots.

Zabbix server actually starts, but frontend says that zabbix wasn't able to conenct to the databse.

If I change the postgresql config to listen on TCP (listen_addresses = '*'), zabbix starts normally, but traffic can be seen on loopback device - which obviously means that zabbix talks to PSQL via TCP.

Expected:
No traffic on loopback device, and zabbix being able to connect to postgresql when postgresql does not listen on any TCP port

I am using version 6.2.3 which somehow isn't available to pick in the dropdown menu.