萬盛學電腦網

 萬盛學電腦網 >> 數據庫 >> mysql教程 >> mysql 隨機RAND和GROUP BY優化

mysql 隨機RAND和GROUP BY優化

隨機RAND和GROUP BY如果我們不進行優化那麼它的運行是非常的緩慢了特別是在數據多的時候會碰到這種問題了,今天我們就一起來看此問題的解決辦法哦。


最近看到一篇文章關於mysql的優化的,其中兩個塊內容平時用到的優化方法,記錄和分享一下

group by 優化

SELECT goods_id,count(*) FROM t GROUP BY goods_id;

默認情況下,MySQL對所有GROUP BY col1,col2…的字段進行排序。如果查詢包括GROUP BY,想要避免排序結果的消耗,則可以指定ORDER BY NULL禁止排序。
修改成

SELECT goods_id,count(*) FROM t GROUP BY goods_id ORDER BY NULL;

RAND隨機優化

SELECT * FROM t1 WHERE 1=1 ORDER BY RAND() LIMIT 4;

MySQL不支持函數索引,會導致全表掃描

修改成

SELECT * FROM t1 WHERE id >= CEIL(RAND()*1000) LIMIT 4;


補充:

看下建表DDL,這是一個沒有顯式自增主鍵的InnoDB表:

 

[yejr@imysql]> show create table t_innodb_random\G
*************************** 1. row ***************************
Table: t_innodb_random
Create Table: CREATE TABLE `t_innodb_random` (
`id` int(10) unsigned NOT NULL,
`user` varchar(64) NOT NULL DEFAULT '',
KEY `idx_id` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1
往這個表裡灌入一些測試數據,至少10萬以上, id 字段也是亂序的。


[yejr@imysql]> select count(*) from t_innodb_random\G
*************************** 1. row ***************************
count(*): 393216
1、常量等值檢索:

 

[yejr@imysql]> explain select id from t_innodb_random where id = 13412\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: t_innodb_random
type: ref
possible_keys: idx_id
key: idx_id
key_len: 4
ref: const
rows: 1
Extra: Using index

[yejr@imysql]> select id from t_innodb_random where id = 13412;
1 row in set (0.00 sec)

可以看到執行計劃很不錯,是常量等值查詢,速度非常快。

2、使用RAND()函數乘以常量,求得隨機數後檢索:

 

[yejr@imysql]> explain select id from t_innodb_random where id = round(rand()*13241324)\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: t_innodb_random
type: index
possible_keys: NULL
key: idx_id
key_len: 4
ref: NULL
rows: 393345
Extra: Using where; Using index

[yejr@imysql]> select id from t_innodb_random where id = round(rand()*13241324)\G
Empty set (0.26 sec)

可以看到執行計劃很糟糕,雖然是只掃描索引,但是做了全索引掃描,效率非常差。因為WHERE條件中包含了RAND(),使得MySQL把它當做變量來處理,無法用常量等值的方式查詢,效率很低。

我們把常量改成取t_innodb_random表的最大id值,再乘以RAND()求得隨機數後檢索看看什麼情況:

 

[yejr@imysql]> explain select id from t_innodb_random where id = round(rand()*(select max(id) from t_innodb_random))\G
*************************** 1. row ***************************
id: 1
select_type: PRIMARY
table: t_innodb_random
type: index
possible_keys: NULL
key: idx_id
key_len: 4
ref: NULL
rows: 393345
Extra: Using where; Using index
*************************** 2. row ***************************
id: 2
select_type: SUBQUERY
table: NULL
type: NULL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: NULL
Extra: Select tables optimized away

[yejr@imysql]> select id from t_innodb_random where id = round(rand()*(select max(id) from t_innodb_random))\G
Empty set (0.27 sec)

可以看到,執行計劃依然是全索引掃描,執行耗時也基本相當。

3、改造成普通子查詢模式 ,這裡有兩次子查詢

 

[yejr@imysql]> explain select id from t_innodb_random where id = (select round(rand()*(select max(id) from t_innodb_random)) as nid)\G
*************************** 1. row ***************************
id: 1
select_type: PRIMARY
table: t_innodb_random
type: index
possible_keys: NULL
key: idx_id
key_len: 4
ref: NULL
rows: 393345
Extra: Using where; Using index
*************************** 2. row ***************************
id: 3
select_type: SUBQUERY
table: NULL
type: NULL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: NULL
Extra: Select tables optimized away

[yejr@imysql]> select id from t_innodb_random where id = (select round(rand()*(select max(id) from t_innodb_random)) as nid)\G
Empty set (0.27 sec)

可以看到,執行計劃也不好,執行耗時較慢。

4、改造成JOIN關聯查詢,不過最大值還是用常量表示

 

[yejr@imysql]> explain select id from t_innodb_random t1 join (select round(rand()*13241324) as id2) as t2 where t1.id = t2.id2\G
*************************** 1. row ***************************
id: 1
select_type: PRIMARY
table: <derived2>
type: system
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: 1
Extra:
*************************** 2. row ***************************
id: 1
select_type: PRIMARY
table: t1
type: ref
possible_keys: idx_id
key: idx_id
key_len: 4
ref: const
rows: 1
Extra: Using where; Using index
*************************** 3. row ***************************
id: 2
select_type: DERIVED
table: NULL
type: NULL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: NULL
Extra: No tables used

[yejr@imysql]> select id from t_innodb_random t1 join (select round(rand()*13241324) as id2) as t2 where t1.id = t2.id2\G
Empty set (0.00 sec)

這時候執行計劃就非常完美了,和最開始的常量等值查詢是一樣的了,執行耗時也非常之快。

這種方法雖然很好,但是有可能查詢不到記錄,改造范圍查找,但結果LIMIT 1就可以了:

 

[yejr@imysql]> explain select id from t_innodb_random where id > (select round(rand()*(select max(id) from t_innodb_random)) as nid) limit 1\G
*************************** 1. row ***************************
id: 1
select_type: PRIMARY
table: t_innodb_random
type: index
possible_keys: NULL
key: idx_id
key_len: 4
ref: NULL
rows: 393345
Extra: Using where; Using index
*************************** 2. row ***************************
id: 3
select_type: SUBQUERY
table: NULL
type: NULL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: NULL
Extra: Select tables optimized away

[yejr@imysql]> select id from t_innodb_random where id > (select round(rand()*(select max(id) from t_innodb_random)) as nid) limit 1\G
*************************** 1. row ***************************
id: 1301
1 row in set (0.00 sec)

可以看到,雖然執行計劃也是全索引掃描,但是因為有了LIMIT 1,只需要找到一條記錄,即可終止掃描,所以效率還是很快的。

小結:

從數據庫中隨機取一條記錄時,可以把RAND()生成隨機數放在JOIN子查詢中以提高效率。

5、再來看看用ORDRR BY RAND()方式一次取得多個隨機值的方式:

 

[yejr@imysql]> explain select id from t_innodb_random order by rand() limit 1000\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: t_innodb_random
type: index
possible_keys: NULL
key: idx_id
key_len: 4
ref: NULL
rows: 393345
Extra: Using index; Using temporary; Using filesort

[yejr@imysql]> select id from t_innodb_random order by rand() limit 1000;
1000 rows in set (0.41 sec)

全索引掃描,生成排序臨時表,太差太慢了。

6、把隨機數放在子查詢裡看看:

 

[yejr@imysql]> explain select id from t_innodb_random where id > (select rand() * (select max(id) from t_innodb_random) as nid) limit 1000\G
*************************** 1. row ***************************
id: 1
select_type: PRIMARY
table: t_innodb_random
type: index
possible_keys: NULL
key: idx_id
key_len: 4
ref: NULL
rows: 393345
Extra: Using where; Using index
*************************** 2. row ***************************
id: 3
select_type: SUBQUERY
table: NULL
type: NULL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: NULL
Extra: Select tables optimized away

[yejr@imysql]> select id from t_innodb_random where id > (select rand() * (select max(id) from t_innodb_random) as nid) limit 1000\G
1000 rows in set (0.04 sec)

嗯,提速了不少,這個看起來還不賴:)

7、仿照上面的方法,改成JOIN和隨機數子查詢關聯

 

[yejr@imysql]> explain select id from t_innodb_random t1 join (select rand() * (select max(id) from t_innodb_random) as nid) t2 on t1.id > t2.nid limit 1000\G
*************************** 1. row ***************************
id: 1
select_type: PRIMARY
table: <derived2>
type: system
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: 1
Extra:
*************************** 2. row ***************************
id: 1
select_type: PRIMARY
table: t1
type: range
possible_keys: idx_id
key: idx_id
key_len: 4
ref: NULL
rows: 196672
Extra: Using where; Using index
*************************** 3. row ***************************
id: 2
select_type: DERIVED
table: NULL
type: NULL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: NULL
Extra: No tables used
*************************** 4. row ***************************
id: 3
select_type: SUBQUERY
table: NULL
type: NULL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: NULL
Extra: Select tables optimized away

[yejr@imysql]> select id from t_innodb_random t1 join (select rand() * (select max(id) from t_innodb_random) as nid) t2 on t1.id > t2.nid limit 1000\G
1000 rows in set (0.00 sec)

可以看到,全索引檢索,發現符合記錄的條件後,直接取得1000行,這個方法是最快的。

綜上,想從MySQL數據庫中隨機取一條或者N條記錄時,最好把RAND()生成隨機數放在JOIN子查詢中以提高效率。
上面說了那麼多的廢話,最後簡單說下,就是把下面這個SQL:

 

SELECT id FROM table ORDER BY RAND() LIMIT n;
改造成下面這個:


SELECT id FROM table t1 JOIN (SELECT RAND() * (SELECT MAX(id) FROM table) AS nid) t2 ON t1.id > t2.nid LIMIT n;
就可以享受在SQL中直接取得隨機數了,不用再在程序中構造一串隨機數去檢索了。

copyright © 萬盛學電腦網 all rights reserved