如何重写（或），（和）条件的查询替换

Question

1. 查询执行时间过长，大约需要 130 分钟。
2. 具有索引但查询未使用索引的表。
3. 表大小约为 3000 万条记录。
4. 请帮助我通过更改（或）、（和）条件来重写查询。
5. 查询未在正常执行中使用，因此我们尝试使用强制索引但没有成功。

查询：

SELECT customer_history.customer_id,
        customer_history.row_mod,
        customer_history.row_create,
        customer_history.event_id,
        customer_history.event_type,
        customer_history.new_value,
        customer_history.old_value
FROM customer_history FORCE INDEX (customer_history_n2)
WHERE customer_id >= 1
        AND customer_id < 5000000
        AND (customer_history.row_mod >= '2012-10-01')
        OR (
                customer_history.row_create >= '2012-10-01'
                AND customer_history.row_create < '2012-10-13'
                );


Table structure

    CREATE TABLE `customer_history` (
      `customer_id` int(11) NOT NULL,
      `row_mod` datetime DEFAULT NULL,
      `row_create` datetime DEFAULT NULL,
      `event_id` int(11) DEFAULT NULL,
      `event_type` varchar(45) DEFAULT NULL,
      `new_value` varchar(255) DEFAULT NULL,
      `old_value` varchar(255) DEFAULT NULL,
      KEY `customer_id1` (`customer_id`),
      KEY `new_value_n1` (`new_value`),
      KEY `customer_history_n1` (`row_create`),
      KEY `customer_history_n2` (`row_mod`)
    ) ENGINE=InnoDB DEFAULT CHARSET=utf8

解释计划：

+----+-------------+------------------+------+---------------------+------+---------+------+----------+-------------+
| id | select_type | table            | type | possible_keys       | key  | key_len | ref  | rows     | Extra       |
+----+-------------+------------------+------+---------------------+------+---------+------+----------+-------------+
|  1 | SIMPLE      | customer_history | ALL  | customer_history_n2 | NULL | NULL    | NULL | 18490530 | Using where |
+----+-------------+------------------+------+---------------------+------+---------+------+----------+-------------+
1 row in set (0

Answer 1

您可以尝试将此查询分成两部分：

SELECT customer_history.customer_id,
        customer_history.row_mod,
        customer_history.row_create,
        customer_history.event_id,
        customer_history.event_type,
        customer_history.new_value,
        customer_history.old_value
FROM customer_history
WHERE customer_id >= 1
        AND customer_id < 5000000
        AND (customer_history.row_mod >= '2012-10-01')

还有

SELECT customer_history.customer_id,
        customer_history.row_mod,
        customer_history.row_create,
        customer_history.event_id,
        customer_history.event_type,
        customer_history.new_value,
        customer_history.old_value
FROM customer_history
WHERE  customer_history.row_create >= '2012-10-01'
       AND customer_history.row_create < '2012-10-13';

并查看其中哪一个需要更长的时间来执行和优化它们。为了获得与第一个查询相同的结果集，您可以在 MySQL 中UNION 这两个查询，或者简单地从您的应用程序层单独执行它们并在那里加入结果集。

Answer 2

啊，我想我知道你的问题是什么。根据MySQL documentation：

通过指定 USE INDEX (index_list)，你可以告诉 MySQL 只使用 用于在表中查找行的命名索引之一。替代方案 语法 IGNORE INDEX (index_list) 可用于告诉 MySQL 不要使用 一些特定的索引或索引。如果 EXPLAIN，这些提示很有用 表明 MySQL 使用了可能列表中的错误索引 索引。

您也可以使用 FORCE INDEX，其作用类似于 USE INDEX (index_list) 但另外假设表扫描非常 昂贵的。换句话说，只有在没有 使用给定索引之一在表中查找行的方法。

换句话说，通过使用 FORCE INDEX，您告诉 MySQL 仅使用customer_history_n2 索引并跳过所有其他可用索引。您需要从查询中删除 FORCE INDEX 子句，或者将其指定为 this 以便它将使用其他可用列的索引：

SELECT customer_history.customer_id,
    customer_history.row_mod,
    customer_history.row_create,
    customer_history.event_id,
    customer_history.event_type,
    customer_history.new_value,
    customer_history.old_value
FROM customer_history
    FORCE INDEX (customer_history_n2, customer_history_n1, customer_id1)
WHERE customer_id >= 1
    AND customer_id < 5000000
    AND (customer_history.row_mod >= '2012-10-01')
    OR (
        customer_history.row_create >= '2012-10-01'
        AND customer_history.row_create < '2012-10-13' );