MySQL SELECT/JOIN query cause deadlocks on production DB

Question

Following SQL query cause deadlocks on production MySQL DB on a Wordpress site with heavy load.

    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;

With exactly the same DB on a test environment the query is executed in 0.005 seconds. The mv_postmeta table has over 3 million rows and the mv_post has over 1 million rows.

Probably concurrent INSERTS cause this problem and maybe a DB cache overload.

I tried to do READ COMMITED through wordpress wp_query with no success.

Do anyone have a tip for this? The dealocks shuts down the site for around 2 minutes.