MySQL SELECT / JOIN查询导致生产数据库死锁

Question

以下SQL查询导致Wordpress站点上生产MySQL数据库的负载很重。

    SELECT      
        lessons.ID AS lesson_id,
        resultmeta.meta_value AS status,
        lessons.post_title AS title,
        lessons.post_name AS slug,
        count(resultmeta.meta_value) AS nr_of_results
    FROM        
        mv_posts AS lessons
    LEFT JOIN   
        mv_posts AS results
    ON      
        results.post_parent = lessons.ID AND
        results.post_type = 'we_result' AND
        results.post_author IN($students_string)
    LEFT JOIN   
        mv_postmeta AS resultmeta
    ON      
        results.ID = resultmeta.post_id AND
        resultmeta.meta_key = 'we_result_status_code'AND
        results.post_author IN($students_string)
    WHERE       
        lessons.post_type = 'we_lesson' AND
        lessons.post_status = 'publish' AND
        lessons.ID IN($lessons_id)
    GROUP BY
        lessons.ID,
        resultmeta.meta_value;

在测试环境中使用完全相同的DB，查询将在0.005秒内执行。 mv_postmeta表有超过300万行，mv_post有超过100万行。

可能并发INSERTS导致此问题，并且可能是数据库缓存过载。

我尝试通过wordpress wp_query做READ COMMITED但没有成功。

有人有这方面的提示吗？这些交易关闭了网站大约2分钟。