技术分享 | InnoDB Handler_read_* 变量解释「建议收藏」

大家好，我是考100分的小小码，祝大家学习进步，加薪顺利呀。今天说一说技术分享 | InnoDB Handler_read_* 变量解释「建议收藏」,希望您对编程的造诣更进一步.

作者：高鹏文章末尾有他著作的《深入理解 MySQL 主从原理 32 讲》，深入透彻理解 MySQL 主从，GTID 相关技术知识。源码版本：percona 5.7.14 本文为学习记录，可能有误请谅解。

本文建议PC端观看，效果更佳。

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},

代码100分

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 querying 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 server 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 are 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

代码100分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

代码100分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。

7. 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，下次再说。

五、参考栈帧

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)

第一次:
#0 row_search_mvcc (buf=0x7fff2ccc9380 "377", mode=PAGE_CUR_G, prebuilt=0x7fff2cd4bb40, match_mode=0, direction=0)
at /root/mysql5.7.14/percona-server-5.7.14-7/storage/innobase/row/row0sel.cc:4479
#1 0x00000000019b3051 in ha_innobase::index_read (this=0x7fff2cd32480, buf=0x7fff2ccc9380 "377", key_ptr=0x0, key_len=0, find_flag=HA_READ_AFTER_KEY)
at /root/mysql5.7.14/percona-server-5.7.14-7/storage/innobase/handler/ha_innodb.cc:9104
#2 0x00000000019b4374 in ha_innobase::index_first (this=0x7fff2cd32480, buf=0x7fff2ccc9380 "377")
at /root/mysql5.7.14/percona-server-5.7.14-7/storage/innobase/handler/ha_innodb.cc:9551
#3 0x00000000019b462c in ha_innobase::rnd_next (this=0x7fff2cd32480, buf=0x7fff2ccc9380 "377")
at /root/mysql5.7.14/percona-server-5.7.14-7/storage/innobase/handler/ha_innodb.cc:9656
#4 0x0000000000f66fa2 in handler::ha_rnd_next (this=0x7fff2cd32480, buf=0x7fff2ccc9380 "377") at /root/mysql5.7.14/percona-server-5.7.14-7/sql/handler.cc:3099
#5 0x00000000014c61b6 in rr_sequential (info=0x7fff2c0026a0) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/records.cc:520
#6 0x000000000155f2a4 in join_init_read_record (tab=0x7fff2c002650) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/sql_executor.cc:2481
#7 0x000000000155c381 in sub_select (join=0x7fff2c001f70, qep_tab=0x7fff2c002650, end_of_records=false)
at /root/mysql5.7.14/percona-server-5.7.14-7/sql/sql_executor.cc:1271
#8 0x000000000155bd

最后推荐高鹏的专栏《深入理解MySQL主从原理 32讲》，想要透彻了解学习MySQL 主从原理的朋友不容错过。

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