[Dovecot] Expire plugin with Mysql

Hey guys, got a question about the expire plugin. According to the wiki, the expire plugin "keeps an internal database (Berkeley DB or SQL)". I was wondering what fields in the mysql table are needed for expire to work. I assume it is setup is similar to the quota dict setup, so was just wondering what fields are required (i.e: select_field, where_field, username_field, for quota).

Thanks