MYSQL的全表扫描,主键索引(聚集索引、第一索引),非主键索引(非聚集索引、第二索引),覆盖索引四种不同查询的分析...

 

MYSQL的全表扫描,主键索引(聚集索引、第一索引),非主键索引(非聚集索引、第二索引),覆盖索引四种不同查询的分析

 

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。

 

MYSQL的全表扫描,主键索引(聚集索引、第一索引),非主键索引(非聚集索引、第二索引),覆盖索引四种不同查询的分析..._第1张图片

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。

 

MYSQL的全表扫描,主键索引(聚集索引、第一索引),非主键索引(非聚集索引、第二索引),覆盖索引四种不同查询的分析..._第2张图片

 

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Initstatistics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;

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