不要将Order By放入主查询中，而是将其包装起来，如下所示：
SELECT * FROM (
... your query
) ORDER BY `created at`
看一下查询计划。您会发现，在这种情况下，将在执行mtrt_items外部联接之前在表上执行排序。在我提供的部分重写中，排序是在外部联接之后应用的，并且在更小的集合上应用。
假设将LIMIT应用于大型集合（500,000？），您似乎可以在执行任何联接之前执行顶部操作。
SELECT * from (
SELECT
`id`, ... `created_at`, ...
ORDER BY `i`.`created_at` DESC
LIMIT 100 OFFSET 0) as i
LEFT JOIN `mtrt_users` AS `u` ON i.user_id =u.id
LEFT JOIN `twt_tweets_content` AS `t` ON t.id =i.id
LEFT JOIN `twt_users` AS `tu` ON t.user_id = tu.id
INNER JOIN `mtrt_items_searches` AS `r` ON i.id =r.item_id
INNER JOIN `mtrt_searches` AS `s` ON s.id =r.search_id
INNER JOIN `mtrt_searches_groups` AS `sg` ON sg.search_id =s.id
INNER JOIN `mtrt_search_groups` AS `g` ON sg.group_id =g.id
INNER JOIN `account_clients` AS `c` ON g.client_id =c.id
GROUP BY i.id
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