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本文介紹了Mysql應用MySQL優化之對RAND()的優化方法，希望對您有用。如果有疑問，可以聯系我們。
眾所周知,在MySQL中,如果直接 ORDER BY RAND() 的話,效率非常差,因為會多次執行.事實上,如果等值查詢也是用 RAND() 的話也如此,我們先來看看下面這幾個SQL的分歧執行計劃和執行耗時.MYSQL學習
首先,看下建表DDL,這是一個沒有顯式自增主鍵的InnoDB表：
MYSQL學習
代碼如下:
[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
往這個內外灌入一些測試數據,至少10萬以上, id 字段也是亂序的.
代碼如下:
[yejr@imysql]> select count(*) from t_innodb_random\G
*************************** 1. row ***************************
count(*): 393216
1、常量等值檢索：MYSQL學習
代碼如下:
[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)MYSQL學習
可以看到執行計劃很不錯,是常量等值查詢,速度非常快.MYSQL學習
2、使用RAND()函數乘以常量,求得隨機數后檢索：MYSQL學習
代碼如下:
[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)MYSQL學習
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條件中包括了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.MYSQL學習
我們把常量改成取t_innodb_random表的最大id值,再乘以RAND()求得隨機數后檢索看看什么情況：
MYSQL學習
代碼如下:
[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)MYSQL學習
可以看到,執行計劃依然是全索引掃描,執行耗時也基原形當.MYSQL學習
3、改造成普通子查詢模式 ,這里有兩次子查詢MYSQL學習
代碼如下:
[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)MYSQL學習
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現
MYSQL學習
代碼如下:
[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)MYSQL學習
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.
這種辦法雖然很好,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：
MYSQL學習
代碼如下:
[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)MYSQL學習
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,只必要找到一條記錄,即可終止掃描,所以效率還是很快的.MYSQL學習
小結：MYSQL學習
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.MYSQL學習
5、再來看看用ORDRR BY RAND()方式一次取得多個隨機值的方式：
MYSQL學習
代碼如下:
[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)MYSQL學習
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里看看：
MYSQL學習
代碼如下:
[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)MYSQL學習
嗯,提速了不少,這個看起來還不賴：)
7、仿照上面的辦法,改成JOIN和隨機數子查詢關聯
MYSQL學習
代碼如下:
[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)MYSQL學習
可以看到,全索引檢索,發現符合記錄的條件后,直接取得1000行,這個辦法是最快的.
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.
上面說了那么多的廢話,最后簡單說下,就是把下面這個SQL：
MYSQL學習
代碼如下:
SELECT id FROM table ORDER BY RAND() LIMIT n;
改造成下面這個：
代碼如下:
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,不消再在程序中構造一串隨機數去檢索了.
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