when deinstalled but with purge case the packages of zabbix-server postrm process do not remove the database:

dbconfig-common: dropping pgsql database zabbix.
dropping database zabbix: does not exist.
dbconfig-common: revoking privileges for user zabbix on zabbix.
revoking access to database zabbix from zabbix@localhost: failed.

i'll try to also post a solution here but i cannot see the problem when occurs.. so i only post the error when uninstall

for mysql case do not have errors... so this are my suspect.. i dont know why this poor dbms (related to scaled dbms such like oracle or postgres) have so support and do not take cares on cases such postgresql