Question: For the user who gave the most number of long reviews, what is his/her averge ratings of all review?
SQL: SELECT CAST(SUM(T1.review_stars) AS REAL) / COUNT(T1.review_stars) FROM Reviews AS T1 INNER JOIN Users AS T2 ON T1.user_id = T2.user_id WHERE T1.review_length LIKE 'Long' GROUP BY T1.user_id ORDER BY COUNT(T1.review_length) DESC LIMIT 1