Why do I need MySQL whena I want to have OpenLDAP backend?

==== Required information ====
- iRedMail version: 0.8.4
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): LDAP!!!
- Linux/BSD distribution name and version: FreeBSD 9.1
- Related log if you're reporting an issue: see subject
====
I have chosen LDAP backend, but MySQL gets installed as well.
If a sql db is needed anyway for some reason, can I have postgresql instead?

Thanks