我正在尝试构建一个查询，以便从连接到查找表的3个不同表中检索数据。
我的问题是查询需要很长时间才能运行（花了1个小时，我放弃了等待，假设出现问题，python脚本正在运行查询并将结果写入文件中）
查询如下所示：
SELECT a.id,
GROUP_CONCAT(DISTINCT b.product_id SEPARATOR ':') AS b_pids,
GROUP_CONCAT(DISTINCT c.product_id SEPARATOR ':') AS c_pids,
GROUP_CONCAT(DISTINCT d.product_id SEPARATOR ':') AS d_ids
FROM a_table a
LEFT JOIN b_table b ON a.customer_account_guid = b.customer_account_guid
LEFT JOIN c_table c ON b.customer_account_guid = c.customer_account_guid
LEFT JOIN d_table d ON c.customer_account_guid = d.customer_account_guid
WHERE a.category = 'product_category' AND a.location = 'UK' GROUP BY a.id
此查询的解释结果为：
id | select_type | table | type | possible_keys | key | key_len | ref | rows |filtered |Extra
1 | SIMPLE | a | ref | PRIMARY,idx_id,idx_customer_account_guid,idx_category_location,idx_category,idx_location | idx_category_location | 47 | const,const | 1211986 |100 |Using index condition
1 | SIMPLE | b | ref | idx_customer_account_guid | idx_customer_account_guid | 300 | a.customer_account_guid | 6 |100 |NULL
1 | SIMPLE | c | ref | idx_customer_account_guid | idx_customer_account_guid | 300 | b.customer_account_guid | 18 |100 |NULL
1 | SIMPLE | d | ref | idx_customer_account_guid | idx_customer_account_guid | 300 | c.customer_account_guid | 2 |100 |NULL
表记录（计数）是：
a_table - 3 million
b_table - 8 million
c_table - 2 million
d_table - 4 million
正如你所看到的那样，索引似乎没有问题，而且查询并不复杂，任何想法为什么需要超过1小时？我认为上述计数不应该花那么长时间，或者我错了吗？
答案 0 :(得分：2)
您正在使用连接生成荒谬的行数。相反，在加入之前聚合：
DISTINCT我从GROUP_CONCAT()删除了SELECT a.id,
(SELECT GROUP_CONCAT(b.product_id SEPARATOR ':') as b_pids
FROM b_table b
WHERE a.customer_account_guid = b.customer_account_guid
) b_pids,
(SELECT GROUP_CONCAT(c.product_id SEPARATOR ':') as b_pids
FROM c_table c
WHERE a.customer_account_guid = c.customer_account_guid
) b_pids,
(SELECT GROUP_CONCAT(d.product_id SEPARATOR ':')
FROM d_table d
WHERE a.customer_account_guid = d.customer_account_guid
) d_pids
FROM a_table a LEFT JOIN
WHERE a.category = 'product_category' AND a.location = 'UK';
。你可能仍然需要它。
编辑：
鉴于您在外部查询中进行过滤，相关子查询可能是最佳方法：
a_table(category, location, customer_account_guid)对于此版本，您需要(customer_account_guid, product_id)上的索引以及其他每个表{1}}的索引。