由淺入深探究mysql索引結構原理、性能分析與優化（二）

時間  2019-11-07
標籤 由淺入深 探究 mysql 索引 結構 原理 性能 分析 優化 欄目 MySQL 简体版
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(1.1)能正確的利用索引mysql

l  Where子句表達式順序是(username)web

  1. mysql> explain select * from one where username='abgvwfnt';  sql

  2. +----+-------------+-------+------+---------------+----------+---------+-------+------+-------------+  性能

  3. | id | select_type | table | type | possible_keys | key      | key_len | ref   |rows | Extra       |  測試

  4. +----+-------------+-------+------+---------------+----------+---------+-------+------+-------------+  優化

  5. |  1 | SIMPLE      | one   | ref  | username      | username | 24      | const |5 | Using where |  編碼

  6. +----+-------------+-------+------+---------------+----------+---------+-------+------+-------------+  orm

  7. 1 row in set (0.00 sec)  索引

l  Where子句表達式順序是(username,password)it

  1. mysql> explain select * from one where username='abgvwfnt' and password='123456';  

  2. +----+-------------+-------+------+---------------+----------+---------+-------------+------+-------------+  

  3. | id | select_type | table | type | possible_keys | key      | key_len | ref | rows | Extra       |  

  4. +----+-------------+-------+------+---------------+----------+---------+-------------+------+-------------+  

  5. |  1 | SIMPLE      | one   | ref  | username      | username | 43      | const,const |    1 | Using where |  

  6. +----+-------------+-------+------+---------------+----------+---------+-------------+------+-------------+  

  7. 1 row in set (0.00 sec)  

l  Where子句表達式順序是(username,password, last_login)

  1. mysql> explain select * from one where username='abgvwfnt' and password='123456'and last_login='1338251170';  

  2. +----+-------------+-------+------+---------------+----------+---------+-------------------+------+-------------+  

  3. | id | select_type | table | type | possible_keys | key      | key_len | ref| rows | Extra       |  

  4. +----+-------------+-------+------+---------------+----------+---------+-------------------+------+-------------+  

  5. |  1 | SIMPLE   | one   | ref  | username     | username | 83      | const,const,const |    1 | Using where |  

  6. +----+-------------+-------+------+---------------+----------+---------+-------------------+------+-------------+  

  7. 1 row in set (0.00 sec)  

上面能夠看出type=ref 是多列索引，key_len分別是2四、4三、83，這說明用到的索引分別是(username), (username,password), (username,password, last_login );row分別是五、一、1檢索的數據行都不多，由於這三個查詢都按照索引前綴原則，能夠利用到索引。

 

(1.2)不能正確的利用索引

l  Where子句表達式順序是(password, last_login)

  1. mysql> explain select * from one where password='123456'and last_login='1338251170';  

  2. +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+  

  3. | id | select_type | table | type | possible_keys | key  | key_len | ref  | rows| Extra       |  

  4. +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+  

  5. |  1 | SIMPLE      | one   | ALL  | NULL          | NULL | NULL    | NULL | 20146 | Using where |  

  6. +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+  

  7. 1 row in set (0.00 sec)  

l  Where 子句表達式順序是(last_login)

  1. mysql> explain select * from one where last_login='1338252525';  

  2. +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+  

  3. | id | select_type | table | type | possible_keys | key  | key_len | ref  | rows| Extra       |  

  4. +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+  

  5. |  1 | SIMPLE      | one   | ALL  | NULL          | NULL | NULL    | NULL | 20146 | Using where |  

  6. +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+  

  7. 1 row in set (0.00 sec)  

以上的兩條語句都不是以username開始，這樣是用不了索引，經過type=all(全表掃描)，key_len=null，rows都很大20146

Ps：one表裏只有20003條數據，爲何出現20146，這是優化器對錶的一個估算值，不精確的。

l  Where 子句表達式雖然順序是(username,password, last_login)或(username,password)但第一個是有範圍’<’、’>’，’<=’，’>=’等出現

  1. mysql> explain select * from one where username>'abgvwfnt' and password ='123456'and last_login='1338251170';  

  2. +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+  

  3. | id | select_type | table | type | possible_keys | key  | key_len | ref  | rows| Extra       |  

  4. +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+  

  5. |  1 | SIMPLE      | one   | ALL  | username      | NULL | NULL    | NULL | 20146 | Using where |  

  6. +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+  

  7. 1 row in set (0.00 sec)  

這個查詢很明顯是遍歷全部表，一個索引都沒用到，非第一列出現範圍(password列或last_login列)，則能利用索引到首先出現範圍的一列，也就是「where username='abgvwfnt' and password >'123456'and last_login='1338251170';」或則「where username='abgvwfnt' and password >'123456'and last_login<'1338251170';」索引長度ref_len=43,索引檢索到password列，因此考慮多列索引的時候把那些查詢語句用的比較的列放在最後(或非第一位)。

l  斷層，便是where順序(username, last_login)

  1. mysql> explain select * from one where username='abgvwfnt' and last_login='1338252525';  

  2. +----+-------------+-------+------+---------------+----------+---------+-------+------+-------------+  

  3. | id | select_type | table | type | possible_keys | key      | key_len | ref   | rows | Extra       |  

  4. +----+-------------+-------+------+---------------+----------+---------+-------+------+-------------+  

  5. |  1 | SIMPLE   | one   | ref  | username   | username | 24     | const |5 | Using where |  

  6. +----+-------------+-------+------+---------------+----------+---------+-------+------+-------------+  

  7. 1 row in set (0.00 sec)  

注意這裏的key_len=24=8*3(8是username的長度，3是utf8編碼)，rows=5，和下面一條sql語句搜索出來同樣

  1. mysql>  select * from one where username='abgvwfnt';  

  2. +-------+----------+----------+-------+------------+  

  3. | id    | username | password | level | last_login |  

  4. +-------+----------+----------+-------+------------+  

  5. |  3597 | abgvwfnt | 234567   |     0 | 1338251420 |  

  6. |  7693 | abgvwfnt | 456789   |     0 | 1338251717 |  

  7. | 11789 | abgvwfnt | 456789   |     0 | 1338251992 |  

  8. | 15885 | abgvwfnt | 456789   |     0 | 1338252258 |  

  9. | 19981 | abgvwfnt | 456789   |     0 | 1338252525 |  

  10. +-------+----------+----------+-------+------------+  

  11. 5 rows in set (0.00 sec)  

  12.  

  13. mysql>  select * from one where username='abgvwfnt' and last_login='1338252525';  

  14. +-------+----------+----------+-------+------------+  

  15. | id    | username | password | level | last_login |  

  16. +-------+----------+----------+-------+------------+  

  17. | 19981 | abgvwfnt | 456789   |     0 | 1338252525 |  

  18. +-------+----------+----------+-------+------------+  

  19. 1 row in set (0.00 sec)  

這個就是要的返回結果，因此能夠知道斷層(username,last_login)，這樣只用到username索引，把用到索引的數據再從新檢查last_login條件，這個相對全表查詢來講仍是有性能上優化，這也是不少sql優化文章中提到的where 範圍查詢要放在最後(這不絕對，但能夠利用一部分索引)

(1.3)若是一個查詢where子句中確實不須要password列，那就用「補洞」。

  1. mysql> select distinct(password) from one;  

  2. +----------+  

  3. | password |  

  4. +----------+  

  5. | 234567   |  

  6. | 345678   |  

  7. | 456789   |  

  8. | 123456   |  

  9. +----------+  

  10. 4 rows in set (0.08 sec)

能夠看出password列中只有這幾個值，固然在現實中不可能密碼有這麼多同樣的，再說數據也可能不斷更新，這裏只是舉例說明補洞的方法

  1. mysql> explain select * from one where username='abgvwfnt' and password in('123456','234567','345678','456789')

  2. and last_login='1338251170';  

  3. +----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+  

  4. | id | select_type | table | type  | possible_keys | key      | key_len | ref  | rows | Extra       |  

  5. +----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+  

  6. |  1 | SIMPLE    | one | range | username    | username| 83      | NULL |4 | Using where |  

  7. +----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+  

  8. 1 row in set (0.00 sec)  

能夠看出ref=83 全部的索引都用到了，type=range是由於用了in子句。

這個被「補洞」列中的值應該是有限的，可預知的，如性別，其值只有男和女(加多一個不男不女也無妨)。

「補洞」方法也有瓶頸，當不少列，且須要補洞的相應列(能夠多列)的值雖有限但不少(如中國城市)的時候，優化器在優化時組合起來的數量是很大，這樣的話就要作好基準測試和性能分析，權衡得失，取得一個合理的優化方法。

(1.4)like

  1. mysql> explain select * from one where username like 'abgvwfnt%';  

  2. +----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+  

  3. | id | select_type | table | type  | possible_keys | key      | key_len | ref  |  

  4. rows | Extra       |  

  5. +----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+  

  6. |  1 | SIMPLE      | one   | range | username      | username | 24      | NULL |  

  7. 5 | Using where |  

  8. +----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+  

  9. 1 row in set (0.00 sec)  

  10. mysql> explain select * from one where username like '%abgvwfnt%';  

  11. +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+  

  12. | id | select_type | table | type | possible_keys | key  | key_len | ref  | rows| Extra       |  

  13. +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+  

  14. |  1 | SIMPLE      | one   | ALL  | NULL          | NULL | NULL    | NULL | 20259 | Using where |  

  15. +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+  

  16. 1 row in set (0.01 sec)  

對比就知道like操做abgvwfnt%能用到索引，%abgvwfnt%用不到

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