[got][/usr/local/etc/postfix]# cat mysql_virtual_mailboxes_maps.cf
user = postfix
password = postfix
dbname = postfix
hosts = localhost
query = SELECT maildir FROM mailbox WHERE username='%s' AND active = 1

[got][/usr/local/etc/postfix]# cat mysql_virtual_mailbox_limit_maps.cf
user = postfix
password = postfix
hosts = localhost
dbname = postfix
query = SELECT quota FROM mailbox WHERE username='%s' AND active = '1'

[got][/usr/local/etc/postfix]# cat mysql_virtual_domains_maps.cf
user = postfix
password = postfix
dbname = postfix
hosts = localhost
query = SELECT domain FROM domain WHERE domain='%s'

[got][/usr/local/etc/postfix]# cat mysql_virtual_alias_maps.cf
user = postfix
password = postfix
hosts = localhost
dbname = postfix
query = SELECT goto FROM alias WHERE address='%s' AND active = 1

[got][/usr/local/etc/postfix]# cat mysql_virtual_alias_domain_maps.cf
user = postfix
password = postfix
hosts = localhost
dbname = postfix
query = SELECT goto FROM alias,alias_domain WHERE alias_domain.alias_domain = '%d' and alias.address = CONCAT('%u', '@', alias_domain.target_domain) AND alias.active = 1 AND alias_domain.active='1'

[got][/usr/local/etc/postfix]# cat mysql_virtual_alias_domain_mailboxes_maps.cf
user = postfix
password = postfix
hosts = localhost
dbname = postfix
query = SELECT maildir FROM mailbox,alias_domain WHERE alias_domain.alias_domain = '%d' and mailbox.username = CONCAT('%u', '@', alias_domain.target_domain) AND mailbox.active = 1 AND alias_domain.active='1'

[got][/usr/local/etc/postfix]# cat mysql_virtual_alias_domain_catchall_maps.cf
# handles catch-all settings of target-domain
user = postfix
password = postfix
hosts = localhost
dbname = postfix
query  = SELECT goto FROM alias,alias_domain WHERE alias_domain.alias_domain = '%d' and alias.address = CONCAT('@', alias_domain.target_domain) AND alias.active = 1 AND alias_domain.active='1'

[got][/usr/local/etc/postfix]#

Relevant postfix main.cf portion:

virtual_mailbox_domains = proxy:mysql:/usr/local/etc/postfix/mysql_virtual_domains_maps.cf
virtual_alias_maps =
        proxy:mysql:/usr/local/etc/postfix/mysql_virtual_alias_maps.cf
        proxy:mysql:/usr/local/etc/postfix/mysql_virtual_alias_domain_maps.cf
        proxy:mysql:/usr/local/etc/postfix/mysql_virtual_alias_domain_catchall_maps.cf
virtual_mailbox_maps =
        proxy:mysql:/usr/local/etc/postfix/mysql_virtual_mailboxes_maps.cf
        proxy:mysql:/usr/local/etc/postfix/mysql_virtual_alias_domain_mailboxes_maps.cf
transport_maps = hash:/usr/local/etc/postfix/transport
vacation_destination_recipient_limit = 1

virtual_create_maildirsize = yes
virtual_mailbox_extended = yes
virtual_mailbox_limit_maps = mysql:/usr/local/etc/postfix/mysql_virtual_mailbox_limit_maps.cf
virtual_mailbox_limit_override = yes
virtual_maildir_limit_message = Sorry, the user's maildir has overdrawn their diskspace quota, please try again later.
virtual_overquota_bounce = yes

proxy_read_maps = $local_recipient_maps $mydestination $virtual_alias_maps
  $virtual_alias_domains $virtual_mailbox_maps $virtual_mailbox_domains
  $relay_recipient_maps $relay_domains $canonical_maps $sender_canonical_maps
  $recipient_canonical_maps $relocated_maps $mynetworks
  $virtual_mailbox_limit_maps