mysql慢查詢優化之從理論和實踐說明limit的優點

阿新 • • 發佈：2020-01-09

很多時候，我們預期查詢的結果最多是1條記錄資料，那麼這個時候，最好用上limit 1,當查到這條資料後， mysql會立即終止繼續查詢，不進行更多的無用查詢，從而提升了效率。

我們來實際測試一下，在一個擁有10萬的mysql表中，查詢lily的分數（假設系統中只有1個lily,而我們預期也只需要這條資料）。為了顯示出時間的差別，我並不對錶的name欄位建索引。

先看看錶結構：

mysql> show create table tb_province;
+-------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table    | Create Table                                                                                                                                                                                                                                                                                                                           |
+-------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tb_province | CREATE TABLE `tb_province` (
 `id` bigint(10) unsigned NOT NULL AUTO_INCREMENT,`name` varchar(32) NOT NULL,`score` int(10) unsigned DEFAULT '0',`x` int(10) unsigned DEFAULT '0',`x1` int(10) unsigned DEFAULT '0',`x2` int(10) unsigned DEFAULT '0',`x3` int(10) unsigned DEFAULT '0',`x4` int(10) unsigned DEFAULT '0',`x5` int(10) unsigned DEFAULT '0',`x6` int(10) unsigned DEFAULT '0',`x7` int(10) unsigned DEFAULT '0',`x8` int(10) unsigned DEFAULT '0',`x9` int(10) unsigned DEFAULT '0',`x10` int(10) unsigned DEFAULT '0',PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=124178 DEFAULT CHARSET=latin1 |
+-------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

我們開啟set profiling=1;的開關，執行mysql語句來對比：

mysql> select score from tb_province where name='lily';
+-------+
| score |
+-------+
|  100 |
+-------+
1 row in set (0.03 sec)

mysql> select score from tb_province where name='lily';
+-------+
| score |
+-------+
|  100 |
+-------+
1 row in set (0.03 sec)

mysql> select score from tb_province where name='lily';
+-------+
| score |
+-------+
|  100 |
+-------+
1 row in set (0.04 sec)

mysql> select score from tb_province where name='lily';
+-------+
| score |
+-------+
|  100 |
+-------+
1 row in set (0.02 sec)

mysql> select score from tb_province where name='lily';
+-------+
| score |
+-------+
|  100 |
+-------+
1 row in set (0.03 sec)

mysql> select score from tb_province where name='lily' limit 1;
+-------+
| score |
+-------+
|  100 |
+-------+
1 row in set (0.00 sec)

mysql> select score from tb_province where name='lily' limit 1;
+-------+
| score |
+-------+
|  100 |
+-------+
1 row in set (0.00 sec)

mysql> select score from tb_province where name='lily' limit 1;
+-------+
| score |
+-------+
|  100 |
+-------+
1 row in set (0.00 sec)

mysql> select score from tb_province where name='lily' limit 1;
+-------+
| score |
+-------+
|  100 |
+-------+
1 row in set (0.01 sec)

mysql> select score from tb_province where name='lily' limit 1;
+-------+
| score |
+-------+
|  100 |
+-------+
1 row in set (0.00 sec)

可見，我們針對是否採用limit 1進行了5次對比測試，來看看結果吧：

mysql> show profiles;
+----------+------------+---------------------------------------------------------+
| Query_ID | Duration  | Query                          |
+----------+------------+---------------------------------------------------------+
|    5 | 0.02686000 | select score from tb_province where name='lily'     |
|    6 | 0.02649050 | select score from tb_province where name='lily'     |
|    7 | 0.03413500 | select score from tb_province where name='lily'     |
|    8 | 0.02601350 | select score from tb_province where name='lily'     |
|    9 | 0.02785775 | select score from tb_province where name='lily'     |
|    10 | 0.00042300 | select score from tb_province where name='lily' limit 1 |
|    11 | 0.00043250 | select score from tb_province where name='lily' limit 1 |
|    12 | 0.00044350 | select score from tb_province where name='lily' limit 1 |
|    13 | 0.00053200 | select score from tb_province where name='lily' limit 1 |
|    14 | 0.00043250 | select score from tb_province where name='lily' limit 1 |
+----------+------------+---------------------------------------------------------+
14 rows in set,1 warning (0.00 sec)

可見，採用limit 1後， mysql語句的效率確實提升很多。當表更大時，效率提升會更加明顯。

我們已經從理論和實踐的指令碼都說明了limit的優點，所以，建議是：在可用limit的時候要用limit （當然，如果結果是多個，肯定不能limit 1啊）

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