在大型数据库中通过RAND()mysql查询优化JOIN ORDER

Question

我正在开发一个拥有大型QuestionBank 的项目,对于Tests addedSystem,在运行时根据以下查询动态获取20个问题:

SELECT Question.* from Question JOIN Test 
ON Question.Subject_ID = Test.Subject_ID 
AND Question.Question_Level = Test.Test_Level 
ORDER BY RAND() 
LIMIT 20;

但是,众所周知,RAND()MySQL杀死你的服务器的功能我一直在寻找更好的解决方案.

结果 EXPLAIN [above query]:

+----+-------------+----------+------+---------------+------+---------+------+------+----------------------------------------------------+
| id | select_type | table    | type | possible_keys | key  | key_len | ref  | rows | Extra                                              |
+----+-------------+----------+------+---------------+------+---------+------+------+----------------------------------------------------+
|  1 | SIMPLE      | Test     | ALL  | NULL          | NULL | NULL    | NULL |    5 | Using temporary; Using filesort                    |
|  1 | SIMPLE      | Question | ALL  | NULL          | NULL | NULL    | NULL |    7 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+----------+------+---------------+------+---------+------+------+----------------------------------------------------+

结果 EXPLAIN Question:

+-------------------+------------------------------------------+------+-----+---------+----------------+
| Field             | Type                                     | Null | Key | Default | Extra          |
+-------------------+------------------------------------------+------+-----+---------+----------------+
| Question_ID       | int(11)                                  | NO   | PRI | NULL    | auto_increment |
| Questions         | varchar(100)                             | NO   |     | NULL    |                |
| Available_Options | varchar(200)                             | NO   |     | NULL    |                |
| Correct_Answer    | varchar(50)                              | NO   |     | NULL    |                |
| Subject_ID        | int(11)                                  | NO   |     | NULL    |                |
| Question_Level    | enum('Beginner','Intermediate','Expert') | NO   |     | NULL    |                |
| Created_By        | int(11)                                  | NO   |     | NULL    |                |
+-------------------+------------------------------------------+------+-----+---------+----------------+

结果 EXPLAIN Test:

+----------------+------------------------------------------+------+-----+---------+----------------+
| Field          | Type                                     | Null | Key | Default | Extra          |
+----------------+------------------------------------------+------+-----+---------+----------------+
| Test_ID        | int(11)                                  | NO   | PRI | NULL    | auto_increment |
| Test_Name      | varchar(50)                              | NO   |     | NULL    |                |
| Test_Level     | enum('Beginner','Intermediate','Expert') | NO   |     | NULL    |                |
| Subject_ID     | int(11)                                  | NO   |     | NULL    |                |
| Question_Count | int(11)                                  | NO   |     | NULL    |                |
| Created_By     | int(11)                                  | NO   |     | NULL    |                |
+----------------+------------------------------------------+------+-----+---------+----------------+

任何帮助将被赞赏优化查询以减少服务器负载和执行时间.

PS系统也具有删除功能,因此QUESTION和TEST表的AUTO_INCREMENT PRIMARY KEY可能有很大的间隙.

Answer 1

为什么不在 PHP 中对数字进行随机排列，然后按 id 选择问题？我的观点的逻辑是这样的：

$MIN       = 1;
$MAX       = 50000; // You may want to get the MAX from your database
$questions = '';

for($i = 0; $i < 20; $i++)
   $questions .= mt_rand($MIN, $MAX) . ',';

// Removes last comma
$questions = rtrim($questions, ',');

$query = "SELECT * FROM Question WHERE Question.id IN ($questions)";

编辑1：

我正在考虑这个问题，我发现您可以从数据库中选择所有 ID，然后使用array_rand()函数选择 20 个项目。

$values    = array(1, 5, 10000, 102021, 1000000); // Your database ID's
$questions = array_rand($values, 20);

$questions[0];
$questions[1];
$questions[2]; // etc