一条SQL查询语句的优化（效率提升1000倍）

weike_student_wrong_questions表大概有150万行数据，其中(tid, cid)做了索引，weike_exam_files不到100行，tid做了索引。下面一条语句执行时间大概是1.3秒。

 

SELECT weike_student_wrong_questions.tid, name, exam_time,
update_time, cid
FROM weike_student_wrong_questions
INNER JOIN weike_exam_files
ON weike_student_wrong_questions.tid = weike_exam_files.tid
GROUP BY weike_student_wrong_questions.tid, cid
ORDER BY exam_time DESC, cid;

把语句换成下面之后，查询效率大概提高了1000倍。 

     

      SELECT name, exam_time, update_time, e.tid, cid
      FROM weike_exam_files e
      INNER JOIN
        (SELECT tid, cid
         FROM weike_student_wrong_questions
         GROUP BY tid, cid) tc
      ON e.tid = tc.tid
      ORDER BY exam_time DESC, cid;