Question

对不起，这是一个特定的，可能是陈词滥调的问题，但它确实给我带来了重大问题。

每天我必须做几十万个看起来像这两个的选择语句（这是一个例子，但它们几乎只有不同的word1）：

SELECT pibn,COUNT(*) AS aaa FROM research_storage1
USE INDEX (word21pibn)
WHERE word1=270299 AND word2=0
GROUP BY pibn
ORDER BY aaa DESC
LIMIT 1000;

SELECT pibn,page FROM research_storage1
USE INDEX (word12num)
WHERE word1=270299 AND word2=0
ORDER BY num DESC
LIMIT 1000;

第一个声明是快速闪存，只需要几分之一秒。第二个声明需要大约2秒钟，考虑到我有数十万人要做，这太长了。

索引是：

word21pibn: word2, word1, pibn
word12num: word1, word2, num

解释结果（对于扩展和分区都是）：

mysql> explain extended SELECT pibn,COUNT(*) AS aaa FROM research_storage1 USE INDEX (word21pibn) WHERE word1=270299 AND word2=0 GROUP BY pibn ORDER BY aaa DESC LIMIT 1000;
+----+-------------+-------------------+------+---------------+------------+---------+-------------+------+----------+-----------------------------------------------------------+
| id | select_type | table             | type | possible_keys | key        | key_len | ref         | rows | filtered | Extra                                                     |
+----+-------------+-------------------+------+---------------+------------+---------+-------------+------+----------+-----------------------------------------------------------+
|  1 | SIMPLE      | research_storage1 | ref  | word21pibn    | word21pibn | 6       | const,const | 1549 |   100.00 | Using where; Using index; Using temporary; Using filesort |
+----+-------------+-------------------+------+---------------+------------+---------+-------------+------+----------+-----------------------------------------------------------+
1 row in set, 1 warning (0.00 sec)

mysql> explain partitions SELECT pibn,COUNT(*) AS aaa FROM research_storage1 USE INDEX (word21pibn) WHERE word1=270299 AND word2=0 GROUP BY pibn ORDER BY aaa DESC LIMIT 1000;
+----+-------------+-------------------+------------+------+---------------+------------+---------+-------------+------+-----------------------------------------------------------+
| id | select_type | table             | partitions | type | possible_keys | key        | key_len | ref         | rows | Extra                                                     |
+----+-------------+-------------------+------------+------+---------------+------------+---------+-------------+------+-----------------------------------------------------------+
|  1 | SIMPLE      | research_storage1 | p99        | ref  | word21pibn    | word21pibn | 6       | const,const | 1549 | Using where; Using index; Using temporary; Using filesort |
+----+-------------+-------------------+------------+------+---------------+------------+---------+-------------+------+-----------------------------------------------------------+
1 row in set (0.00 sec)

mysql> explain extended SELECT pibn,page FROM research_storage1 USE INDEX (word12num) WHERE word1=270299 AND word2=0 ORDER BY num DESC LIMIT 1000;
+----+-------------+-------------------+------+---------------+-----------+---------+-------------+------+----------+-------------+
| id | select_type | table             | type | possible_keys | key       | key_len | ref         | rows | filtered | Extra       |
+----+-------------+-------------------+------+---------------+-----------+---------+-------------+------+----------+-------------+
|  1 | SIMPLE      | research_storage1 | ref  | word12num     | word12num | 6       | const,const |  818 |   100.00 | Using where |
+----+-------------+-------------------+------+---------------+-----------+---------+-------------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

mysql> explain partitions SELECT pibn,page FROM research_storage1 USE INDEX (word12num) WHERE word1=270299 AND word2=0 ORDER BY num DESC LIMIT 1000;
+----+-------------+-------------------+------------+------+---------------+-----------+---------+-------------+------+-------------+
| id | select_type | table             | partitions | type | possible_keys | key       | key_len | ref         | rows | Extra       |
+----+-------------+-------------------+------------+------+---------------+-----------+---------+-------------+------+-------------+
|  1 | SIMPLE      | research_storage1 | p99        | ref  | word12num     | word12num | 6       | const,const |  818 | Using where |
+----+-------------+-------------------+------------+------+---------------+-----------+---------+-------------+------+-------------+
1 row in set (0.00 sec)

我看到的唯一区别是第二个语句在describe的额外列中没有Using index。虽然这没有意义，因为索引是为该语句设计的，所以我不明白为什么它不会被使用。

有什么想法吗？

Answer 1

尝试将pbin和page列添加到word12num复合索引中。然后，您查询所需的所有信息都将在索引中，就像您在第一次查询中一样。

修改我错过了您选择的pbin列;对于那个很抱歉。

如果您的复合索引结果包含(word1, word2, num, pbin, page)，则第二个查询中的所有内容都可以来自索引。

如果您查看第一个查询Extra下的EXPLAIN列，其中一个简介Using index. @sebas指出了这一点。实际上，这意味着Using index only.这意味着服务器可以通过查询索引来满足您的查询，而无需查阅表。这就是为什么它如此之快：服务器不必随意访问磁盘头来获取额外的列。您的第二个查询的Using index

中不存在EXPLAIN.

WHERE中提到的列首先出现。然后我们在ORDER BY中有列。最后，我们只列出SELECT个列。为什么要在索引中使用此特定顺序？服务器找到匹配SELECT的第一个索引条目，然后可以按顺序读取索引以满足查询。

在大桌面上构建和维护复合索引确实很昂贵。您正在寻找DBMS设计的基本权衡：您是否想花时间构建表格或在其中查找内容？只有你知道在制作桌子或查看桌面时是否更好地承担费用。

为什么这个MySQL查询没有正确使用索引？

1 个答案: