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本文为学习笔记，有误请指出。

本文使用源码版本：Percona 5.7.14

一、Handler_read_*值的实质

内部表示如下：

{"Handler_read_first",       (char*) offsetof(STATUS_VAR, ha_read_first_count),     SHOW_LONGLONG_STATUS,    SHOW_SCOPE_ALL},  {"Handler_read_key",         (char*) offsetof(STATUS_VAR, ha_read_key_count),       SHOW_LONGLONG_STATUS,    SHOW_SCOPE_ALL},  {"Handler_read_last",        (char*) offsetof(STATUS_VAR, ha_read_last_count),      SHOW_LONGLONG_STATUS,    SHOW_SCOPE_ALL},  {"Handler_read_next",        (char*) offsetof(STATUS_VAR, ha_read_next_count),      SHOW_LONGLONG_STATUS,    SHOW_SCOPE_ALL},  {"Handler_read_prev",        (char*) offsetof(STATUS_VAR, ha_read_prev_count),      SHOW_LONGLONG_STATUS,    SHOW_SCOPE_ALL},  {"Handler_read_rnd",         (char*) offsetof(STATUS_VAR, ha_read_rnd_count),       SHOW_LONGLONG_STATUS,    SHOW_SCOPE_ALL},  {"Handler_read_rnd_next",    (char*) offsetof(STATUS_VAR, ha_read_rnd_next_count),  SHOW_LONGLONG_STATUS,    SHOW_SCOPE_ALL},

实际上这些变量都是MySQL层定义出来的，因为MySQL可以包含多个存储引擎。因此这些值如何增加需要在引擎层的接口中自行实现，也就是说各个引擎都有自己的实现，在MySQL层进行汇总，因此这些值不是某个引擎特有的，打个比方如果有Innodb和MyISAM引擎，那么这些值是两个引擎的总和。本文将以Innodb为主要学习对象进行解释。

二、各个值的解释

1、Handler_read_key

• 内部表示：ha_read_key_count
• Innodb更改接口：ha_innobase::index_read
• 文档解释：The number of requests to read a row based on a key. If this value is high, it is a good indication that your tables are properly indexed for your queries.
• 源码函数解释：Positions an index cursor to the index specified in the handle. Fetches the row if any.
• 作者解释：这个函数是访问索引的时候定位到值所在的位置用到的函数，因为必须要知道读取索引的开始位置才能向下访问。

2、Handler_read_next

• 内部表示：ha_read_next_count
• Innodb更改接口：ha_innobase::index_next_same ha_innobase::index_next
• 文档解释：The number of requests to read the next row in key order. This value is incremented if you are

1. an index column with a range constraint or if you are doing an index scan.

• 源码函数解释：
  index_next - Reads the next row from a cursor, which must have previously been positioned using index_read.

index_next_same - Reads the next row matching to the key value given as the parameter.

• 作者解释：访问索引的下一条数据封装的ha_innobase::general_fetch函数，index_next_same和index_next不同在于访问的方式不一样，比如范围range查询需要用到和索引全扫描也会用到index_next，而ref访问方式会使用index_next_same

3、Handler_read_first

• 内部表示：ha_read_first_count
• Innodb更改接口：ha_innobase::index_first
• 文档解释：The number of times the first entry in an index was read. If this value is high, it suggests that the

1. is doing a lot of full index scans; for example, SELECT col1 FROM foo, assuming that col1

is indexed

• 源码函数解释：Positions a cursor on the first record in an index and reads the corresponding row to buf.
• 作者解释：定位索引的第一条数据，实际上也是封装的ha_innobase::index_read 函数(如全表扫描/全索引扫描调用)

4、Handler_read_rnd_next

• 内部表示：ha_read_rnd_next_count
• Innodb更改接口：ha_innobase::rnd_next
• 文档解释：The number of requests to read the next row in the data file. This value is high if you are doing a lot of table scans. Generally this suggests that your tables are not properly indexed or that your queries

1. not written to take advantage of the indexes you have.

• 源码函数解释：Reads the next row in a table scan (also used to read the FIRST row in a table scan).
• 作者解释：全表扫描访问下一条数据，实际上也是封装的ha_innobase::general_fetch，在访问之前会调用ha_innobase::index_first

5、Handler_read_rnd

• 内部表示：ha_read_rnd_count
• Innodb更改接口：ha_innobase::rnd_pos
• Memory更改接口：ha_heap::rnd_pos
• 文档解释：The number of requests to read a row based on a fixed position. This value is high if you are doing a lot of queries that require sorting of the result. You probably have a lot of queries that require MySQL to scan entire tables or you have joins that do not use keys properly.
• 作者解释：这个状态值在我测试期间只发现对临时表做排序的时候会用到，而且是Memory引擎的，具体只能按照文档理解了。

6、其他

最后2个简单说一下

• Handler_read_prev
  Innodb接口为 ha_innobase::index_prev 访问索引的上一条数据，实际上也是封装的ha_innobase::general_fetch函数，用于ORDER BY DESC 索引扫描避免排序，内部状态值ha_read_prev_count增加。
• Handler_read_last
  Innodb接口为ha_innobase::index_last 访问索引的最后一条数据作为定位，实际上也是封装的ha_innobase::index_read函数，用于ORDER BY DESC 索引扫描避免排序，内部状态值ha_read_last_count增加。

三、常用查询测试

1、测试用例

mysql> show create table z1;+-------+-------------------------------------------------------------------------------------------------------------------------------------------+| Table | Create Table                                                                                                                              |+-------+-------------------------------------------------------------------------------------------------------------------------------------------+| z1    | CREATE TABLE `z1` (  `a` int(11) DEFAULT NULL,  `name` varchar(20) DEFAULT NULL,  KEY `a` (`a`)) ENGINE=InnoDB DEFAULT CHARSET=latin1 |+-------+-------------------------------------------------------------------------------------------------------------------------------------------+1 row in set (0.00 sec)mysql> show create table z10;+-------+------------------------------------------------------------------------------------------------------------------------------------------------+| Table | Create Table                                                                                                                                   |+-------+------------------------------------------------------------------------------------------------------------------------------------------------+| z10   | CREATE TABLE `z10` (  `a` int(11) DEFAULT NULL,  `name` varchar(20) DEFAULT NULL,  KEY `a_idx` (`a`)) ENGINE=InnoDB DEFAULT CHARSET=latin1 |+-------+------------------------------------------------------------------------------------------------------------------------------------------------+1 row in set (0.00 sec)mysql> select count(*) from z1;+----------+| count(*) |+----------+|    56415 |+----------+1 row in set (5.27 sec)mysql> select count(*) from z10;+----------+| count(*) |+----------+|       10 |+----------+1 row in set (0.00 sec)

2、全表扫描

mysql> desc select * from z1;+----+-------------+-------+------------+------+---------------+------+---------+------+-------+----------+-------+| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows  | filtered | Extra |+----+-------------+-------+------------+------+---------------+------+---------+------+-------+----------+-------+|  1 | SIMPLE      | z1    | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 56650 |   100.00 | NULL  |+----+-------------+-------+------------+------+---------------+------+---------+------+-------+----------+-------+1 row in set, 1 warning (0.00 sec)mysql> pager cat >>/dev/nullPAGER set to 'cat >>/dev/null'mysql> flush status;Query OK, 0 rows affected (0.10 sec)mysql> select * from z1;56415 rows in set (4.05 sec)mysql> pager;Default pager wasn't set, using stdout.mysql> show status like 'Handler_read%';+-----------------------+-------+| Variable_name         | Value |+-----------------------+-------+| Handler_read_first    | 1     || Handler_read_key      | 1     || Handler_read_last     | 0     || Handler_read_next     | 0     || Handler_read_prev     | 0     || Handler_read_rnd      | 0     || Handler_read_rnd_next | 56416 |+-----------------------+-------+7 rows in set (0.01 sec)

Handler_read_first增加1次用于初次定位，Handler_read_key增加1次，Handler_read_rnd_next增加扫描行数。我们前面说过因为ha_innobase::index_first也是封装的ha_innobase::index_read因此都需要+1。

3、全索引扫描

mysql> desc select a from z1;+----+-------------+-------+------------+-------+---------------+------+---------+------+-------+----------+-------------+| id | select_type | table | partitions | type  | possible_keys | key  | key_len | ref  | rows  | filtered | Extra       |+----+-------------+-------+------------+-------+---------------+------+---------+------+-------+----------+-------------+|  1 | SIMPLE      | z1    | NULL       | index | NULL          | a    | 5       | NULL | 56650 |   100.00 | Using index |+----+-------------+-------+------------+-------+---------------+------+---------+------+-------+----------+-------------+1 row in set, 1 warning (0.00 sec)mysql> flush status;Query OK, 0 rows affected (0.12 sec)mysql> pager cat >>/dev/nullPAGER set to 'cat >>/dev/null'mysql> select a from z1;56415 rows in set (4.57 sec)mysql> pagerDefault pager wasn't set, using stdout.mysql> show status like 'Handler_read%';+-----------------------+-------+| Variable_name         | Value |+-----------------------+-------+| Handler_read_first    | 1     || Handler_read_key      | 1     || Handler_read_last     | 0     || Handler_read_next     | 56415 || Handler_read_prev     | 0     || Handler_read_rnd      | 0     || Handler_read_rnd_next | 0     |+-----------------------+-------+7 rows in set (0.01 sec)

Handler_read_first增加1次用于初次定位，Handler_read_key增加1次，Handler_read_next增加扫描行数用于连续访问接下来的行。我们前面说过因为ha_innobase::index_first也是封装的ha_innobase::index_read因此都需要+1。

4、索引ref访问

我这里因为是测试索引全是等于10的加上了force index

mysql>  desc select  * from z1 force index(a) where a=10;+----+-------------+-------+------------+------+---------------+------+---------+-------+-------+----------+-------+| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref   | rows  | filtered | Extra |+----+-------------+-------+------------+------+---------------+------+---------+-------+-------+----------+-------+|  1 | SIMPLE      | z1    | NULL       | ref  | a             | a    | 5       | const | 28325 |   100.00 | NULL  |+----+-------------+-------+------------+------+---------------+------+---------+-------+-------+----------+-------+1 row in set, 1 warning (0.01 sec)mysql> flush status;Query OK, 0 rows affected (0.13 sec)mysql> pager cat >>/dev/nullPAGER set to 'cat >>/dev/null'mysql> select  * from z1 force index(a) where a=10;56414 rows in set (32.39 sec)mysql> pagerDefault pager wasn't set, using stdout.mysql> show status like 'Handler_read%';+-----------------------+-------+| Variable_name         | Value |+-----------------------+-------+| Handler_read_first    | 0     || Handler_read_key      | 1     || Handler_read_last     | 0     || Handler_read_next     | 56414 || Handler_read_prev     | 0     || Handler_read_rnd      | 0     || Handler_read_rnd_next | 0     |+-----------------------+-------+7 rows in set (0.06 sec)

Handler_read_key增加1次这是用于初次定位，Handler_read_next增加扫描行数次数用于接下来的数据访问。

5、索引range访问

mysql> desc select  * from z1 force index(a) where a>9 and a<12;+----+-------------+-------+------------+-------+---------------+------+---------+------+-------+----------+-----------------------+| id | select_type | table | partitions | type  | possible_keys | key  | key_len | ref  | rows  | filtered | Extra                 |+----+-------------+-------+------------+-------+---------------+------+---------+------+-------+----------+-----------------------+|  1 | SIMPLE      | z1    | NULL       | range | a             | a    | 5       | NULL | 28325 |   100.00 | Using index condition |+----+-------------+-------+------------+-------+---------------+------+---------+------+-------+----------+-----------------------+1 row in set, 1 warning (0.00 sec)mysql>  pager cat >>/dev/nullPAGER set to 'cat >>/dev/null'mysql> select  * from z1 force index(a) where a>9 and a<12;56414 rows in set (47.54 sec)mysql> show status like 'Handler_read%';7 rows in set (0.03 sec)mysql>  pagerDefault pager wasn't set, using stdout.mysql> show status like 'Handler_read%';+-----------------------+-------+| Variable_name         | Value |+-----------------------+-------+| Handler_read_first    | 0     || Handler_read_key      | 1     || Handler_read_last     | 0     || Handler_read_next     | 56414 || Handler_read_prev     | 0     || Handler_read_rnd      | 0     || Handler_read_rnd_next | 0     |+-----------------------+-------+7 rows in set (0.02 sec)

Handler_read_key增加1次这是用于初次定位，Handler_read_next增加扫描行数次数用于接下来的数据访问。

6、被驱动表带索引访问

mysql> desc select * from z1 STRAIGHT_JOIN z10 force index(a_idx) on z1.a=z10.a;+----+-------------+-------+------------+------+---------------+-------+---------+-----------+-------+----------+-------------+| id | select_type | table | partitions | type | possible_keys | key   | key_len | ref       | rows  | filtered | Extra       |+----+-------------+-------+------------+------+---------------+-------+---------+-----------+-------+----------+-------------+|  1 | SIMPLE      | z1    | NULL       | ALL  | a             | NULL  | NULL    | NULL      | 56650 |   100.00 | Using where ||  1 | SIMPLE      | z10   | NULL       | ref  | a_idx         | a_idx | 5       | test.z1.a |    10 |   100.00 | NULL        |+----+-------------+-------+------------+------+---------------+-------+---------+-----------+-------+----------+-------------+2 rows in set, 1 warning (0.01 sec)mysql> flush status;Query OK, 0 rows affected (0.47 sec)mysql> pager cat >> /dev/nullPAGER set to 'cat >> /dev/null'mysql>  select * from z1 STRAIGHT_JOIN z10 force index(a_idx) on z1.a=z10.a;112828 rows in set (1 min 21.21 sec)mysql> pagerDefault pager wasn't set, using stdout.mysql>  show status like 'Handler_read%';+-----------------------+--------+| Variable_name         | Value  |+-----------------------+--------+| Handler_read_first    | 1      || Handler_read_key      | 56416  || Handler_read_last     | 0      || Handler_read_next     | 112828 || Handler_read_prev     | 0      || Handler_read_rnd      | 0      || Handler_read_rnd_next | 56416  |+-----------------------+--------+7 rows in set (0.00 sec)

Handler_read_first 增加一次作为驱动表z1全表扫描定位的开始，接下来Handler_read_rnd_next扫描全部记录，每次扫描一次在z10表通过索引a_idx定位一次Handler_read_key增加1次，然后接下来进行索引a_idx进行数据查找Handler_read_next增加为扫描的行数。

6、索引避免排序正向和反向

mysql>  flush status;Query OK, 0 rows affected (0.05 sec)mysql> pager cat >> /dev/nullPAGER set to 'cat >> /dev/null'mysql> select * from z1 force index(a) order by a;56415 rows in set (27.39 sec)mysql> pagerDefault pager wasn't set, using stdout.mysql> show status like 'Handler_read%';+-----------------------+-------+| Variable_name         | Value |+-----------------------+-------+| Handler_read_first    | 1     || Handler_read_key      | 1     || Handler_read_last     | 0     || Handler_read_next     | 56415 || Handler_read_prev     | 0     || Handler_read_rnd      | 0     || Handler_read_rnd_next | 0     |+-----------------------+-------+7 rows in set (0.01 sec)mysql> flush status;Query OK, 0 rows affected (0.10 sec)mysql> desc  select * from z1 force index(a) order by a desc;+----+-------------+-------+------------+-------+---------------+------+---------+------+-------+----------+-------+| id | select_type | table | partitions | type  | possible_keys | key  | key_len | ref  | rows  | filtered | Extra |+----+-------------+-------+------------+-------+---------------+------+---------+------+-------+----------+-------+|  1 | SIMPLE      | z1    | NULL       | index | NULL          | a    | 5       | NULL | 56650 |   100.00 | NULL  |+----+-------------+-------+------------+-------+---------------+------+---------+------+-------+----------+-------+1 row in set, 1 warning (0.00 sec)mysql> pager cat >> /dev/nullPAGER set to 'cat >> /dev/null'mysql>  select * from z1 force index(a) order by a desc;56415 rows in set (24.94 sec)mysql> pagerDefault pager wasn't set, using stdout.mysql>  show status like 'Handler_read%';+-----------------------+-------+| Variable_name         | Value |+-----------------------+-------+| Handler_read_first    | 0     || Handler_read_key      | 1     || Handler_read_last     | 1     || Handler_read_next     | 0     || Handler_read_prev     | 56415 || Handler_read_rnd      | 0     || Handler_read_rnd_next | 0     |+-----------------------+-------+7 rows in set (0.01 sec)

不用过多解释，可以看到Handler_read_last 和Handler_read_prev的用途。

四、总结

• Handler_read_rnd_next 通常代表着全表扫描。
• Handler_read_first 通常代表着全表或者全索引扫描。
• Handler_read_next 通常代表着合理的使用了索引或者全索引扫描。
• Handler_read_key 不管全表全索引或者正确使用的索引实际上都会增加，只是一次索引定位而已。
• Innodb中全表扫描也是主键的全索引扫描。
• 顺序访问的一条记录实际上都是调用ha_innobase::general_fetch函数，另外一个功能innodb_thread_concurrency参数的功能就在里面实现，下次在说。

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