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
- 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
- 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
- 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/null
PAGER 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/null
PAGER set to 'cat >>/dev/null'
mysql> select a from z1;
56415 rows in set (4.57 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 | 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/null
PAGER set to 'cat >>/dev/null'
mysql> select * from z1 force index(a) where a=10;
56414 rows in set (32.39 sec)
mysql> pager
Default 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/null
PAGER 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> pager
Default 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/null
PAGER 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> pager
Default 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/null
PAGER set to 'cat >> /dev/null'
mysql> select * from z1 force index(a) order by a;
56415 rows in set (27.39 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 | 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/null
PAGER set to 'cat >> /dev/null'
mysql> select * from z1 force index(a) order by a desc;
56415 rows in set (24.94 sec)
mysql> pager
Default 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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