OAT and MON$

Just as a "for your information", should anybody be interested.
The following query on Firebird V2.x gets the attachment that holds the oldest active transaction via the monitoring tables.

SELECT *
FROM MON$ATTACHMENTS
where MON$ATTACHMENT_ID =
(select MON$ATTACHMENT_ID FROM MON$TRANSACTIONS
where MON$TRANSACTION_ID =
(SELECT min(MON$TRANSACTION_ID) FROM MON$TRANSACTIONS))