MySQL 通過多個示例學習索引

阿新 • • 發佈：2018-09-02

cde 註意實例 query affect 大於等於篩選條件 notice highlight

　　最近在準備面試，關於索引這一塊，發現很多以前忽略的點，這裏好好整理一下

首先為什麽要建立索引

　　一本書，有章、節、段、行這種單位。

　　如果現在需要找一個內容：第9章>第2節>第3段>第4行>第5個字。

　　如果沒有索引（目錄），那麽，就必須從第一頁開始，按照這個順序：第1章>第1節>第1段>第1行>第1個字開始，順序查找，也就是說，如果運氣足夠壞，那麽，找到目標的時候，已經將整本書都差不多掃了一遍。

　　但是現在，如果建立了索引（目錄），只是建立了 “章” 的索引：每一章的第一頁，那麽，我們開始查找的時候，並不是從第1章開始，我們可以根據目錄，查找到第9章的頁數，然後直接翻到第9章的那章內容，初始就是第9章>第1節>第1段>第1行>第1個字，然後仍舊開始順序查找，那麽這個時候，雖然仍然有點慢，但是已經快了很多很多。

　　現在，如果建立（章-節）的索引，記錄每一章的每一個小節的第1頁，那麽我們開始查找的時候，可以通過讀一下索引，直接找到第9章的第2小節，然後從這1小節的第1段的第1行的第1個字開始查找。仍舊順序查找。

　　很好，你會想，可以建立（章-節-段）的索引，那麽，一開始就可以直接定位到第9章>第2小節>第3段，然後從這段的第1行的第1個字開始順序查找。

　　那麽，有沒有必要建立（章-節-段-行）的索引呢？可以根據情況而定。

索引類型

　　有很多中索引，比如：

　　1、普通索引：最基本的索引，沒有任何約束限制

　　2、唯一索引：與主鍵索引類似，但是不允許出現相同的值（唯一性約束）。

　　3、主鍵索引：特殊的唯一索引，不允許有空值。

　　4、復蓋索引（組合索引）：將多個列組合在一起創建索引，可以覆蓋多個列，比如“章-節-段”建立索引

　　5、外鍵索引：只有Innodb支持，保證數據一致性、完整性，實現級聯操作。

　　6、全文索引：只有MyISAM引擎支持，並且只支持對英文進行全文檢索。

建立索引

　　假設有一個user表（用戶表）

mysql> desc user;
+-------+----------+------+-----+---------+----------------+
| Field | Type     | Null | Key | Default | Extra          |
+-------+----------+------+-----+---------+----------------+
| uid   | int(11)  | NO   | PRI | NULL    | auto_increment |
| uname | char(20) | NO   | UNI | NULL    |                |
| addr  | char(20) | NO   | MUL | NULL    |                |
| token | char(20) | NO   |     | NULL    |                |
+-------+----------+------+-----+---------+----------------+

　　uid因為是主鍵，所以已經默認創建了主鍵索引　　

　　為uname字段建立一個唯一索引，為addr字段建立普通索引：

　　token字段不建立索引

mysql> create unique index uname_index on user(uname);
mysql> create index addr_index on user(addr);

查看索引

mysql> show index from user;
*************************** 1. row ***************************
        Table: user
   Non_unique: 0
     Key_name: PRIMARY
 Seq_in_index: 1
  Column_name: uid
    Collation: A
  Cardinality: 0
     Sub_part: NULL
       Packed: NULL
         Null:
   Index_type: BTREE
      Comment:
Index_comment:
*************************** 2. row ***************************
        Table: user
   Non_unique: 0
     Key_name: uname_index
 Seq_in_index: 1
  Column_name: uname
    Collation: A
  Cardinality: 0
     Sub_part: NULL
       Packed: NULL
         Null:
   Index_type: BTREE
      Comment:
Index_comment:
*************************** 3. row ***************************
        Table: user
   Non_unique: 1
     Key_name: addr_index
 Seq_in_index: 1
  Column_name: addr
    Collation: A
  Cardinality: NULL
     Sub_part: NULL
       Packed: NULL
         Null:
   Index_type: BTREE
      Comment:
Index_comment:
3 rows in set (0.00 sec)

插入數據

　　使用pdo循環插入10000條記錄，各個字段除了uid自增之外，都是隨機值。

<?php
    $pdo = new PDO("mysql:host=127.0.0.1;dbname=MyDb;charset=utf8", "root", "root");

    $stmt = $pdo->prepare("insert into user (uname, addr, token) values (?, ?, ?)");

    $str = "abcdefghijklmnopqrstuvwxzy";

    for ($j = 0; $j < 10000; $j++) {
        $name = "";
        $addr = "";
        $token = "";
        for ($i = 0; $i < 7; $i++) {
            $name .= $str[mt_rand(0, 25)];
            $addr .= $str[mt_rand(0, 25)];
            $token .= $str[mt_rand(0, 25)];
        }
        $stmt->execute(array($name, $addr, $token));
    }

?>

是否會使用索引？

　　示例1：

mysql> explain select count(*) from user;
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra                        |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
|  1 | SIMPLE      | NULL  | NULL | NULL          | NULL | NULL    | NULL | NULL | Select tables optimized away |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
1 row in set (0.00 sec)

　　未使用索引，因為沒有使用where子句，索引是在查找的時候（有where的時候才使用索引）。

　　示例2：

mysql> explain select * from user where uname=‘outalwa‘;
+----+-------------+-------+-------+---------------+-------------+---------+-------+------+-------+
| id | select_type | table | type  | possible_keys | key         | key_len | ref   | rows | Extra |
+----+-------------+-------+-------+---------------+-------------+---------+-------+------+-------+
|  1 | SIMPLE      | user  | const | uname_index   | uname_index | 60      | const |    1 |       |
+----+-------------+-------+-------+---------------+-------------+---------+-------+------+-------+
1 row in set (0.00 sec)

mysql> explain select * from user where uname!=‘outalwa‘;
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows  | Extra       |
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
|  1 | SIMPLE      | user  | ALL  | uname_index   | NULL | NULL    | NULL | 10000 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
1 row in set (0.00 sec)

mysql> explain select * from user where uname<>‘outalwa‘;
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows  | Extra       |
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
|  1 | SIMPLE      | user  | ALL  | uname_index   | NULL | NULL    | NULL | 10000 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
1 row in set (0.00 sec)

　　對於一個建了索引的列來說，如果使用=、!=、<>來判斷字段值是否與一個值相等的時候，只有使用=的時候會使用索引，而!=和<>不會使用索引。

　　示例3：

mysql> explain select * from user where uid=‘9999‘ and uname=‘muqehaq‘;                              
+----+-------------+-------+-------+---------------------+---------+---------+-------+------+-------+
| id | select_type | table | type  | possible_keys       | key     | key_len | ref   | rows | Extra |
+----+-------------+-------+-------+---------------------+---------+---------+-------+------+-------+
|  1 | SIMPLE      | user  | const | PRIMARY,uname_index | PRIMARY | 4       | const |    1 |       |
+----+-------------+-------+-------+---------------------+---------+---------+-------+------+-------+
1 row in set (0.06 sec)

　　因為uid建立了主鍵索引，uname建立了唯一索引，使用and表示（並且）的關系時，會使用各自的索引。只掃描了1行數據就找到了

　　示例4：

mysql> explain select * from user where uid=‘9999‘ or uname=‘muqehaq‘;
+----+-------------+-------+-------------+---------------------+---------------------+---------+------+------+-----------------------------------------------+
| id | select_type | table | type        | possible_keys       | key                 | key_len | ref  | rows | Extra                                         |
+----+-------------+-------+-------------+---------------------+---------------------+---------+------+------+-----------------------------------------------+
|  1 | SIMPLE      | user  | index_merge | PRIMARY,uname_index | PRIMARY,uname_index | 4,60    | NULL |    2 | Using union(PRIMARY,uname_index); Using where |
+----+-------------+-------+-------------+---------------------+---------------------+---------+------+------+-----------------------------------------------+
1 row in set (0.00 sec)

　　因為uid建立了主鍵索引，uname建立了唯一索引，使用or表示（或）的關系時，使用了索引。

　　示例5

mysql> explain select * from user where uid=‘9999‘ and token=‘dakghzz‘;
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref   | rows | Extra |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
|  1 | SIMPLE      | user  | const | PRIMARY       | PRIMARY | 4       | const |    1 |       |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+

　　uid有主鍵索引，token沒有建立索引，使用and來連接條件，可以使用索引。

　　如果條件是token=‘dakghzz‘ and uid=9999 ，同樣會使用索引。

　　示例6

mysql> explain select * from user where uid=‘9999‘ or token=‘dakghzz‘;
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows  | Extra       |
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
|  1 | SIMPLE      | user  | ALL  | PRIMARY       | NULL | NULL    | NULL | 10000 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
1 row in set (0.00 sec)

　　uid有主鍵索引，但是token沒有建立索引，使用or來連接的時候，可以看到，查詢並沒有使用索引。

　　示例7

mysql> explain select count(*) from user where uid < 1000;
+----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows | Extra                    |
+----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+
|  1 | SIMPLE      | user  | range | PRIMARY       | PRIMARY | 4       | NULL | 1401 | Using where; Using index |
+----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+
1 row in set (0.00 sec)

mysql> explain select count(*) from user where uid between 10 and 1000;
+----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows | Extra                    |
+----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+
|  1 | SIMPLE      | user  | range | PRIMARY       | PRIMARY | 4       | NULL | 1388 | Using where; Using index |
+----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+
1 row in set (0.00 sec)

mysql> explain select count(*) from user where uid >10 and uid < 1000;
+----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows | Extra                    |
+----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+
|  1 | SIMPLE      | user  | range | PRIMARY       | PRIMARY | 4       | NULL | 1386 | Using where; Using index |
+----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+
1 row in set (0.00 sec)

　　可以看到，對一個創建了索引的列，使用大於、小於、大於等於、小於等於、between and 最為篩選條件，都會使用索引。

　　示例8

mysql> explain select * from user where uname=null;
+----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra                                               |
+----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+
|  1 | SIMPLE      | NULL  | NULL | NULL          | NULL | NULL    | NULL | NULL | Impossible WHERE noticed after reading const tables |
+----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+
1 row in set (0.00 sec)

mysql> explain select * from user where uname!=null;
+----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra                                               |
+----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+
|  1 | SIMPLE      | NULL  | NULL | NULL          | NULL | NULL    | NULL | NULL | Impossible WHERE noticed after reading const tables |
+----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+
1 row in set (0.00 sec)

mysql> explain select * from user where uname is null;
+----+-------------+-------+------+---------------+------+---------+------+------+------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra            |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------+
|  1 | SIMPLE      | NULL  | NULL | NULL          | NULL | NULL    | NULL | NULL | Impossible WHERE |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------+
1 row in set (0.00 sec)

mysql> explain select * from user where uname is not null;
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows  | Extra       |
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
|  1 | SIMPLE      | user  | ALL  | uname_index   | NULL | NULL    | NULL | 10000 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
1 row in set (0.00 sec)

　　　　對於一個建立了索引的列來說，判斷字段是否為0時，使用=，!=，is null這幾種方式都不會使用索引，但是is not null可以使用索引。

　　示例9

mysql> explain select * from user where uname like ‘%wolwgrc‘;
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows  | Extra       |
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
|  1 | SIMPLE      | user  | ALL  | NULL          | NULL | NULL    | NULL | 10000 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
1 row in set (0.00 sec)

mysql> explain select * from user where uname like ‘%wolwgrc%‘;
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows  | Extra       |
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
|  1 | SIMPLE      | user  | ALL  | NULL          | NULL | NULL    | NULL | 10000 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+

mysql> explain select * from user where uname like ‘wolwgrc‘;
+----+-------------+-------+-------+---------------+-------------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key         | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+-------------+---------+------+------+-------------+
|  1 | SIMPLE      | user  | range | uname_index   | uname_index | 60      | NULL |    1 | Using where |
+----+-------------+-------+-------+---------------+-------------+---------+------+------+-------------+
1 row in set (0.00 sec)

mysql> explain select * from user where uname like ‘wolwgrc%‘;
+----+-------------+-------+-------+---------------+-------------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key         | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+-------------+---------+------+------+-------------+
|  1 | SIMPLE      | user  | range | uname_index   | uname_index | 60      | NULL |    1 | Using where |
+----+-------------+-------+-------+---------------+-------------+---------+------+------+-------------+
1 row in set (0.00 sec)

　　對一個已經建立了索引的列使用模式匹配的時候，%keyword、%keyword%不會使用索引，而keyword和keyword%會使用索引。

普通索引的註意事項

　　1、對於一個沒有where的查詢，不會使用索引，所以，在創建索引的時候，應該以where子句的字段為準，而不是select後面的字段為準。

　　2、對於一個建了索引的列來說，如果使用=、!=、<>來判斷字段值是否與一個值相等的時候，只有使用=的時候會使用索引，而!=和<>不會使用索引。

　　3、where包含多個字段，如果每個字段都建了索引，那麽使用and和or連接的時候，查詢會使用索引。

　　4、where包含多個字段，如果一部分字段建立了索引，而有一部分字段沒有建立索引，那麽使用and連接的時候，查詢的時候，建立了索引的字段會使用索引，而沒有建立索引的字段不會使用索引。

　　5、where包含多個字段，如果一部分字段建立了索引，而有一部分字段沒有建立索引，那麽使用or連接的時候，查詢的時候，不會使用索引。

　　6、對一個創建了索引的列，使用大於、小於、大於等於、小於等於、between and 最為篩選條件，都會使用索引。

　　7、對於一個建立了索引的列來說，判斷字段是否為0時，使用=，!=，is null這幾種方式都不會使用索引，但是is not null可以使用索引。

　　8、對一個已經建立了索引的列使用模式匹配的時候，%keyword、%keyword%不會使用索引，而keyword和keyword%會使用索引。

復合索引

　　復合索引有前綴原則，後面示例會解釋。

　　刪除之前為uname字段和addr字段創建的索引。

　　創建一個復合索引，包含uname、addr、token，命令如下：

mysql> drop index uname_index on user;                                                                                                                               
Query OK, 10000 rows affected (0.27 sec)                                                                                                                             
Records: 10000  Duplicates: 0  Warnings: 0                                                                                                                           
                                                                                                                                                                     
mysql> drop index addr_index on user;                                                                                                                                
Query OK, 10000 rows affected (0.24 sec)                                                                                                                             
Records: 10000  Duplicates: 0  Warnings: 0                                                                                                                           
                                                                                                                                                                     
mysql> create index three_fields_index on user(uname, addr, token);                                                                                                  
Query OK, 10000 rows affected (0.25 sec)                                                                                                                             
Records: 10000  Duplicates: 0  Warnings: 0                                                                                                                           
                                                                                                                                                                     
mysql> show index from user;                                                                                                                                         
+-------+------------+--------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ 
| Table | Non_unique | Key_name           | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | 
+-------+------------+--------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ 
| user  |          0 | PRIMARY            |            1 | uid         | A         |       10000 |     NULL | NULL   |      | BTREE      |         |               | 
| user  |          1 | three_fields_index |            1 | uname       | A         |       10000 |     NULL | NULL   |      | BTREE      |         |               | 
| user  |          1 | three_fields_index |            2 | addr        | A         |       10000 |     NULL | NULL   |      | BTREE      |         |               | 
| user  |          1 | three_fields_index |            3 | token       | A         |       10000 |     NULL | NULL   |      | BTREE      |         |               | 
+-------+------------+--------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

　　可以從上面索引查詢結果中看出，uname、addr、token都包含在一個索引中（Key_name相同）。

覆蓋索引的使用示例

　　下面的實例，簡化一下，創建的索引是key(uname, addr, token)，那麽對應key(A, B, C)

　　示例1

mysql> explain select * from user where uname=‘muqehaq‘ and addr=‘hgommlp‘ and token=‘dakghzz‘;
+----+-------------+-------+------+--------------------+--------------------+---------+-------------------+------+-------------+
| id | select_type | table | type | possible_keys      | key                | key_len | ref               | rows | Extra       |
+----+-------------+-------+------+--------------------+--------------------+---------+-------------------+------+-------------+
|  1 | SIMPLE      | user  | ref  | three_fields_index | three_fields_index | 180     | const,const,const |    1 | Using where |
+----+-------------+-------+------+--------------------+--------------------+---------+-------------------+------+-------------+
1 row in set (0.00 sec)

mysql> explain select * from user where addr=‘hgommlp‘ and token=‘dakghzz‘ and uname=‘muqehaq‘;
+----+-------------+-------+------+--------------------+--------------------+---------+-------------------+------+-------------+
| id | select_type | table | type | possible_keys      | key                | key_len | ref               | rows | Extra       |
+----+-------------+-------+------+--------------------+--------------------+---------+-------------------+------+-------------+
|  1 | SIMPLE      | user  | ref  | three_fields_index | three_fields_index | 180     | const,const,const |    1 | Using where |
+----+-------------+-------+------+--------------------+--------------------+---------+-------------------+------+-------------+
1 row in set (0.00 sec)

　　因為建立了uname-addr-token的聯合索引，在where中，三個字段都出現了（和建立索引時使用的字段列相同時），此時，和每個字段的順序無關，都會使用索引。

　　結論：where A=‘xx‘ and B=‘xx‘ and 或者where C=‘xx‘ and B=‘xx‘ and A=‘xx‘會使用索引。

　　示例2

mysql> explain select * from user where uname=‘muqehaq‘ and addr=‘hgommlp‘;
+----+-------------+-------+------+--------------------+--------------------+---------+-------------+------+-------------+
| id | select_type | table | type | possible_keys      | key                | key_len | ref         | rows | Extra       |
+----+-------------+-------+------+--------------------+--------------------+---------+-------------+------+-------------+
|  1 | SIMPLE      | user  | ref  | three_fields_index | three_fields_index | 120     | const,const |    1 | Using where |
+----+-------------+-------+------+--------------------+--------------------+---------+-------------+------+-------------+
1 row in set (0.00 sec)

　　示例：where A=‘xx‘ and B=‘xx‘

　　結論：會使用索引，使用復合索引中的前兩個字段的索引。

　　示例3：

mysql> explain select * from user where uname=‘muqehaq‘ and token=‘dakghzz‘;
+----+-------------+-------+------+--------------------+--------------------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys      | key                | key_len | ref   | rows | Extra       |
+----+-------------+-------+------+--------------------+--------------------+---------+-------+------+-------------+
|  1 | SIMPLE      | user  | ref  | three_fields_index | three_fields_index | 60      | const |    1 | Using where |
+----+-------------+-------+------+--------------------+--------------------+---------+-------+------+-------------+
1 row in set (0.00 sec)

mysql> explain select * from user where token=‘dakghzz‘ and uname=‘muqehaq‘;
+----+-------------+-------+------+--------------------+--------------------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys      | key                | key_len | ref   | rows | Extra       |
+----+-------------+-------+------+--------------------+--------------------+---------+-------+------+-------------+
|  1 | SIMPLE      | user  | ref  | three_fields_index | three_fields_index | 60      | const |    1 | Using where |
+----+-------------+-------+------+--------------------+--------------------+---------+-------+------+-------------+
1 row in set (0.00 sec)

　　示例：where A=‘xxx‘ and C=‘xx‘ 或者 where C=‘xx‘ and A=‘xxx‘

　　結論：查詢A字段的時候，會使用復合索引中的A那一部分，但是查詢C字段的時候，並不會使用復合索引。

　　原因：因為沒有使用復合索引中的B字段。

　　示例4

mysql> explain select * from user where addr=‘hgommlp‘ and token=‘dakghzz‘;
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows  | Extra       |
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
|  1 | SIMPLE      | user  | ALL  | NULL          | NULL | NULL    | NULL | 10000 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
1 row in set (0.00 sec)

　　示例：where B=‘xxx‘ and C=‘xxx‘

　　結論：不會使用索引。

　　原因：沒有使用復合索引中B前面的A字段索引。

　　示例5

mysql> explain select * from user where uname=‘muqehaq‘;                                                              
+----+-------------+-------+------+--------------------+--------------------+---------+-------+------+-------------+  
| id | select_type | table | type | possible_keys      | key                | key_len | ref   | rows | Extra       |  
+----+-------------+-------+------+--------------------+--------------------+---------+-------+------+-------------+  
|  1 | SIMPLE      | user  | ref  | three_fields_index | three_fields_index | 60      | const |    1 | Using where |  
+----+-------------+-------+------+--------------------+--------------------+---------+-------+------+-------------+  
1 row in set (0.00 sec)                                                                                               
                                                                                                 
mysql> explain select * from user where addr=‘hgommlp‘;                                                               
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+                     
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows  | Extra       |                     
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+                     
|  1 | SIMPLE      | user  | ALL  | NULL          | NULL | NULL    | NULL | 10000 | Using where |                     
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+                     
1 row in set (0.00 sec)                                                                                               
                                                                                                                      
mysql> explain select * from user where token=‘dakghzz‘;                                                              
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+                     
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows  | Extra       |                     
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+                     
|  1 | SIMPLE      | user  | ALL  | NULL          | NULL | NULL    | NULL | 10000 | Using where |                     
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+

　　示例：where A=‘xxx‘ 結論：會使用索引。

　　示例：where B=‘xxx‘ 結論：不會使用索引。

　　示例：where C=‘xxx‘ 結論：不會使用索引。

復合索引的使用總結

　　假設創建了key（a，b，c）復合索引，那麽：

　　where a = 1 and b = 2 and c = 3; 會使用索引

　　where a = 1 and c = 3 and b = 2; 會使用索引

　　where a = 1 and b = 2 ；會使用索引

　　where a = 1;會使用索引

　　where a = 1 and c = 3; 不會使用索引，因為c前面的b沒有使用

　　where b = 2 and c = 3; 不會使用索引，因為b前面的a沒有使用

MySQL 通過多個示例學習索引

cde 註意實例 query affect 大於等於篩選條件 notice highlight 　　最近在準備面試，關於索引這一塊，發現很多以前忽略的點，這裏好好整理一下　　首先為什麽要建立索引　　一本書，有章、節、段、行這種單位。　　如果現在需要找一個內容：第

MySQL 通過多個示例學習索引

首先為什麽要建立索引

索引類型

建立索引

查看索引

插入數據

是否會使用索引？

示例1：

示例2：

示例3：

示例4：

示例5

示例6

示例7

示例8

示例9

普通索引的註意事項

復合索引

覆蓋索引的使用示例

示例1

示例2

示例3：

示例4

示例5

復合索引的使用總結

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　　示例9

　　示例1

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　　示例3：

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　　示例5