MySQL Handler变量解析

 

http://blog.itpub.net/29254281/viewspace-1159014/

To see the effect of a query do the following steps:

FLUSH STATUS;
Execute the query
SHOW SESSION STATUS LIKE 'handler_read%';
Do an EXPLAIN of the query

实验数据初始化:

CREATE TABLE test (
    id INT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY
  , data VARCHAR(32)
  , ts TIMESTAMP
  , INDEX (data)
);

INSERT INTO test
VALUES (NULL, 'abc', NOW()), (NULL, 'abc', NOW()), (NULL, 'abd', NOW())
     , (NULL, 'acd', NOW()), (NULL, 'def', NOW()), (NULL, 'pqr', NOW())
     , (NULL, 'stu', NOW()), (NULL, 'vwx', NOW()), (NULL, 'yza', NOW())
     , (NULL, 'def', NOW())
;

SELECT * FROM test;
+----+------+---------------------+
| id | data | ts                  |
+----+------+---------------------+
| 1  | abc  | 2008-01-18 16:28:40 |
| 2  | abc  | 2008-01-18 16:28:40 |
| 3  | abd  | 2008-01-18 16:28:40 |
| 4  | acd  | 2008-01-18 16:28:40 |
| 5  | def  | 2008-01-18 16:28:40 |
| 6  | pqr  | 2008-01-18 16:28:40 |
| 7  | stu  | 2008-01-18 16:28:40 |
| 8  | vwx  | 2008-01-18 16:28:40 |
| 9  | yza  | 2008-01-18 16:28:40 |
| 10 | def  | 2008-01-18 16:28:40 |
+----+------+---------------------+

Handler_read_first
全索引扫描的次数
The number of times the first entry was read from an index. 
If this value is high, it suggests that the server is doing a lot of full index scans.

Handler_read_key
走索引的次数
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.

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.

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.

Handler_read_rnd
文件排序或者没有使用索引
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 don't use keys properly.

Handler_read_rnd_next
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。
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.

可以看到全表扫描其实也是走了key,可能是因为索引组织表的原因。因为limit 2 所以rnd_next为2.这个Stop Key在执行计划中是看不出来的。


使用索引消除排序，因为是升序，所以read first为1,由于limit 4,所以read_next为3.通过这个也可以看出Stop Key.


也是使用索引消除排序，因为是倒序，所以read_last为1,read_prev为2.因为往回读了两个key.

ALTER TABLE test ADD COLUMN file_sort text;

UPDATE test SET file_sort = 'abcdefghijklmnopqrstuvwxyz' WHERE id = 1;
UPDATE test SET file_sort = 'bcdefghijklmnopqrstuvwxyza' WHERE id = 2;
UPDATE test SET file_sort = 'cdefghijklmnopqrstuvwxyzab' WHERE id = 3;
UPDATE test SET file_sort = 'defghijklmnopqrstuvwxyzabc' WHERE id = 4;
UPDATE test SET file_sort = 'efghijklmnopqrstuvwxyzabcd' WHERE id = 5;
UPDATE test SET file_sort = 'fghijklmnopqrstuvwxyzabcde' WHERE id = 6;
UPDATE test SET file_sort = 'ghijklmnopqrstuvwxyzabcdef' WHERE id = 7;
UPDATE test SET file_sort = 'hijklmnopqrstuvwxyzabcdefg' WHERE id = 8;
UPDATE test SET file_sort = 'ijklmnopqrstuvwxyzabcdefgh' WHERE id = 9;
UPDATE test SET file_sort = 'jklmnopqrstuvwxyzabcdefghi' WHERE id = 10;

Handler_read_rnd为4 说明没有使用索引
rnd_next为11说明扫描了所有的数据
但是read first和read key的数据，不能解释.不知道为什么会是这个数据
read key总是read_rnd+1

参考:
http://www.fromdual.com/mysql-handler-read-status-variables
http://hi.baidu.com/thinkinginlamp/item/8d038333c6b0674a3075a1d3