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

[Rick James]

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As for using the "wrong" index, I am puzzled. Please try one more thing:

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.