MySQL之SQL优化一

一、随机查询优化：
4.6G -rw-rw---- 1 mysql mysql 4.6G Oct  7 14:13 task_log.ibd
select sql_no_cache * from task_log order by rand() limit 1;
mysql> explain select sql_no_cache * from task_log order by rand() limit 1 ;
+----+-------------+----------+------+---------------+------+---------+------+---------+---------------------------------+
| id | select_type | table    | type | possible_keys | key  | key_len | ref  | rows    | Extra                           |
+----+-------------+----------+------+---------------+------+---------+------+---------+---------------------------------+
|  1 | SIMPLE      | task_log | ALL  | NULL          | NULL | NULL    | NULL | 8822652 | Using temporary; Using filesort |
+----+-------------+----------+------+---------------+------+---------+------+---------+---------------------------------+
1.考虑不周的初始方法(没有加入min(id) 查询总是返回id靠近最大的一些值)
SELECT *
FROM task_log AS t1 JOIN (SELECT ROUND(RAND() * (SELECT MAX(id) FROM task_log)) AS id) AS t2
WHERE t1.id >= t2.id
ORDER BY t1.id ASC LIMIT 1;
2.完善后的方法
select * from task_log  where id>=(
select floor(RAND()*((SELECT MAX(id) FROM task_log) - (SELECT MIN(id) FROM task_log)) + (SELECT MIN(id) FROM task_log))
) limit 1;
+----+-------------+----------+------+---------------+------+---------+------+---------+------------------------------+
| id | select_type | table    | type | possible_keys | key  | key_len | ref  | rows    | Extra                        |
+----+-------------+----------+------+---------------+------+---------+------+---------+------------------------------+
|  1 | PRIMARY     | task_log | ALL  | NULL          | NULL | NULL    | NULL | 8825321 | Using where                  |
|  5 | SUBQUERY    | NULL     | NULL | NULL          | NULL | NULL    | NULL |    NULL | Select tables optimized away |
|  4 | SUBQUERY    | NULL     | NULL | NULL          | NULL | NULL    | NULL |    NULL | Select tables optimized away |
|  3 | SUBQUERY    | NULL     | NULL | NULL          | NULL | NULL    | NULL |    NULL | Select tables optimized away |
+----+-------------+----------+------+---------------+------+---------+------+---------+------------------------------+

mysql> select sql_no_cache * from task_log  where id>=( select floor(RAND()*((SELECT MAX(id) FROM task_log) - (SELECT MIN(id) FROM task_log)) + (SELECT MIN(id) FROM task_log)) )  limit 1\G
*************************** 1. row ***************************
         id: 202439383
update_time: 20130907000344
       dist: 北京古都
    account: (undefined)
        gid:
  task_type: xiux
  task_name:
     action: jiangl
         ip: (undefined)
       line: 北京古都十线
      level: 0
        tao: 0
  gold_coin: 0
    insider: 0
      para1: 花妖
      para2: :5226584700060AE34F76:
      para3: 1
       memo: martial:66
1 row in set (0.08 sec)
二、分页limit优化：
缺点：数据必须是连续的，可以说不能有where条件，where条件会筛选数据，导致数据失去连续性。
优化的关键：当limit的offset 值到很大时，limit的效率就下降，要求找准这个阀值。
145M -rw-rw---- 1 mysql mysql 144M Oct  7 15:08 block_log.ibd
mysql> select count(*) from block_log ; 
+----------+
| count(*) |
+----------+
|   411923 |
select sql_no_cache * from block_log limit 401922,5;
5 rows in set (0.98 sec)
select * from block_log
where id between
         (select sql_no_cache id from block_log order by id asc limit 401923,1)
         and
         (select sql_no_cache id from block_log order by id asc limit 401927,1);
5 rows in set (0.34 sec)
select * from block_log
where id >= ( select sql_no_cache id from block_log order by id asc limit 401923,1)
limit 5;
5 rows in set (0.17 sec)