Re: MySQL 5.1 does not seem to be using the index that I expect it to.

[Tom Muldoon]

The explain plan seems to be unchanged...

'1', 'SIMPLE', 'activity_event', 'ref', 'recipient_id__status__trigger_time__id,recipient_id__trigger_time__id', 'recipient_id__status__trigger_time__id', '8', 'const', '333', 'Using where; Using filesort'

... after trying the query you suggested...

EXPLAIN
select  id, trigger_time, note, type, status,
        notification_status, submission_id,
        actor_id, actor_display_name, actor_user_type,
        recipient_id, recipient_display_name,
        recipient_user_type, system_id,
    from  activity_event
    where  recipient_id=1
    order by  trigger_time;   -- note: just one field here.