mysql status 解释 Handler_read%

mysql status 解释 Handler_read%

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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，可惜实际很多人并不理解它们的实际意义，本文简单介绍一下

(root@db01) [hyip]>show status like '%Handler%';
+-----------------------+-------+
| Variable_name         | Value |
+-----------------------+-------+
| Handler_read_first    | 1     |
| Handler_read_key      | 4     |
| Handler_read_last     | 0     |
| Handler_read_next     | 0     |
| Handler_read_prev     | 0     |
| Handler_read_rnd      | 0     |
| Handler_read_rnd_next | 2223  |
+-----------------------+———+

Handler_read_first

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 col1is

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，简便是索引文件，做一次完整的扫描也是很费时的

Handler_read_key

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

Handler_read_last

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 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.

Handler_read_next

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,貌似也是越小越好，至少官方文档的例子是这样说的：

The Handler_read_next[647]value decreases from 5 to 1, indicating more efficient use of the index

Handler_read_prev

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

Handler_read_rnd

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

Handler_read_rnd_next

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.

此选项表明在进行数据文件扫描时，从数据文件里取数据的次数,这个涉及到table scans，肯定是越小越好

后记：

不同平台，不同版本的MySQL，在运行上面例子的时候，Handler_read_*的数值可能会有所不同，这并不要紧，关键是你要意识到Handler_read_*可以协助你理解MySQL处理查询的过程，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

参考链接:
http://www.cnblogs.com/wangchy0927/p/3301665.html http://www.2cto.com/database/201203/123031.html