Mysql优化--Limit查询的优化考虑

本篇文章主要是结合我之前面试的各种经历和实战开发中遇到的问题解决经验整理的，希望这篇《Mysql优化--Limit查询的优化考虑》对你有很大帮助！欢迎收藏，分享给更多的需要的朋友学习~

在实际业务中对于分页来说是一个比较常见的业务需求。那么就会使用到

limit

offset

limit 100000,20

Limit

order by

order by

mysql>   SELECT * FROM student    LIMIT 1000000,1;
+---------+------------+------------+------------+-------+---------------------+
| id      | first_name | last_name  | created_at | score | updated_at          |
+---------+------------+------------+------------+-------+---------------------+
| 1000001 | kF9DxBgnUi | yLXnPSHJpH | 2019-07-11 |    97 | 2019-07-11 14:29:59 | |
+---------+------------+------------+------------+-------+---------------------+
1 rows in set (0.31 sec)

可以看到时间

mysql>  EXPLAIN SELECT score,first_name FROM student  ORDER BY created_at   LIMIT 1000000,20 \G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: student
   partitions: NULL
         type: index
possible_keys: NULL
          key: time_sorce_name
      key_len: 69
          ref: NULL
         rows: 1000001
     filtered: 100.00
        Extra: Using index
1 row in set, 1 warning (0.00 sec)

mysql>

这样的话查询的列使用到的了覆盖索引，扫描行数会减少很多，但是这样的效果也不是很尽人意，但是如果有其他的查询的话，这样的查询也会变的很慢。
比如我们加上

last_name

mysql>  SELECT score,first_name,last_name FROM student  ORDER BY created_at   LIMIT 1000000,1;
+-------+------------+------------+
| score | first_name | last_name  |
+-------+------------+------------+
|    86 | knKsV2g2fY | WB5qJeLZuk |
+-------+------------+------------+
1 row in set (4.81 sec)

mysql>

这个查询需要执行 4秒多的时间。通过分析可以看到这个查询是没有办法使用索引的

mysql> explain SELECT score,first_name,last_name FROM student  ORDER BY created_at   LIMIT 1000000,1\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: student
   partitions: NULL
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 6489221
     filtered: 100.00
        Extra: Using filesort
1 row in set, 1 warning (0.00 sec)

mysql>

那么我们现在把查询修改如下

mysql> SELECT student.score,student.first_name FROM student INNER JOIN (SELECT id FROM student  ORDER BY created_at   LIMIT 1000000,1 ) AS temp USING(id);
+-------+------------+
| score | first_name |
+-------+------------+
|    15 | 2QWZ       |
+-------+------------+
1 row in set (0.18 sec)

mysql> EXPLAIN SELECT student.score,student.first_name,last_name FROM student INNER JOIN (SELECT id FROM student  ORDER BY created_at   LIMIT 1000000,1 ) AS temp USING(id);
+----+-------------+------------+------------+--------+---------------+-----------------+---------+---------+---------+----------+-------------+
| id | select_type | table      | partitions | type   | possible_keys | key             | key_len | ref     | rows    | filtered | Extra       |
+----+-------------+------------+------------+--------+---------------+-----------------+---------+---------+---------+----------+-------------+
|  1 | PRIMARY     |  | NULL       | ALL    | NULL          | NULL            | NULL    | NULL    | 1000001 |   100.00 | NULL        |
|  1 | PRIMARY     | student    | NULL       | eq_ref | PRIMARY       | PRIMARY         | 4       | temp.id |       1 |   100.00 | NULL        |
|  2 | DERIVED     | student    | NULL       | index  | NULL          | time_sorce_name | 69      | NULL    | 1000001 |   100.00 | Using index |
+----+-------------+------------+------------+--------+---------------+-----------------+---------+---------+---------+----------+-------------+
3 rows in set, 1 warning (0.00 sec)

分析结果,可以看到这个时候只查询了1000001一条数据记录。为什么会有这样的变化呢。这种叫

延时关联

在某些特定的场合，其实有另外一种优化方案的。比如要获取最新的几条插入记录。那么在上一次查询的时候我们可以记录下最后一条记录的主键ID(

last_id

SELECT score,first_name,last_name,id FROM student  WHERE id>=last_id ORDER BY id ASC   LIMIT 1

比如

last_id=1000000

1000000

offset

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