[Dovecot] userdb syntax

[suzuki]

Hello,

I have a question about userdb lookup when using expire-tool.

Dovecot version 1.2.15
In dovecot.conf,
  mail_location = maildir:/var/mail/%u:INDEX=/var/indexes/%u

I run dovecot expire-tool, target user's directory becomes
executing user's one.
According to wiki, userdb setting overrides mail_location.

  userdb passwd {
    mail=maildir:/var/mail/%u:INDEX=/var/indexes/%u
  }

With this setting, it seems working.
But, there are no examples with ":INDEX=.." setting.
Is this syntax right?

Regards