Re: Sudden drop in performance on MyISAM DB

[Rick James]

explain 
select  message_id
    from  email_message em, message m
    where  m.id=em.message_id
      and  m.sent_on is null    -- Missing from proposed index
      and  m.publisher_id=2     -- In index
    order by  m.created_on      -- Last in index

ALTER TABLE message ADD INDEX(publisher_id, created_on);

Jørgen, I am weak on this subtle issue...
Can you elaborate on when this pattern works and when it might not? I see that part of the index is being used for the WHERE filtering, but then it skips over to the ORDER BY.
Also (if you happen to know), I would like to know how far back in old versions this index would actually use both fields.