mysql回表简介

之前写的《mysql B+Tree索引的一点理解》一文中，介绍了MySQL在使用辅助索引的原理，通过辅助索引进行回表不难理解就相当于Oracle的index skip scan.但是mysql5.6版本中推出了mrr功能，其实就是将随机访问的数据，通过内部机制缓存到线程内存read_rnd_buffer_size中，然后进行排序，排序后的数据再访问主键索引，将随机访问改变为了顺序访问。近似理解为Oracle中的index range scan

一：优点

1.磁盘和磁头不再需要来回做机械运动

如果没有这个功能，那么每获取一个辅助索引的叶子块就会遍历一下主键，找到对应的数据--该过程我们又称为回表。
mrr功能，将这些辅助索引扫描后的数据同一进行缓存，然后一次性访问主键索引，然后找到对应的数据，这样就大大减少了访问数据块的数量

2.可以充分利用磁盘预读

mysql数据库有一个预读功能，也就是访问一个页的数据时，将临近页也会加载到内存中，刚好需要下一页的数据时就不再需要进行物理IO

二：案例演示

说明：本测试在mysql 5.7.35中进行测试。

1.表结构

Create Table: CREATE TABLE `salaries` (
  `emp_no` int(11) NOT NULL,
  `salary` int(11) NOT NULL,
  `from_date` date NOT NULL,
  `to_date` date NOT NULL,
  PRIMARY KEY (`emp_no`,`from_date`),
  KEY `idx_salaries_salary` (`salary`),
  CONSTRAINT `salaries_ibfk_1` FOREIGN KEY (`emp_no`) REFERENCES `employees` (`emp_no`) ON DELETE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8

2.优化器参数

root@localhost [employees]>show variables like '%optimizer_switch%'G;
*************************** 1. row ***************************
Variable_name: optimizer_switch
        Value: index_merge=on,index_merge_union=on,index_merge_sort_union=on,index_merge_intersection=on,engine_condition_pushdown=on,index_condition_pushdown=on,mrr=on,mrr_cost_based=on,block_nested_loop=on,batched_key_access=off,materialization=on,semijoin=on,loosescan=on,firstmatch=on,duplicateweedout=on,subquery_materialization_cost_based=on,use_index_extensions=on,condition_fanout_filter=on,derived_merge=on,prefer_ordering_index=on
1 row in set (0.00 sec)

3.查询SQL并查看执行计划

从这里并没有发现该执行步骤使用了mrr功能，还是每行检索之后访问主键索引，然后进行回表

root@localhost [employees]>explain select * from salaries where salary>10000 and salary<40000;
+----+-------------+----------+------------+-------+---------------------+---------------------+---------+------+-------+----------+-----------------------+
| id | select_type | table    | partitions | type  | possible_keys       | key                 | key_len | ref  | rows  | filtered | Extra                 |
+----+-------------+----------+------------+-------+---------------------+---------------------+---------+------+-------+----------+-----------------------+
|  1 | SIMPLE      | salaries | NULL       | range | idx_salaries_salary | idx_salaries_salary | 4       | NULL | 21450 |   100.00 | Using index condition |
+----+-------------+----------+------------+-------+---------------------+---------------------+---------+------+-------+----------+-----------------------+

4.关闭MySQL成本控制

root@localhost [employees]>set optimizer_switch='mrr_cost_based=off';
Query OK, 0 rows affected (0.00 sec)

root@localhost [employees]>show variables like '%optimizer_switch%'G;
*************************** 1. row ***************************
Variable_name: optimizer_switch
        Value: index_merge=on,index_merge_union=on,index_merge_sort_union=on,index_merge_intersection=on,engine_condition_pushdown=on,index_condition_pushdown=on,mrr=on,mrr_cost_based=off,block_nested_loop=on,batched_key_access=off,materialization=on,semijoin=on,loosescan=on,firstmatch=on,duplicateweedout=on,subquery_materialization_cost_based=on,use_index_extensions=on,condition_fanout_filter=on,derived_merge=on,prefer_ordering_index=on
1 row in set (0.00 sec)

5.再次执行查看执行计划

这时我们发现，执行计划已经使用了mrr功能，对辅助索引数据进行缓存之后，一次回表，

root@localhost [employees]>explain select * from salaries where salary>10000 and salary<40000;
+----+-------------+----------+------------+-------+---------------------+---------------------+---------+------+-------+----------+----------------------------------+
| id | select_type | table    | partitions | type  | possible_keys       | key                 | key_len | ref  | rows  | filtered | Extra                            |
+----+-------------+----------+------------+-------+---------------------+---------------------+---------+------+-------+----------+----------------------------------+
|  1 | SIMPLE      | salaries | NULL       | range | idx_salaries_salary | idx_salaries_salary | 4       | NULL | 21450 |   100.00 | Using index condition; Using MRR |
+----+-------------+----------+------------+-------+---------------------+---------------------+---------+------+-------+----------+----------------------------------+

但是上面基于成本MySQL为什么没有使用这种方式呢？

显然上面回表效率是高效的，但是MySQL优化器对于MRR功能又是相当的悲观。还是尽可能的选择索引扫描回表。这是我们需要注意的地方

6.mysql5.7版本之后支持hint

mysql5.7版本之后，我们可以使用hint的方式来强制SQL走mrr

root@localhost [employees]>explain select /*+ mrr(salaries) */ * from salaries where salary>10000 and salary<40000;
+----+-------------+----------+------------+-------+---------------------+---------------------+---------+------+-------+----------+----------------------------------+
| id | select_type | table    | partitions | type  | possible_keys       | key                 | key_len | ref  | rows  | filtered | Extra                            |
+----+-------------+----------+------------+-------+---------------------+---------------------+---------+------+-------+----------+----------------------------------+
|  1 | SIMPLE      | salaries | NULL       | range | idx_salaries_salary | idx_salaries_salary | 4       | NULL | 21450 |   100.00 | Using index condition; Using MRR |
+----+-------------+----------+------------+-------+---------------------+---------------------+---------+------+-------+----------+----------------------------------+