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java.sql.SQLException: Out of sort memory, consider increasing server sort buffer size

Query being executed when exception was thrown:
SELECT t.topic_id, t.topic_title AS subject, p.post_id, t.forum_id, pt.post_text, p.post_time, p.user_id, u.username FROM jforum_topics t, jforum_posts p, jforum_posts_text pt, jforum_users u WHERE p.post_id = t.topic_first_post_id AND p.topic_id = t.topic_id AND p.user_id = u.user_id AND p.post_id = pt.post_id AND p.need_moderate = 0  ORDER BY topic_first_post_id DESC LIMIT 15