1.前置条件：

本次是基于小数据量，且数据块在一个页中的最理想情况进行分析，可能无具体的实际意义，但是可以借鉴到各种复杂条件下，因为原理是相同的,知小见大，见微知著！

打开语句分析并确认是否已经打开

mysql> set profiling=1;     Query OK, 0 rows affected (0.00 sec)   mysql> select @@profiling;  +-------------+  | @@profiling |  +-------------+  |           1 |  +-------------+  1 row in set (0.01 sec)

2.数据准备：

2.1全表扫描数据

create table person4all(id int not null  auto_increment, name varchar(30) not null, gender varchar(10) not null ,primary key(id));  insert into person4all(name,gender) values("zhaoming","male");  insert into person4all(name,gender) values("wenwen","female");

2.2根据主键查看数据

create table person4pri(id int not null  auto_increment, name varchar(30) not null, gender varchar(10) not null ,primary key(id));  insert into person4pri(name,gender) values("zhaoming","male");  insert into person4pri(name,gender) values("wenwen","female");

2.3根据非聚集索引查数据

create table person4index(id int not null  auto_increment, name varchar(30) not null, gender varchar(10) not null ,primary key(id) , index(gender));  insert into person4index(name,gender) values("zhaoming","male");  insert into person4index(name,gender) values("wenwen","female");

2.4根据覆盖索引查数据

create table person4cindex(id int not null  auto_increment, name varchar(30) not null, gender varchar(10) not null ,primary key(id) , index(name,gender));    insert into person4cindex(name,gender) values("zhaoming","male");    insert into person4cindex(name,gender) values("wenwen","female");

主要从以下几个方面分析：查询消耗的时间，走的执行计划等方面。

3.开工测试：

第一步：全表扫描

mysql> select * from person4all ;  +----+----------+--------+  | id | name     | gender |  +----+----------+--------+  |  1 | zhaoming | male   |  |  2 | wenwen   | female |  +----+----------+--------+  2 rows in set (0.00 sec)

查看其执行计划：

mysql> explain select * from person4all;  +----+-------------+------------+------+---------------+------+---------+------+------+-------+  | id | select_type | table      | type | possible_keys | key  | key_len | ref  | rows | Extra |  +----+-------------+------------+------+---------------+------+---------+------+------+-------+  |  1 | SIMPLE      | person4all | ALL  | NULL          | NULL | NULL    | NULL |    2 |       |  +----+-------------+------------+------+---------------+------+---------+------+------+-------+  1 row in set (0.01 sec)

我们可以很清晰的看到走的是全表扫描，而没有走索引！

查询消耗的时间：

mysql> show profiles;  +----------+------------+-----------------------------------------------------------------------------------------------------------------------------------+  | Query_ID | Duration   | Query                                                                                                                             |  |       54 | 0.00177300 | select * from person4all                                                                                                          |  |       55 | 0.00069200 | explain select * from person4all                                                                                                  |  +----------+------------+-----------------------------------------------------------------------------------------------------------------------------------+

全表扫描总共话了0.0017730秒

各个阶段消耗的时间是：

mysql> show profile for query 54;  +--------------------------------+----------+  | Status                         | Duration |  +--------------------------------+----------+  | starting                       | 0.000065 |  | checking query cache for query | 0.000073 |  | Opening tables                 | 0.000037 |  | System lock                    | 0.000024 |  | Table lock                     | 0.000053 |  | init                           | 0.000044 |  | optimizing                     | 0.000022 |  | statistics                     | 0.000032 |  | preparing                      | 0.000030 |  | executing                      | 0.000020 |  | Sending data                   | 0.001074 |  | end                            | 0.000091 |  | query end                      | 0.000020 |  | freeing items                  | 0.000103 |  | storing result in query cache  | 0.000046 |  | logging slow query             | 0.000019 |  | cleaning up                    | 0.000020 |  +--------------------------------+----------+  17 rows in set (0.00 sec)

第一次不走缓存的话，需要检查是否存在缓存中，打开表，初始化等操作，最大的开销在于返回数据。

第二步：根据主键查询数据。

mysql> select name ,gender from person4pri where id in (1,2);  +----------+--------+  | name     | gender |  +----------+--------+  | zhaoming | male   |  | wenwen   | female |  +----------+--------+  2 rows in set (0.01 sec)

查看其执行计划：

mysql> explain select name ,gender from person4pri where id in (1,2);  +----+-------------+------------+-------+---------------+---------+---------+------+------+-------------+  | id | select_type | table      | type  | possible_keys | key     | key_len | ref  | rows | Extra       |  +----+-------------+------------+-------+---------------+---------+---------+------+------+-------------+  |  1 | SIMPLE      | person4pri | range | PRIMARY       | PRIMARY | 4       | NULL |    2 | Using where |  +----+-------------+------------+-------+---------------+---------+---------+------+------+-------------+  1 row in set (0.00 sec)

从执行计划中我们可以看出，走的是范围索引。

再看其执行消耗的时间：

mysql> show profiles;  +----------+------------+-----------------------------------------------------------------------------------------------------------------------------------+  | Query_ID | Duration   | Query                                                                                                                             |  +----------+------------+-----------------------------------------------------------------------------------------------------------------------------------+  |       63 | 0.00135700 | select name ,gender from person4pri where id in (1,2)                                                                             |  |       64 | 0.00079200 | explain select name ,gender from person4pri where id in (1,2)                                                                     |  +----------+------------+-----------------------------------------------------------------------------------------------------------------------------------+  15 rows in set (0.01 sec)

这次查询消耗时间为0.00079200。

查看各个阶段消耗的时间：

mysql> show profile for query 63;  +--------------------------------+----------+  | Status                         | Duration |  +--------------------------------+----------+  | starting                       | 0.000067 |  | checking query cache for query | 0.000146 |  | Opening tables                 | 0.000342 |  | System lock                    | 0.000027 |  | Table lock                     | 0.000115 |  | init                           | 0.000056 |  | optimizing                     | 0.000032 |  | statistics                     | 0.000069 |  | preparing                      | 0.000039 |  | executing                      | 0.000022 |  | Sending data                   | 0.000100 |  | end                            | 0.000075 |  | query end                      | 0.000022 |  | freeing items                  | 0.000158 |  | storing result in query cache  | 0.000045 |  | logging slow query             | 0.000019 |  | cleaning up                    | 0.000023 |  +--------------------------------+----------+  17 rows in set (0.00 sec)

看出最大的消耗也是在Sending data，第一次也是需要一些初始化操作。

第三步：根据非聚集索引查询

mysql> select name ,gender from person4index where gender in ("male","female");  +----------+--------+  | name     | gender |  +----------+--------+  | wenwen   | female |  | zhaoming | male   |  +----------+--------+  2 rows in set (0.00 sec)

查看器执行计划：

mysql> explain select name ,gender from person4index where gender in ("male","female");  +----+-------------+--------------+-------+---------------+--------+---------+------+------+-------------+  | id | select_type | table        | type  | possible_keys | key    | key_len | ref  | rows | Extra       |  +----+-------------+--------------+-------+---------------+--------+---------+------+------+-------------+  |  1 | SIMPLE      | person4index | range | gender        | gender | 12      | NULL |    2 | Using where |  +----+-------------+--------------+-------+---------------+--------+---------+------+------+-------------+  1 row in set (0.00 sec)

可以看出，走的也是范围索引。同主键查询，那么就看其消耗时间了

mysql> show profiles;  +----------+------------+-----------------------------------------------------------------------------------------------------------------------------------------------------+  | Query_ID | Duration   | Query                                                                                                                                               |  +----------+------------+-----------------------------------------------------------------------------------------------------------------------------------------------------+  |       68 | 0.00106600 | select name ,gender from person4index where gender in ("male","female")                                                                             |  |       69 | 0.00092500 | explain select name ,gender from person4index where gender in ("male","female")                                                                     |  +----------+------------+-----------------------------------------------------------------------------------------------------------------------------------------------------+  15 rows in set (0.00 sec)

这个非主键索引消耗的时间为：0.00106600，可以看出略大于组件索引消耗的时间。

看其具体消耗的阶段：

mysql> show profile for query 68 ;  +--------------------------------+----------+  | Status                         | Duration |  +--------------------------------+----------+  | starting                       | 0.000059 |  | checking query cache for query | 0.000111 |  | Opening tables                 | 0.000085 |  | System lock                    | 0.000023 |  | Table lock                     | 0.000067 |  | init                           | 0.000183 |  | optimizing                     | 0.000031 |  | statistics                     | 0.000139 |  | preparing                      | 0.000035 |  | executing                      | 0.000020 |  | Sending data                   | 0.000148 |  | end                            | 0.000024 |  | query end                      | 0.000019 |  | freeing items                  | 0.000043 |  | storing result in query cache  | 0.000042 |  | logging slow query             | 0.000017 |  | cleaning up                    | 0.000020 |  +--------------------------------+----------+  17 rows in set (0.00 sec)

看几个关键词的点；init,statistics,Sending data 这几个关键点上的消耗向比较主键的查询要大很多，特别是Sending data。因为若是走的非聚集索引，那么就需要回表进行再进行一次查询，多消耗一次IO。

第四部：根据覆盖索引查询数据

mysql> select gender ,name from person4cindex where gender in ("male","female");  +--------+----------+  | gender | name     |  +--------+----------+  | female | wenwen   |  | male   | zhaoming |  +--------+----------+  2 rows in set (0.01 sec)

这里需要注意的是，我的字段查询顺序变了，是gender,name而不在是前面的name,gender，这样是为了走覆盖索引。具体看效果吧

还是先看执行计划：

mysql> explain select gender ,name from person4cindex where gender in ("male","female");  +----+-------------+---------------+-------+---------------+------+---------+------+------+--------------------------+  | id | select_type | table         | type  | possible_keys | key  | key_len | ref  | rows | Extra                    |  +----+-------------+---------------+-------+---------------+------+---------+------+------+--------------------------+  |  1 | SIMPLE      | person4cindex | index | NULL          | name | 44      | NULL |    2 | Using where; Using index |  +----+-------------+---------------+-------+---------------+------+---------+------+------+--------------------------+  1 row in set (0.00 sec)

最后栏Extra中表示走的就是覆盖索引。

看消耗的时间吧：

mysql> show profiles;  +----------+------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------+  | Query_ID | Duration   | Query                                                                                                                                                            |  +----------+------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------+  |       83 | 0.00115400 | select gender ,name from person4cindex where gender in ("male","female")                                                                                         |  |       84 | 0.00074000 | explain select gender ,name from person4cindex where gender in ("male","female")                                                                                 |  +----------+------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------+

我们看到消耗的时间是0.00115400，看这个数字好像挺高的，那么都花在什么地方了呢？

看下具体的消耗情况：

mysql> show profile for query 83 ;  +--------------------------------+----------+  | Status                         | Duration |  +--------------------------------+----------+  | starting                       | 0.000083 |  | checking query cache for query | 0.000113 |  | Opening tables                 | 0.000039 |  | System lock                    | 0.000026 |  | Table lock                     | 0.000075 |  | init                           | 0.000128 |  | optimizing                     | 0.000193 |  | statistics                     | 0.000056 |  | preparing                      | 0.000038 |  | executing                      | 0.000021 |  | Sending data                   | 0.000121 |  | end                            | 0.000042 |  | query end                      | 0.000021 |  | freeing items                  | 0.000112 |  | storing result in query cache  | 0.000043 |  | logging slow query             | 0.000021 |  | cleaning up                    | 0.000022 |  +--------------------------------+----------+  17 rows in set (0.00 sec)

很惊奇吧，在初始化和优化上消耗了这么多时间，取数据基恩差不多。

总  结：

有了上面这些数据，那么我们整理下吧。未存在缓存下的数据。

看这个表，全表扫描最慢，我们可以理解，同时主键查询比覆盖所有扫描慢也还能接受，但是为什么主键扫描会比非主键扫描慢？而且非主键查询需要消耗的1次查询的io+一次回表的查询IO，理论上是要比主键扫描慢，而出来的数据缺不是如此。那么就仔细看下是个查询方式在各个主要阶段消耗的时间吧。

查询是否存在缓存，打开表及锁表这些操作时间是差不多，我们不会计入。具体还是看init，optimizing等环节消耗的时间。

1.从这个表中，我们看到非主键索引和覆盖索引在准备时间上需要开销很多的时间，预估这两种查询方式都需要进行回表操作，所以花在准备上更多时间。

2.第二项optimizing上，可以清晰知道，覆盖索引话在优化上大量的时间，这样在二级索引上就无需回表。

3. Sendingdata，全表扫描慢就慢在这一项上，因为是加载所有的数据页，所以花费在这块上时间较大，其他三者都差不多。

4. 非主键查询话在freeingitems上时间最少，那么可以看出它在读取数据块的时候最少。

5.相比较主键查询和非主键查询，非主键查询在Init，statistics都远高于主键查询，只是在freeingitems开销时间比主键查询少。因为这里测试数据比较少，但是我们可以预见在大数据量的查询上，不走缓存的话，那么主键查询的速度是要快于非主键查询的，本次数据不过是太小体现不出差距而已。

6.在大多数情况下，全表扫描还是要慢于索引扫描的。

tips:

过程中的辅助命令：

1.清楚缓存

reset query cache ;

flush tables;

2.查看表的索引：

show index from tablename;

原文链接：http://inter12.iteye.com/blog/1430144