作者：蒋乐兴

MySQL DBA，擅长 python 和 SQL，目前维护着 github 的两个开源项目：mysqltools 、dbmc 以及独立博客：https://www.sqlpy.com。

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MRR 要解决的问题

MRR 是 MySQL 针对特定查询的一种优化手段。假设一个查询有二级索引可用，读完二级索引后要回表才能查到那些不在当前二级索引上的列值，由于二级索引上引用的主键值不一定是有序的，因此就有可能造成大量的随机 IO，如果回表前把主键值给它排一下序，那么在回表的时候就可以用顺序 IO 取代原本的随机 IO。

环境准备

为了实验我们要准备一下表结构和数据。

-- 创建表

mysql> show create table t;

+----------------------------------------------------------------------+

| Table | Create Table |

+----------------------------------------------------------------------+

| t | CREATE TABLE `t` (

`id` int NOT NULL AUTO_INCREMENT,

`i0` int NOT NULL,

`i1` int NOT NULL,

`i2` int NOT NULL,

`i3` int NOT NULL,

`c0` varchar(128) NOT NULL,

`c1` varchar(128) NOT NULL,

`f0` float NOT NULL,

`f1` float NOT NULL,

PRIMARY KEY (`id`),

KEY `idx_i0` (`i0`)

) ENGINE=InnoDB

+----------------------------------------------------------------------+

1 row in set (0.00 sec)

-- 造数据

mysql> select count(*) from t;

+----------+

| count(*) |

+----------+

| 1120000 |

+----------+

1 row in set (0.77 sec)

--

update t set i0 = id % 100;

MRR 的优化效果

1. 有 MRR 优化(Using MRR)时 SQL 的耗时情况。

mysql> explain select i0,i3 from t where i0 between 1 and 2;

+----+-------------+-------+------------+-------+---------------+--------+---------+------+-------+----------+----------------------------------+

| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |

+----+-------------+-------+------------+-------+---------------+--------+---------+------+-------+----------+----------------------------------+

| 1 | SIMPLE | t | NULL | range | idx_i0 | idx_i0 | 4 | NULL | 43968 | 100.00 | Using index condition; Using MRR |

+----+-------------+-------+------------+-------+---------------+--------+---------+------+-------+----------+----------------------------------+

1 row in set, 1 warning (0.00 sec)

mysql> select i0,i3 from t where i0 between 1 and 2;

22400 rows in set (0.80 sec)

2. 关闭 MRR 优化。

set optimizer_switch = '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=off,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,use_invisible_indexes=off,skip_scan=on,hash_join=on';

mysql> explain select i0,i3 from t where i0 between 1 and 2;

+----+-------------+-------+------------+-------+---------------+--------+---------+------+-------+----------+-----------------------+

| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |

+----+-------------+-------+------------+-------+---------------+--------+---------+------+-------+----------+-----------------------+

| 1 | SIMPLE | t | NULL | range | idx_i0 | idx_i0 | 4 | NULL | 43968 | 100.00 | Using index condition |

+----+-------------+-------+------------+-------+---------------+--------+---------+------+-------+----------+-----------------------+

1 row in set, 1 warning (0.00 sec)

mysql> select i0,i3 from t where i0 between 1 and 2;

22400 rows in set (2.56 sec)结论

就刚才的测试场景开启 MRR 优化可以得到 3 倍的性能提升。

MRR 的优化器参数调整

如果想关闭 MRR 优化的话，就要把优化器开关 mrr 设置为 off。

默认只有在优化器认为 MRR 可以带来优化的情况下才会走 MRR，如果你想不管什么时候能走 MRR 的都走 MRR 的话，你要把 mrr_cost_based 设置为 off，不过最好不要这么干，因为这确实是一个坑，MRR 不一定什么时候都好，全表扫描有时候会更加快，如果在这种场景下走 MRR 就完成了。

开启 MRR 关闭基于开销的优化。

-- mrr=on,mrr_cost_based=off

set optimizer_switch = '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,use_invisible_indexes=off,skip_scan=on,hash_join=on';

mysql> explain select i0,i3 from t where i0 between 1 and 10;

+----+-------------+-------+------------+-------+---------------+--------+---------+------+--------+----------+----------------------------------+

| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |

+----+-------------+-------+------------+-------+---------------+--------+---------+------+--------+----------+----------------------------------+

| 1 | SIMPLE | t | NULL | range | idx_i0 | idx_i0 | 4 | NULL | 218492 | 100.00 | Using index condition; Using MRR |

+----+-------------+-------+------------+-------+---------------+--------+---------+------+--------+----------+----------------------------------+

1 row in set, 1 warning (0.00 sec)

select i0,i3 from t where i0 between 1 and 10;

112000 rows in set (4.86 sec)

开启 MRR 开启基于开销的优化。

-- mrr=on,mrr_cost_based=on

set optimizer_switch = '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,use_invisible_indexes=off,skip_scan=on,hash_join=on';

mysql> explain select i0,i3 from t where i0 between 1 and 10;

+----+-------------+-------+------------+------+---------------+------+---------+------+---------+----------+-------------+

| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |

+----+-------------+-------+------------+------+---------------+------+---------+------+---------+----------+-------------+

| 1 | SIMPLE | t | NULL | ALL | idx_i0 | NULL | NULL | NULL | 1121902 | 19.48 | Using where |

+----+-------------+-------+------------+------+---------------+------+---------+------+---------+----------+-------------+

1 row in set, 1 warning (0.00 sec)

mysql> select i0,i3 from t where i0 between 1 and 10;

112000 rows in set (1.52 sec)

可以看到当 mrr_cost_based = OFF 的情况下用时 4.86s，mrr_cost_based = ON 的情况下用时 1.52s，总的来说 mrr_cost_based 是非常关键的建议始终打开。

MRR 的参数优化

MRR 要把主键排个序，这样之后对磁盘的操作就是由顺序读代替之前的随机读。从资源的使用情况上来看就是让 CPU 和内存多做点事，来换磁盘的顺序读。然而排序是需要内存的，这块内存的大小就由参数 read_rnd_buffer_size 来控制。

read_rnd_buffer_size 太小无法启用 MRR 功能。

mysql> select @@read_rnd_buffer_size;

+------------------------+

| @@read_rnd_buffer_size |

+------------------------+

| 262144 |

+------------------------+

1 row in set (0.00 sec)

mysql> explain select i0,i3 from t where i0 between 1 and 12;

+----+-------------+-------+------------+------+---------------+------+---------+------+---------+----------+-------------+

| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |

+----+-------------+-------+------------+------+---------------+------+---------+------+---------+----------+-------------+

| 1 | SIMPLE | t | NULL | ALL | idx_i0 | NULL | NULL | NULL | 1121902 | 23.57 | Using where |

+----+-------------+-------+------------+------+---------------+------+---------+------+---------+----------+-------------+

1 row in set, 1 warning (0.00 sec)

放大 read_rnd_buffer_size 让 MySQL 有足够的资源用于 MRR 。

mysql> set read_rnd_buffer_size = 32 * 1024 * 1024;

Query OK, 0 rows affected (0.00 sec)

mysql> explain select i0,i3 from t where i0 between 1 and 12;

+----+-------------+-------+------------+-------+---------------+--------+---------+------+--------+----------+----------------------------------+

| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |

+----+-------------+-------+------------+-------+---------------+--------+---------+------+--------+----------+----------------------------------+

| 1 | SIMPLE | t | NULL | range | idx_i0 | idx_i0 | 4 | NULL | 264436 | 100.00 | Using index condition; Using MRR |

+----+-------------+-------+------------+-------+---------------+--------+---------+------+--------+----------+----------------------------------+

1 row in set, 1 warning (0.00 sec)