0227浅谈MySQL之 Handler_read

转自博客http://www.path8.net/tn/archives/5613

1.监控语法：

在MySQL里，使用SHOW STATUS查询服务器状态，语法一般来说如下：

SHOW [GLOBAL | SESSION] STATUS [LIKE 'pattern' | WHERE expr]

执行命令后会看到很多内容，其中有一部分是Handler_read_*，它们显示了数据库处理SELECT查询语句的状态，对于调试SQL语句有很大意义。

mysql> show session status like 'Handler_read%';

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

| Variable_name | Value |

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

| Handler_read_first | 0 |

| Handler_read_key | 1 |

| Handler_read_last | 0 |

| Handler_read_next | 1 |

| Handler_read_prev | 0 |

| Handler_read_rnd | 0 |

| Handler_read_rnd_next | 0 |

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

7 rows in set (0.00 sec)

2.测试流程及用例

每次执行SQL时按照如下过程执行：

FLUSH STATUS;

SELECT ...;

SHOW SESSION STATUS LIKE 'Handler_read%';

EXPLAIN SELECT ...;

测试用例：

mysql> show create table artist\G;

*************************** 1. row ***************************

Table: artist

Create Table: CREATE TABLE `artist` (

`artist_id` int(10) unsigned NOT NULL,

`type` enum('Band','Person','Unknown','Combination') NOT NULL,

`name` varchar(255) NOT NULL,

`name_reverse` varchar(255) DEFAULT NULL,

`gender` enum('Male','Female') DEFAULT NULL,

`founded` year(4) DEFAULT NULL,

`country_id` smallint(5) unsigned DEFAULT NULL,

PRIMARY KEY (`artist_id`),

KEY `name` (`name`),

KEY `idx_founded` (`founded`),

KEY `type` (`type`),

KEY `idx_name_reverse` (`name_reverse`)

) ENGINE=InnoDB DEFAULT CHARSET=latin1

1 row in set (0.00 sec)

3.参数测试

3.1 Handler_read_first

Handler_read_first原意：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.

此选项表明SQL是在做一个全索引扫描，注意是全部，而不是部分，所以说如果存在WHERE语句，这个选项是不会变的。如果这个选项的数值很大，既是好事也是坏事。说它好是因为毕竟查询是在索引里完成的，而不是数据文件里，说它坏是因为大数据量时，即使是索引文件，做一次完整的扫描也是很费时间的。

mysql> flush status;

Query OK, 0 rows affected (0.00 sec)

mysql> select name from artist;

......

577983 rows in set (1.50 sec)

mysql> show session status like 'Handler_read%';

ERROR 1317 (70100): Query execution was interrupted

mysql> show session status like 'Handler_read%';

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

| Variable_name | Value |

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

| Handler_read_first | 1 |

| Handler_read_key | 1 |

| Handler_read_last | 0 |

| Handler_read_next | 577983 |

| Handler_read_prev | 0 |

| Handler_read_rnd | 0 |

| Handler_read_rnd_next | 0 |

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

7 rows in set (0.00 sec)

mysql> explain select name from artist\G;

*************************** 1. row ***************************

id: 1

select_type: SIMPLE

table: artist

type: index

possible_keys: NULL

key: name

key_len: 257

ref: NULL

rows: 585801

Extra: Using index

1 row in set (0.00 sec)

使用了只读索引。

3.2 Handler_read_key

Handler_read_key原意：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.

此选项数值如果很高，那么恭喜你，你的系统高效的使用了索引，一切运转良好。

mysql> flush status;

Query OK, 0 rows affected (0.00 sec)

mysql> select * from artist where name='Enya';

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

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

| 88 | Person | Enya | aynE | Female | 1961 | 103 |

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

1 row in set (0.00 sec)

mysql> show session status like 'Handler_read%';

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

| Variable_name | Value |

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

| Handler_read_first | 0 |

| Handler_read_key | 1 |

| Handler_read_last | 0 |

| Handler_read_next | 1 |

| Handler_read_prev | 0 |

| Handler_read_rnd | 0 |

| Handler_read_rnd_next | 0 |

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

7 rows in set (0.00 sec)

mysql> explain select * from artist where name='Enya'\G;

*************************** 1. row ***************************

id: 1

select_type: SIMPLE

table: artist

type: ref

possible_keys: name

key: name

key_len: 257

ref: const

rows: 1

Extra: Using index condition

1 row in set (0.00 sec)

使用了索引下推技术。

3.3 Handler_read_last

Handler_read_last的原意：The number of requests to read the last key in an index. With ORDER BY, the server will issue a first-key request followed by several next-key requests, whereas with ORDER BY DESC, the server will issue a last-key request followed by several previous-key requests. This variable was added in MySQL 5.6.1.

3.4 Handler_read_next

Handler_read_next的原意：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.

此选项表明在进行索引扫描时，按照索引从数据文件里取数据的次数。

mysql> flush status;

Query OK, 0 rows affected (0.00 sec)

mysql> select name from artist order by name;

......

577983 rows in set (1.55 sec)

mysql> show session status like 'Handler_read%';

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

| Variable_name | Value |

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

| Handler_read_first | 1 |

| Handler_read_key | 1 |

| Handler_read_last | 0 |

| Handler_read_next | 577983 |

| Handler_read_prev | 0 |

| Handler_read_rnd | 0 |

| Handler_read_rnd_next | 0 |

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

7 rows in set (0.00 sec)

mysql> explain select name from artist order by name\G;

*************************** 1. row ***************************

id: 1

select_type: SIMPLE

table: artist

type: index

possible_keys: NULL

key: name

key_len: 257

ref: NULL

rows: 585801

Extra: Using index

1 row in set (0.00 sec)

3.4 Handler_read_prev

Handler_read_prev的原意：The number of requests to read the previous row in key order. This read method is mainly used to optimize ORDER BY ... DESC.

此选项表明在进行索引扫描时，按照索引倒序从数据文件里取数据的次数，一般就是ORDER BY ... DESC。注意Handler_read_next是ORDER BY ... ASC的方式取数据。

mysql> flush status;

Query OK, 0 rows affected (0.00 sec)

mysql> select name from artist order by name desc;

......

577983 rows in set (1.55 sec)

mysql> show session 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 | 577983 |

| Handler_read_rnd | 0 |

| Handler_read_rnd_next | 0 |

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

7 rows in set (0.00 sec)

mysql> explain select name from artist order by name desc\G;

*************************** 1. row ***************************

id: 1

select_type: SIMPLE

table: artist

type: index

possible_keys: NULL

key: name

key_len: 257

ref: NULL

rows: 585801

Extra: Using index

1 row in set (0.00 sec)

3.5 Handler_read_rnd

Handler_read_rnd的原意：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.

简单的说，就是查询直接操作了数据文件，很多时候表现为没有使用索引或者文件排序。

mysql> flush status;

Query OK, 0 rows affected (0.00 sec)

mysql> flush status;

Query OK, 0 rows affected (0.00 sec)

mysql> select * from artist order by country_id asc;

577983 rows in set (1.54 sec)

mysql> show session status like 'Handler_read%';

ERROR 1317 (70100): Query execution was interrupted

mysql> show session 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 | 577984 |

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

7 rows in set (0.00 sec)

mysql> explain select country_id from artist order by country_id asc\G;

*************************** 1. row ***************************

id: 1

select_type: SIMPLE

table: artist

type: ALL

possible_keys: NULL

key: NULL

key_len: NULL

ref: NULL

rows: 585801

Extra: Using filesort

1 row in set (0.00 sec)

3.6 Handler_read_rnd_next

Handler_read_rnd_next表示“在数据文件中读下一行的请求数。如果你正进行大量的表扫描，该值较高。通常说明你的表索引不正确或写入的查询没有利用索引。”

这个说明跟你的SQL语句有很大的关系，你可以通过explain工具或者是慢查询日志找出对应的慢SQL，并对执行慢的SQL语句进行调试，直到找到最优的执行计划，这样Handler_read_rnd_next的值就会往下降了。

很多时候，为了完成一个查询任务，我们往往可以写出几种查询语句，这时，你不妨挨个按照上面的方式执行，根据结果中的Handler_read_*数值，你就能相对容易的判断各种查询方式的优劣。

还有一种监控的方法就是profile

mysql> show variables like 'profil%';

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

| Variable_name | Value |

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

| profiling | OFF |

| profiling_history_size | 15 |

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

2 rows in set (0.01 sec)

mysql> set profiling=on;

Query OK, 0 rows affected, 1 warning (0.00 sec)

mysql> show profiles;

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

| Query_ID | Duration | Query |

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

| 1 | 2.94708450 | select country_id from artist order by country_id asc |

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

1 row in set, 1 warning (0.00 sec)

mysql> show profile;

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

| Status | Duration |

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

| starting | 0.000201 |

| checking permissions | 0.000023 |

| Opening tables | 0.000062 |

| init | 0.000069 |

| System lock | 0.000035 |

| optimizing | 0.000011 |

| statistics | 0.000030 |

| preparing | 0.000019 |

| Sorting result | 0.000014 |

| executing | 0.000010 |

| Sending data | 0.000044 |

| Creating sort index | 1.601273 |

| end | 0.000070 |

| query end | 0.000026 |

| closing tables | 0.000029 |

| freeing items | 1.344915 |

| logging slow query | 0.000207 |

| cleaning up | 0.000048 |

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

18 rows in set, 1 warning (0.00 sec)

巴特西

0227浅谈MySQL之 Handler_read_*参数

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