doveadm director userdb

[G H]

How do you configure an iterate_query SQL lookup for the doveadm director?

I tried:
service director {
 userdb {
  ...
 }
}

but userdb is not a valid stanza for director. I do not have a default userdb 
stanza, all my userdb definitions are specified in service imap/lda/lmtp blocks.

Thanks.

G H