我的查询运行有点慢。它需要一个包含搜索结果的项目的表格，然后获取包含这些项目中的一个或多个的类别。类别（~300）存储在具有大约4个级别的嵌套集模型中。我需要知道每个级别的计数（因此一个项目可能属于大孩子类别，但也需要计入子级和父级类别）。
执行此操作的查询如下： -
SELECT category_parent.id,
category_parent.depth,
category_parent.name AS item_sub_category,
category_parent.left_index,
category_parent.right_index,
COUNT(DISTINCT item.id) as total
FROM search_enquiries_found
INNER JOIN item ON search_enquiries_found.item_id = item.id
INNER JOIN category ON item.mmg_code = category.mmg_code
INNER JOIN category category_parent ON category.left_index BETWEEN category_parent.left_index AND category_parent.right_index
WHERE search_enquiries_found.search_enquiry_id = 35
AND item.cost_price > 0
GROUP BY category_parent.id,
category_parent.depth,
item_sub_category,
category_parent.left_index,
category_parent.right_index
ORDER BY category_parent.left_index
发现大约12,000条记录，这需要大约1.5秒。解释如下： -
id select_type table partitions type possible_keys key key_len ref rows Extra
1 SIMPLE category NULL ALL left_index NULL NULL NULL 337 Using temporary; Using filesort
1 SIMPLE item NULL ref PRIMARY,id,mmg_code mmg_code 27 em_entaonline.category.mmg_code 43 Using where
1 SIMPLE search_enquiries_found NULL eq_ref search_enquiry_id,item_id,search_enquiry_id_2,search_enquiry_id_relevance search_enquiry_id 8 const,em_entaonline.item.id 1 Using index
1 SIMPLE category_parent NULL ALL left_index NULL NULL NULL 337 Range checked for each record (index map: 0x2)
重要的表是分层类别表： -
CREATE TABLE `category` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(128) NOT NULL,
`depth` int(11) NOT NULL,
`is_active` tinyint(1) NOT NULL DEFAULT '1',
`left_index` int(4) NOT NULL,
`right_index` int(4) NOT NULL,
`mmg_code` varchar(25) NOT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `left_index` (`left_index`,`right_index`),
UNIQUE KEY `depth` (`depth`,`left_index`,`right_index`),
KEY `name` (`name`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
使用连接上的任何索引来获取父类别是没有用的。
更改此查询以删除大多数分组字段并仅使用唯一ID（实际上组中的其他字段不是必需的，但我倾向于将它们包括在内以保持严格的标准）减少所需的时间降至~0.5秒： -
SELECT category_parent.id,
category_parent.depth,
category_parent.name AS item_sub_category,
category_parent.left_index,
category_parent.right_index,
COUNT(DISTINCT item.id) as total
FROM search_enquiries_found
INNER JOIN item ON search_enquiries_found.item_id = item.id
INNER JOIN category ON item.mmg_code = category.mmg_code
INNER JOIN category category_parent ON category.left_index BETWEEN category_parent.left_index AND category_parent.right_index
WHERE search_enquiries_found.search_enquiry_id = 35
AND item.cost_price > 0
GROUP BY category_parent.id
ORDER BY category_parent.left_index
对此的解释略有不同： -
id select_type table partitions type possible_keys key key_len ref rows Extra?
1 SIMPLE category NULL ALL left_index NULL NULL NULL 337 Using temporary; Using filesort
1 SIMPLE item NULL ref PRIMARY,id,mmg_code mmg_code 27 em_entaonline.category.mmg_code 43 Using where
1 SIMPLE search_enquiries_found NULL eq_ref search_enquiry_id,item_id,search_enquiry_id_2,search_enquiry_id_relevance search_enquiry_id 8 const,em_entaonline.item.id 1 Using index
1 SIMPLE category_parent NULL ALL PRIMARY,left_index,depth,name NULL NULL NULL 337 Using where; Using join buffer (Block Nested Loop)
现在我有两个问题。
1 - 为什么从GROUP BY中删除字段（完全依赖于组中剩余的主键）以这种方式更改EXPLAIN，性能发生如此大的变化。
2 - 在任何一种情况下，性能都很差，没有用于连接到父类别表的密钥。有关改进的建议吗？
另外一点，我尝试使用STRAIGHT_JOIN来反转连接的顺序并强制它。在这种情况下，这进一步改善了性能（~0.2秒），但这是一种不寻常的情况（因为它从父类别表中返回所有记录 - 大多数时候搜索将返回更少的记录，因此远远少于类别），所以我认为这会在更正常的情况下减慢速度，并且仍然没有在某些连接上使用索引。如果MySQL根据结果集决定加入表的方式会很好。
SELECT category_parent.id,
category_parent.depth,
category_parent.name AS item_sub_category,
category_parent.left_index,
category_parent.right_index,
COUNT(DISTINCT item.id) as total
FROM category category_parent
STRAIGHT_JOIN category ON category.left_index BETWEEN category_parent.left_index AND category_parent.right_index
STRAIGHT_JOIN item ON item.mmg_code = category.mmg_code
STRAIGHT_JOIN search_enquiries_found ON search_enquiries_found.item_id = item.id
WHERE search_enquiries_found.search_enquiry_id = 35
GROUP BY category_parent.id,
category_parent.depth,
item_sub_category,
category_parent.left_index,
category_parent.right_index
ORDER BY category_parent.left_index
id select_type table partitions type possible_keys key key_len ref rows Extra
1 SIMPLE category_parent NULL ALL left_index NULL NULL NULL 337 Using temporary; Using filesort
1 SIMPLE category NULL ALL left_index NULL NULL NULL 337 Range checked for each record (index map: 0x2)
1 SIMPLE item NULL ref PRIMARY,id,mmg_code mmg_code 27 em_entaonline.category.mmg_code 43 Using index
1 SIMPLE search_enquiries_found NULL eq_ref search_enquiry_id,item_id,search_enquiry_id_2,search_enquiry_id_relevance search_enquiry_id 8 const,em_entaonline.item.id 1 Using index