Question

我有一个名为Articles的表。该表的结构如下：id（唯一），类别（文章的类别，即娱乐），标题（文章的标题），图像（文章的图像URL），链接（URL）文章），Counter（文章的观看次数），dateStamp（文章发表的日期）。

假设我要打印6篇随机娱乐文章。这种简单但效率低下的方法就是做

之类的事情

$result = $db->query("SELECT * FROM Articles WHERE category = 'entertainment' ORDER BY RAND() LIMIT 6);

相反，我该如何更有效地解决这个问题？我知道有多个网站在解释ORDER BY RAND（）的替代方案，但我只是不了解这些替代方案。我希望根据我的结构来理解。我已经尝试了将关联数组改组为创建随机生成器的多种方法，但所有这些都让人感到沮丧，因为我无法让它正常工作。如何在不使用ORDER BY RAND（）的情况下获得6个随机播放的娱乐文章？

Answer 1

没有通用的方法，其中一个有效的选择是按照给定（可选）条件选择随机子集数据，category。请注意添加到此列的索引。

SELECT
    r1.*
FROM
    articles AS r1
    INNER JOIN (SELECT(RAND() * (SELECT MAX(id) FROM articles)) AS id) AS r2
WHERE
    r1.id >= r2.id
    AND r1.category = 'entertainment'
LIMIT 6;

这里有示例数据（3.2m行）和执行计划的详细信息：

mysql> SELECT COUNT(*) FROM articles;
+----------+
| COUNT(*) |
+----------+
|  3200000 |
+----------+
1 row in set (0.00 sec)

mysql> SELECT
    r1.*
FROM
    articles AS r1
    INNER JOIN (SELECT(RAND() * (SELECT MAX(id) FROM articles)) AS id) AS r2
WHERE
    r1.id >= r2.id
    AND r1.category = 'entertainment'
LIMIT 6;
+---------+-------------+-----------------------------------------------------------+---------------+
| id      | topic       | message                                                   | category      |
+---------+-------------+-----------------------------------------------------------+---------------+
| 3153910 | JAX68VVH3FZ | Sed eu eros. Nam consequat dolor                          | entertainment |
| 3153911 | NIY23HWV0VM | tortor. Nunc commodo auctor velit. Aliquam nisl. Nulla eu | entertainment |
| 3153912 | LKQ42FRB7LA | mus. Proin vel nisl. Quisque                              | entertainment |
| 3153913 | PFL39VHI9RM | gravida                                                   | entertainment |
| 3153914 | FGV59TUN9TQ | elit, pellentesque a, facilisis non, bibendum sed,        | entertainment |
| 3153915 | OWH73EBZ1GW | ligula. Nullam enim. Sed nulla ante, iaculis              | entertainment |
+---------+-------------+-----------------------------------------------------------+---------------+
6 rows in set (0.473 sec)

mysql> explain extended 
SELECT
    r1.*
FROM
    articles AS r1
    INNER JOIN (SELECT(RAND() * (SELECT MAX(id) FROM articles)) AS id) AS r2
WHERE
    r1.id >= r2.id
    AND r1.category = 'entertainment'
LIMIT 6;
+----+-------------+------------+--------+-----------------+---------+---------+-------+---------+----------+------------------------------+
| id | select_type | table      | type   | possible_keys   | key     | key_len | ref   | rows    | filtered | Extra                        |
+----+-------------+------------+--------+-----------------+---------+---------+-------+---------+----------+------------------------------+
|  1 | PRIMARY     | <derived2> | system | NULL            | NULL    | NULL    | NULL  |       1 |      100 | NULL                         |
|  1 | PRIMARY     | r1         | ref    | PRIMARY,cat_IDX | cat_IDX | 768     | const | 1560229 |      100 | Using index condition        |
|  2 | DERIVED     | NULL       | NULL   | NULL            | NULL    | NULL    | NULL  | NULL    | NULL     | No tables used               |
|  3 | SUBQUERY    | NULL       | NULL   | NULL            | NULL    | NULL    | NULL  | NULL    | NULL     | Select tables optimized away |
+----+-------------+------------+--------+-----------------+---------+---------+-------+---------+----------+------------------------------+
4 rows in set (0.00 sec)

与相同数量的数据相比，性能上的差异比通常的情况要差（超过10倍）：

mysql> SELECT * FROM articles WHERE category = 'entertainment' ORDER BY RAND() LIMIT 6;
+---------+-------------+---------------------------------------------------------------------------+---------------+
| id      | topic       | message                                                                   | category      |
+---------+-------------+---------------------------------------------------------------------------+---------------+
| 2374491 | PZC33VGM0ML | Duis cursus, diam at pretium aliquet, metus urna convallis erat,          | entertainment |
|  382306 | RFN88EPE4MI | malesuada fames ac turpis egestas. Aliquam fringilla cursus purus. Nullam | entertainment |
| 1867986 | KWX30ULB1FR | pede.                                                                     | entertainment |
| 1528863 | ADX52RRJ3MQ | lacus. Mauris non                                                         | entertainment |
| 2188208 | AOD82PXQ6FS | diam luctus lobortis. Class aptent taciti sociosqu ad litora              | entertainment |
|  878426 | ABV08HTB2PG | eu eros. Nam consequat dolor vitae dolor. Donec fringilla. Donec          | entertainment |
+---------+-------------+---------------------------------------------------------------------------+---------------+
6 rows in set (5.726 sec)

希望这会对你有所帮助。

根据类别

1 个答案: