#1349 new
Kevin Watt

HATBM uses group by & order by by default unnecessarily, which is very bad for performance

Reported by Kevin Watt | June 30th, 2010 @ 06:40 PM

Groups HABTM Users. Calling group.users loads this SQL:

~ (0.000226) SELECT id, title, group_apps FROM groups WHERE id = 1 ORDER BY id LIMIT 1

~ (0.000272) SELECT users.id FROM users INNER JOIN groups_users ON users.id = groups_users.user_id INNER JOIN groups ON groups_users.group_id = groups.id WHERE groups_users.group_id = 1 GROUP BY users.id ORDER BY users.id

The Group by / Order by causes this in the explain for the query:
Using index; Using temporary; Using filesort. Which is very slow for large result sets.

I'd be interested in looking into this myself, but I can't figure out where the group by / order by are being loaded from, as searching the dm-core source for it doesn't return anything relevant.

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