MYSQL未使用可能的密钥

Question

我有一个名为“ element”的主表：

CREATE TABLE `element` (
  `elements_id` int(11) NOT NULL AUTO_INCREMENT,
  `elements_code` varchar(32) DEFAULT NULL,
  `elements_name` varchar(128) DEFAULT NULL,
  `elements_description` text,
  `elements_image` varchar(64) DEFAULT NULL,
  `attribute_category_id` int(11) DEFAULT '0',
  `attribute_material_id` int(11) DEFAULT '0',
  `attribute_color_id` int(11) DEFAULT '0',
  `attribute_shape_id` int(11) DEFAULT '0',
  `attribute_surface_id` int(11) DEFAULT '0',
  `attribute_size_id` int(11) DEFAULT '0',
  `attribute_holesize_id` int(11) DEFAULT '0',
  `attribute_cut_id` int(11) DEFAULT '0',
  `attribute_height_id` int(11) NOT NULL DEFAULT '0',
  `attribute_width_id` int(11) NOT NULL DEFAULT '0',
  `attribute_thickness_id` int(11) NOT NULL DEFAULT '0',
  `attribute_clasp_id` int(11) NOT NULL DEFAULT '0',
  `attribute_setting_id` int(11) NOT NULL DEFAULT '0',
  `attribute_chain_id` int(11) NOT NULL DEFAULT '0',
  `elements_weight` decimal(5,3) DEFAULT NULL,
  `elements_weight_goldpure` decimal(5,3) NOT NULL DEFAULT '0.000',
  `elements_supplier` varchar(64) DEFAULT NULL,
  `elements_price` decimal(10,5) DEFAULT NULL,
  `add_date` datetime DEFAULT NULL,
  `add_by` varchar(30) DEFAULT NULL,
  `is_finalized` char(1) DEFAULT '0',
  `stars` tinyint(4) NOT NULL DEFAULT '0',
  `wax_complexity` char(1) DEFAULT NULL,
  `elements_dioh_target` varchar(10) NOT NULL DEFAULT '0',
  PRIMARY KEY (`elements_id`),
  KEY `attribute_category_id` (`attribute_category_id`),
  KEY `attribute_material_id` (`attribute_material_id`),
  KEY `attribute_color_id` (`attribute_color_id`),
  KEY `attribute_shape_id` (`attribute_shape_id`),
  KEY `attribute_surface_id` (`attribute_surface_id`),
  KEY `attribute_size_id` (`attribute_size_id`),
  KEY `attribute_holesize_id` (`attribute_holesize_id`),
  KEY `attribute_cut_id` (`attribute_cut_id`),
  KEY `attribute_height_id` (`attribute_height_id`),
  KEY `attribute_width_id` (`attribute_width_id`),
  KEY `attribute_thickness_id` (`attribute_thickness_id`),
  KEY `is_finalized` (`is_finalized`)
) ENGINE=MyISAM AUTO_INCREMENT=12687 DEFAULT CHARSET=latin1

然后我离开了这个名为“ products_material”的表的联接：

CREATE TABLE `products_materials` (
  `products_materials_id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `material_name` varchar(128) NOT NULL DEFAULT '',
  `active_status` char(1) DEFAULT '0',
  `sort_number` int(4) DEFAULT '0',
  `label_active_status` char(1) DEFAULT '0',
  PRIMARY KEY (`products_materials_id`)
) ENGINE=MyISAM AUTO_INCREMENT=120 DEFAULT CHARSET=latin1

用这样的查询：

SELECT e.*, pm.material_name AS mat_name
FROM element e
  LEFT JOIN products_materials pm ON pm.products_materials_id=e.attribute_material_id
WHERE e.is_finalized='1' AND 1 = 1  AND pm.products_materials_id  =  '1' GROUP BY e.elements_id HAVING 1 = 1 ORDER BY e.elements_id;

说明结果：

+----+-------------+-------+------------+-------+--------------------------------------------+-----------------------+---------+-------+------+----------+------------------------------------+
| id | select_type | table | partitions | type  | possible_keys                              | key                   | key_len | ref   | rows | filtered | Extra                              |
+----+-------------+-------+------------+-------+--------------------------------------------+-----------------------+---------+-------+------+----------+------------------------------------+
|  1 | SIMPLE      | pm    | NULL       | const | PRIMARY                                    | PRIMARY               | 4       | const |    1 |   100.00 | Using filesort                     |
|  1 | SIMPLE      | e     | NULL       | ref   | PRIMARY,attribute_material_id,is_finalized | attribute_material_id | 5       | const |   10 |    98.20 | Using index condition; Using where |
+----+-------------+-------+------------+-------+--------------------------------------------+-----------------------+---------+-------+------+----------+------------------------------------+

如您所见，表“ element”使用键attribute_material_id进行索引。但是，如果我离开这个名为“ elements_attributes_description”的表的联接：

CREATE TABLE `elements_attributes_description` (
  `elements_attributes_decription_id` int(11) NOT NULL AUTO_INCREMENT,
  `elements_attributes_id` int(11) DEFAULT NULL,
  `languages_id` int(11) DEFAULT NULL,
  `elements_attributes_groups` int(11) DEFAULT NULL,
  `name` varchar(64) NOT NULL DEFAULT '',
  `description` text NOT NULL,
  PRIMARY KEY (`elements_attributes_decription_id`),
  UNIQUE KEY `Unique` (`elements_attributes_id`,`languages_id`,`elements_attributes_groups`),
  KEY `index3` (`elements_attributes_groups`),
  KEY `Index 1` (`elements_attributes_id`)
) ENGINE=MyISAM AUTO_INCREMENT=1776 DEFAULT CHARSET=latin1

用这样的查询：

SELECT e.*, ead2.name AS mat_name
FROM element e
  LEFT JOIN elements_attributes_description ead2 ON ead2.elements_attributes_id = e.attribute_material_id AND ead2.elements_attributes_groups = 2
WHERE e.is_finalized='1' AND ead2.elements_attributes_id  =  '1' GROUP BY e.elements_id HAVING 1 = 1 ORDER BY e.elements_id;

解释结果：

+----+-------------+-------+------------+------+--------------------------------------------+---------+---------+-------+------+----------+----------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys                              | key     | key_len | ref   | rows | filtered | Extra                                              |
+----+-------------+-------+------------+------+--------------------------------------------+---------+---------+-------+------+----------+----------------------------------------------------+
|  1 | SIMPLE      | ead2  | NULL       | ref  | Unique,index3,Index 1                      | Index 1 | 5       | const |   30 |    19.08 | Using where; Using temporary; Using filesort       |
|  1 | SIMPLE      | e     | NULL       | ALL  | PRIMARY,attribute_material_id,is_finalized | NULL    | NULL    | NULL  | 5123 |    70.20 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+-------+------------+------+--------------------------------------------+---------+---------+-------+------+----------+----------------------------------------------------+

如您所见，表“ element”或未使用任何可能的键。

第二个查询中的查询或表结构有什么问题？

在此先感谢您看我的案子。

任何提示或批评家表示赞赏！

Answer 1

表结构没有问题，索引可能会得到改进。索引的主要问题是它们都只索引单个字段。MySQL 在查询中每个表只能使用 1 个索引。

MySQL 可能决定不使用任何索引的原因：

1. The element table is small. 5123 records are nothing for an rdbms. MySQL can decide against using an index if the table is small because it may be less efficient to open the index and search based on that and then go to the table and fetch the matching data, rather than simply searching through all records in the table.

2. is_finalised field probably has 2 possible values (or 3 max), so its selectivity is low, therefore MySQL is unlikely to use it anyway.

3. 这两个查询都违反了 sql 标准，因为它们对单个字段进行分组，并在选择列表中包含多个字段，这些字段既不在 group by 子句中，也不进行聚合，在功能上也不依赖于 group by 字段。幸运的是，最新版本的 mysql 默认阻止此类查询。

我会做些什么来使 mysql 更有可能使用索引：

1. 将 更改left join为内部联接（您正在右侧表上进行过滤，将左侧联接有效地转换为内部联接）。

2. 为包含字段的元素表添加多列索引attribute_material_id, elements_id。

显然，您应该考虑重写查询以符合 sql 标准。