Mysql高級4-索引的使用規則

来源:https://www.cnblogs.com/Se7eN-HOU/archive/2023/07/28/17587373.html
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一、最左首碼法則 如果索引了多列（聯合索引），要遵守最左首碼法則。最左首碼法則指的是查詢從索引的最左列開始，並且不跳過索引中的列，如果跳躍某一列，索引將部分失效（後面的欄位索引失效） 示例1：account_transaction表中創建一個聯合索引，使用method欄位+trader_staff_ ...


一、最左首碼法則

　　如果索引了多列（聯合索引），要遵守最左首碼法則。最左首碼法則指的是查詢從索引的最左列開始，並且不跳過索引中的列，如果跳躍某一列，索引將部分失效（後面的欄位索引失效）

　　示例1：account_transaction表中創建一個聯合索引，使用method欄位+trader_staff_id欄位+operator_staff_id欄位三個欄位當做聯合索引

mysql> create index mto on account_transaction(method, trader_staff_id, operator_staff_id);
Query OK, 0 rows affected (5.29 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> show index from account_transaction;
+---------------------+------------+-------------+--------------+-------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| Table               | Non_unique | Key_name    | Seq_in_index | Column_name       | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression |
+---------------------+------------+-------------+--------------+-------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| account_transaction |          0 | PRIMARY     |            1 | id                | A         |     2067077 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
| account_transaction |          1 | trade_index |            1 | trade_no          | A         |     2249115 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
| account_transaction |          1 | mto         |            1 | method            | A         |           3 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
| account_transaction |          1 | mto         |            2 | trader_staff_id   | A         |       31046 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
| account_transaction |          1 | mto         |            3 | operator_staff_id | A         |       15847 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
+---------------------+------------+-------------+--------------+-------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
5 rows in set (0.00 sec)

　　說明1：mto是一個聯合索引，裡面包含了三個欄位method,trader_staff_id,operator_staff_id三個欄位。

　　說明2：method是第1索引欄位，即也是最左索引，trader_staff_id 是第2索引， operator_staff_id 是第3索引，這個順序很重要！

 

　　案例1：同時按順序使用三個欄位查詢一條數據

mysql> select * from account_transaction where method="CASH" and trader_staff_id=275 and operator_staff_id=12;
+--------+--------------------+--------+--------+----------------------------+---------------+--------------+--------+---------+-----------------+-------------------+-----------+--------+
| id     | trade_no           | type   | method | time                       | payment       | out_trade_no | amount | balance | trader_staff_id | operator_staff_id | device_id | remark |
+--------+--------------------+--------+--------+----------------------------+---------------+--------------+--------+---------+-----------------+-------------------+-----------+--------+
|     24 | 156384428075000275 | TOP_UP | CASH   | 2019-07-23 01:11:20.221977 | LOCAL_ACCOUNT |              |  10000 |   10000 |             275 |                12 | 6         |        |
| 747793 | 157370375171000275 | TOP_UP | CASH   | 2019-11-14 03:55:51.480417 | LOCAL_ACCOUNT |              |  10000 |   11000 |             275 |                12 | 6         |        |
+--------+--------------------+--------+--------+----------------------------+---------------+--------------+--------+---------+-----------------+-------------------+-----------+--------+
2 rows in set (0.00 sec)

mysql> explain select * from account_transaction where method="CASH" and trader_staff_id=275 and operator_staff_id=12;
+----+-------------+---------------------+------------+------+---------------+------+---------+-------------------+------+----------+-------+
| id | select_type | table               | partitions | type | possible_keys | key  | key_len | ref               | rows | filtered | Extra |
+----+-------------+---------------------+------------+------+---------------+------+---------+-------------------+------+----------+-------+
|  1 | SIMPLE      | account_transaction | NULL       | ref  | mto           | mto  | 70      | const,const,const |    2 |   100.00 | NULL  |
+----+-------------+---------------------+------------+------+---------------+------+---------+-------------------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

　　說明1：使用method,trader_staff_id,operator_staff_id三個欄位作為查詢條件，查詢時間0.00秒以內

　　說明2：使用explain關鍵字查詢執行計劃，該查詢使用的key是mto 即剛創建的聯合索引，key_len是70長度。記住這個長度，我們在後面還會用到。

 

　　案例2：使用 method 和 trader_staff_id 兩個欄位作為查詢條件

mysql> select * from account_transaction where method="CASH" and trader_staff_id=275;
+--------+--------------------+--------+--------+----------------------------+---------------+--------------+--------+---------+-----------------+-------------------+-----------+--------+
| id     | trade_no           | type   | method | time                       | payment       | out_trade_no | amount | balance | trader_staff_id | operator_staff_id | device_id | remark |
+--------+--------------------+--------+--------+----------------------------+---------------+--------------+--------+---------+-----------------+-------------------+-----------+--------+
|     24 | 156384428075000275 | TOP_UP | CASH   | 2019-07-23 01:11:20.221977 | LOCAL_ACCOUNT |              |  10000 |   10000 |             275 |                12 | 6         |        |
| 747793 | 157370375171000275 | TOP_UP | CASH   | 2019-11-14 03:55:51.480417 | LOCAL_ACCOUNT |              |  10000 |   11000 |             275 |                12 | 6         |        |
+--------+--------------------+--------+--------+----------------------------+---------------+--------------+--------+---------+-----------------+-------------------+-----------+--------+
2 rows in set (0.00 sec)

mysql> explain select * from account_transaction where method="CASH" and trader_staff_id=275;
+----+-------------+---------------------+------------+------+---------------+------+---------+-------------+------+----------+-------+
| id | select_type | table               | partitions | type | possible_keys | key  | key_len | ref         | rows | filtered | Extra |
+----+-------------+---------------------+------------+------+---------------+------+---------+-------------+------+----------+-------+
|  1 | SIMPLE      | account_transaction | NULL       | ref  | mto           | mto  | 66      | const,const |    2 |   100.00 | NULL  |
+----+-------------+---------------------+------------+------+---------------+------+---------+-------------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

　　說明1：通過explain執行計劃，可以查看使用的key仍然是mto,但是key_len只有66，比上一條的key_len少了4位。說明operator_staff_id的索引失效，並且operator_staff_id的長度為4

 

　　案例3：使用method+operator_staff_id查詢

mysql> explain select * from account_transaction where method="CASH" and operator_staff_id=12;
+----+-------------+---------------------+------------+------+---------------+------+---------+-------+-------+----------+-----------------------+
| id | select_type | table               | partitions | type | possible_keys | key  | key_len | ref   | rows  | filtered | Extra                 |
+----+-------------+---------------------+------------+------+---------------+------+---------+-------+-------+----------+-----------------------+
|  1 | SIMPLE      | account_transaction | NULL       | ref  | mto           | mto  | 62      | const | 39916 |    10.00 | Using index condition |
+----+-------------+---------------------+------------+------+---------------+------+---------+-------+-------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

　　說明1：使用了method+operator_staff_id作為查詢條件，跳過了trader_staff_id欄位，但是最左首碼method有使用，所以依然觸發了mto索引。

　　說明2：key_len=62說明索引欄位又變短了，那是因為從跳過的trader_staff_id欄位，所以trader_staff_id及之後的索引欄位就失效，案例2中的key_len是66，而現在又變成了62，說明trader_staff_id的索引長度也為4

　　　　

　　案例4：使用trader_staff_id + operator_staff_id查詢

mysql> select * from account_transaction where trader_staff_id=275 and operator_staff_id=12;
+---------+--------------------+---------------+--------+----------------------------+---------------+--------------+--------+---------+-----------------+-------------------+-----------+--------------------------+
| id      | trade_no           | type          | method | time                       | payment       | out_trade_no | amount | balance | trader_staff_id | operator_staff_id | device_id | remark                   |
+---------+--------------------+---------------+--------+----------------------------+---------------+--------------+--------+---------+-----------------+-------------------+-----------+--------------------------+
|      24 | 156384428075000275 | TOP_UP        | CASH   | 2019-07-23 01:11:20.221977 | LOCAL_ACCOUNT |              |  10000 |   10000 |             275 |                12 | 6         |                          |
|  747793 | 157370375171000275 | TOP_UP        | CASH   | 2019-11-14 03:55:51.480417 | LOCAL_ACCOUNT |              |  10000 |   11000 |             275 |                12 | 6         |                          |
| 1993075 | 160454902688000275 | REFUND        | WEB    | 2020-11-05 04:03:46.980204 | LOCAL_ACCOUNT |              |  -3200 |       0 |             275 |                12 | 43        |                          |
| 3764809 | 162122330931000275 | TOP_UP        | CHEQUE | 2021-05-17 03:48:29.748154 | LOCAL_ACCOUNT |              |  10000 |   10000 |             275 |                12 | 6         |                          |
| 4791205 | 162856536047000275 | CONSUME_LUNCH | WEB    | 2021-08-04 04:46:17.000000 | LOCAL_ACCOUNT |              |    200 |    9400 |             275 |                12 | 35        | 管理後臺補充消費         |
| 4791211 | 162856542884000275 | CONSUME_LUNCH | WEB    | 2021-08-05 04:46:17.000000 | LOCAL_ACCOUNT |              |    200 |    9200 |             275 |                12 | 35        | 管理後臺補充消費         |
| 4791217 | 162856543723000275 | CONSUME_LUNCH | WEB    | 2021-08-06 04:46:17.000000 | LOCAL_ACCOUNT |              |    200 |    9000 |             275 |                12 | 35        | 管理後臺補充消費         |
+---------+--------------------+---------------+--------+----------------------------+---------------+--------------+--------+---------+-----------------+-------------------+-----------+--------------------------+
11 rows in set (4.58 sec)

mysql> explain select * from account_transaction where trader_staff_id=275 and operator_staff_id=12;
+----+-------------+---------------------+------------+------+---------------+------+---------+------+---------+----------+-------------+
| id | select_type | table               | partitions | type | possible_keys | key  | key_len | ref  | rows    | filtered | Extra       |
+----+-------------+---------------------+------------+------+---------------+------+---------+------+---------+----------+-------------+
|  1 | SIMPLE      | account_transaction | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 2249115 |     1.00 | Using where |
+----+-------------+---------------------+------------+------+---------------+------+---------+------+---------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

　　說明1：查詢時間為4.58秒，比之前用時多了很多

　　說明2：通過explain執行計劃，可以發現該查詢語句沒有使用索引，是因為不符合最左首碼原則，即索引的最左邊的method也就是第一索引列，這一列必須要使用，是觸發組合索引的首碼。

 

　　案例5：包含最左首碼，但是最左首碼不在最前面

mysql> select * from account_transaction where trader_staff_id=275 and operator_staff_id=12 and method="CASH";
+--------+--------------------+--------+--------+----------------------------+---------------+--------------+--------+---------+-----------------+-------------------+-----------+--------+
| id     | trade_no           | type   | method | time                       | payment       | out_trade_no | amount | balance | trader_staff_id | operator_staff_id | device_id | remark |
+--------+--------------------+--------+--------+----------------------------+---------------+--------------+--------+---------+-----------------+-------------------+-----------+--------+
|     24 | 156384428075000275 | TOP_UP | CASH   | 2019-07-23 01:11:20.221977 | LOCAL_ACCOUNT |              |  10000 |   10000 |             275 |                12 | 6         |        |
| 747793 | 157370375171000275 | TOP_UP | CASH   | 2019-11-14 03:55:51.480417 | LOCAL_ACCOUNT |              |  10000 |   11000 |             275 |                12 | 6         |        |
+--------+--------------------+--------+--------+----------------------------+---------------+--------------+--------+---------+-----------------+-------------------+-----------+--------+
2 rows in set (0.00 sec)

mysql> explain select * from account_transaction where trader_staff_id=275 and operator_staff_id=12 and method="CASH";
+----+-------------+---------------------+------------+------+---------------+------+---------+-------------------+------+----------+-------+
| id | select_type | table               | partitions | type | possible_keys | key  | key_len | ref               | rows | filtered | Extra |
+----+-------------+---------------------+------------+------+---------------+------+---------+-------------------+------+----------+-------+
|  1 | SIMPLE      | account_transaction | NULL       | ref  | mto           | mto  | 70      | const,const,const |    2 |   100.00 | NULL  |
+----+-------------+---------------------+------------+------+---------------+------+---------+-------------------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

　　　　說明1：通過explain發現依然觸發了mto索引，雖然最左首碼沒有在最左邊，但是只要出現了就可以，複合最左首碼法則。

 

二、範圍查詢

　　聯合查詢索引中，出現範圍查詢(>,<),則在範圍查詢欄位在索引中靠後的索引欄位都會失效

　　案例1：查詢method="CASH" and trader_staff_id<257 and operator_staff_id=12;

mysql> show index from account_transaction;
+---------------------+------------+-------------+--------------+-------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| Table               | Non_unique | Key_name    | Seq_in_index | Column_name       | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression |
+---------------------+------------+-------------+--------------+-------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| account_transaction |          0 | PRIMARY     |            1 | id                | A         |     2067077 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
| account_transaction |          1 | trade_index |            1 | trade_no          | A         |     2249115 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
| account_transaction |          1 | mto         |            1 | method            | A         |           3 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
| account_transaction |          1 | mto         |            2 | trader_staff_id   | A         |       31046 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
| account_transaction |          1 | mto         |            3 | operator_staff_id | A         |       15847 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
+---------------------+------------+-------------+--------------+-------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+

　　說明1：mto索引欄位中method的索引順序是1，trader_staff_id的索引順序是2，operator_staff_idde的索引欄位是3

mysql> explain select * from account_transaction where trader_staff_id>275 and operator_staff_id=12 and method="CASH";
+----+-------------+---------------------+------------+-------+---------------+------+---------+------+-------+----------+----------------------------------+
| id | select_type | table               | partitions | type  | possible_keys | key  | key_len | ref  | rows  | filtered | Extra                            |
+----+-------------+---------------------+------------+-------+---------------+------+---------+------+-------+----------+----------------------------------+
|  1 | SIMPLE      | account_transaction | NULL       | range | mto           | mto  | 66      | NULL | 37708 |    10.00 | Using index condition; Using MRR |
+----+-------------+---------------------+------------+-------+---------------+------+---------+------+-------+----------+----------------------------------+
1 row in set, 1 warning (0.01 sec)

　　說明2，在搜索條件中的trader_staff_id是一個範圍查詢使用的">"，因為trader_staff_id在創建索引的時候在第2順序，所以該查詢語句中，處於第三個欄位的operator_staff_id欄位就失效了，所以key_len是66

mysql> explain select * from account_transaction where trader_staff_id>=275 and operator_staff_id=12 and method="CASH";
+----+-------------+---------------------+------------+-------+---------------+------+---------+------+-------+----------+----------------------------------+
| id | select_type | table               | partitions | type  | possible_keys | key  | key_len | ref  | rows  | filtered | Extra                            |
+----+-------------+---------------------+------------+-------+---------------+------+---------+------+-------+----------+----------------------------------+
|  1 | SIMPLE      | account_transaction | NULL       | range | mto           | mto  | 70      | NULL | 37718 |    10.00 | Using index condition; Using MRR |
+----+-------------+---------------------+------------+-------+---------------+------+---------+------+-------+----------+----------------------------------+
1 row in set, 1 warning (0.00 sec)

　　說明3：如果在不影響業務的時候，最好使用">="或者"<="，這樣就可以保證索引的正常使用   

 

三、索引列運算

　　案例1：不要再索引列上進行運算操作，索引將失效　　　　

mysql> select * from account_transaction where trade_no = "156384395941000265";
+----+--------------------+--------+--------+----------------------------+---------------+--------------+--------+---------+-----------------+-------------------+-----------+--------+
| id | trade_no           | type   | method | time                       | payment       | out_trade_no | amount | balance | trader_staff_id | operator_staff_id | device_id | remark |
+----+--------------------+--------+--------+----------------------------+---------------+--------------+--------+---------+-----------------+-------------------+-----------+--------+
| 16 | 156384395941000265 | TOP_UP | CASH   | 2019-07-23 01:05:59.102933 | LOCAL_ACCOUNT |              |  10000 |   10000 |             265 |                12 | 6         |        |
+----+--------------------+--------+--------+----------------------------+---------------+--------------+--------+---------+-----------------+-------------------+-----------+--------+
1 row in set (0.00 sec)

mysql> explain select * from account_transaction where trade_no = "156384395941000265";
+----+-------------+---------------------+------------+------+---------------+-------------+---------+-------+------+----------+-------+
| id | select_type | table               | partitions | type | possible_keys | key         | key_len | ref   | rows | filtered | Extra |
+----+-------------+---------------------+------------+------+---------------+-------------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | account_transaction | NULL       | ref  | trade_index   | trade_index | 62      | const |    1 |   100.00 | NULL  |
+----+-------------+---------------------+------------+------+---------------+-------------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select * from account_transaction where substring(trade_no, 16,3) = "265";
+----+-------------+---------------------+------------+------+---------------+------+---------+------+---------+----------+-------------+
| id | select_type | table               | partitions | type | possible_keys | key  | key_len | ref  | rows    | filtered | Extra       |
+----+-------------+---------------------+------------+------+---------------+------+---------+------+---------+----------+-------------+
|  1 | SIMPLE      | account_transaction | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 2249115 |   100.00 | Using where |
+----+-------------+---------------------+------------+------+---------------+------+---------+------+---------+----------+-------------+
1 row in set, 1 warning (0.01 sec)

　　說明1：通過 trade_no 直接查詢的時候，會觸發trade_index索引

　　說明2：先對 trade_no 欄位做字元串截取，在查詢的時候，則沒有觸發trader_index索引

 

四、字元串查詢不加引號，索引失效

　　案例1：

mysql> select * from account_transaction where trade_no = "156384395941000265";
+----+--------------------+--------+--------+----------------------------+---------------+--------------+--------+---------+-----------------+-------------------+-----------+--------+
| id | trade_no           | type   | method | time                       | payment       | out_trade_no | amount | balance | trader_staff_id | operator_staff_id | device_id | remark |
+----+--------------------+--------+--------+----------------------------+---------------+--------------+--------+---------+-----------------+-------------------+-----------+--------+
| 16 | 156384395941000265 | TOP_UP | CASH   | 2019-07-23 01:05:59.102933 | LOCAL_ACCOUNT |              |  10000 |   10000 |             265 |                12 | 6         |        |
+----+--------------------+--------+--------+----------------------------+---------------+--------------+--------+---------+-----------------+-------------------+-----------+--------+
1 row in set (0.00 sec)

mysql> explain select * from account_transaction where trade_no = "156384395941000265";
+----+-------------+---------------------+------------+------+---------------+-------------+---------+-------+------+----------+-------+
| id | select_type | table               | partitions | type | possible_keys | key         | key_len | ref   | rows | filtered | Extra |
+----+-------------+---------------------+------------+------+---------------+-------------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | account_transaction | NULL       | ref  | trade_index   | trade_index | 62      | const |    1 |   100.00 | NULL  |
+----+-------------+---------------------+------------+------+---------------+-------------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

mysql> select * from account_transaction where trade_no = 156384395941000265;
+----+--------------------+--------+--------+----------------------------+---------------+--------------+--------+---------+-----------------+-------------------+-----------+--------+
| id | trade_no           | type   | method | time                       | payment       | out_trade_no | amount | balance | trader_staff_id | operator_staff_id | device_id | remark |
+----+--------------------+--------+--------+----------------------------+---------------+--------------+--------+---------+-----------------+-------------------+-----------+--------+
| 16 | 156384395941000265 | TOP_UP | CASH   | 2019-07-23 01:05:59.102933 | LOCAL_ACCOUNT |              |  10000 |   10000 |             265 |                12 | 6         |        |
+----+--------------------+--------+--------+----------------------------+---------------+--------------+--------+---------+-----------------+-------------------+-----------+--------+
1 row in set (3.52 sec)

mysql> explain select * from account_transaction where trade_no = 156384395941000265;
+----+-------------+---------------------+------------+------+---------------+------+---------+------+---------+----------+-------------+
| id | select_type | table               | partitions | type | possible_keys | key  | key_len | ref  | rows    | filtered | Extra       |
+----+-------------+---------------------+------------+------+---------------+------+---------+------+---------+----------+-------------+
|  1 | SIMPLE      | account_transaction | NULL       | ALL  | trade_index   | NULL | NULL    | NULL | 2249115 |    10.00 | Using where |
+----+-------------+---------------------+------------+------+---------------+------+---------+------+---------+----------+-------------+
1 row in set, 3 warnings (0.01 sec)

　　說明1：第一個查詢使用了0.00秒以內，並且觸發了trade_index索引。

　　說明2：第二個查詢使用了3.52秒，沒有觸發索引，因為trade_no是字元串類型的，但是並沒有加“”。

 

五、模糊查詢

　　如果僅僅是尾部模糊匹配，索引不會失效，如果是頭部模糊匹配，索引會失效

mysql> select * from account_transaction where trade_no like "15638439594%";
+----+--------------------+--------+--------+----------------------------+---------------+--------------+--------+---------+-----------------+-------------------+-----------+--------+
|
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