Question

我有以下查询。我们的想法是，它让我知道groups以及随后users可以访问每个component_instance。我想知道是否有更好的方法来执行此操作，因为查询速度很慢，但每次处理此表时都有这些额外的列非常方便：

SELECT component_instances.*, 
GROUP_CONCAT(DISTINCT IF(permissions.view, groups.id, NULL)) AS view_group_ids,
GROUP_CONCAT(DISTINCT IF(permissions.edit, groups.id, NULL)) AS edit_group_ids,
GROUP_CONCAT(DISTINCT IF(permissions.view, users.id, NULL)) AS view_user_ids,
GROUP_CONCAT(DISTINCT IF(permissions.edit, users.id, NULL)) AS edit_user_ids
FROM `component_instances`
LEFT OUTER JOIN permissions ON permissions.component_instance_id = component_instances.id
LEFT OUTER JOIN groups ON groups.id = permissions.group_id
LEFT OUTER JOIN groups_users ON groups_users.group_id = groups.id
LEFT OUTER JOIN users ON users.id = groups_users.user_id
GROUP BY component_instances.id
ORDER BY (case when component_instances.ancestry is null then 0 else 1 end), component_instances.ancestry, position

权限表就像这样（原谅Rails！）：

create_table "permissions", :force => true do |t|
  t.integer "component_instance_id"
  t.integer "group_id"
  t.boolean "view",                  :default => false
  t.boolean "edit",                  :default => false
end

权限类型为edit和view。可以为一个组分配一个或两个。权限也是递归的，如果component_instance上没有组权限，我们必须检查其祖先以找到设置权限的第一个（如果有）。这使得一个查询非常重要，因为我可以将此查询与ancestry gem提供的选择逻辑（物化路径树）结合起来。

更新

我已经更快地找到了这个查询基准：

SELECT component_instances.*,
GROUP_CONCAT(DISTINCT view_groups.id) AS view_group_ids,
GROUP_CONCAT(DISTINCT edit_groups.id) AS edit_group_ids,
GROUP_CONCAT(DISTINCT view_users.id) AS view_user_ids,
GROUP_CONCAT(DISTINCT edit_users.id) AS edit_user_ids
FROM `component_instances`
LEFT OUTER JOIN permissions ON permissions.component_instance_id = component_instances.id
LEFT OUTER JOIN groups view_groups ON view_groups.id = permissions.group_id AND permissions.view = 1
LEFT OUTER JOIN groups edit_groups ON edit_groups.id = permissions.group_id AND permissions.edit = 1
LEFT OUTER JOIN groups_users view_groups_users ON view_groups_users.group_id = view_groups.id
LEFT OUTER JOIN groups_users edit_groups_users ON edit_groups_users.group_id = edit_groups.id
LEFT OUTER JOIN users view_users ON view_users.id = view_groups_users.user_id
LEFT OUTER JOIN users edit_users ON edit_users.id = edit_groups_users.user_id
GROUP BY component_instances.id
ORDER BY (case when component_instances.ancestry is null then 0 else 1 end), component_instances.ancestry, position

这是上面查询的EXPLAIN和表CREATE语句：

+----+-------------+---------------------+--------+-----------------------------------------------+--------------------------------------------+---------+--------------------------------------------+------+------------------------------------------------------+
| id | select_type | table               | type   | possible_keys                                 | key                                        | key_len | ref                                        | rows | Extra                                                |
+----+-------------+---------------------+--------+-----------------------------------------------+--------------------------------------------+---------+--------------------------------------------+------+------------------------------------------------------+
| 1  | SIMPLE      | component_instances | ALL    | PRIMARY,index_component_instances_on_ancestry | NULL                                       | NULL    | NULL                                       | 119  | "Using temporary; Using filesort"                    |
| 1  | SIMPLE      | permissions         | ALL    | NULL                                          | NULL                                       | NULL    | NULL                                       | 6    | "Using where; Using join buffer (Block Nested Loop)" |
| 1  | SIMPLE      | view_groups         | eq_ref | PRIMARY                                       | PRIMARY                                    | 4       | 05707d890df9347c.permissions.group_id      | 1    | "Using where; Using index"                           |
| 1  | SIMPLE      | edit_groups         | eq_ref | PRIMARY                                       | PRIMARY                                    | 4       | 05707d890df9347c.permissions.group_id      | 1    | "Using where; Using index"                           |
| 1  | SIMPLE      | view_groups_users   | ref    | index_groups_users_on_group_id_and_user_id    | index_groups_users_on_group_id_and_user_id | 5       | 05707d890df9347c.view_groups.id            | 1    | "Using index"                                        |
| 1  | SIMPLE      | edit_groups_users   | ref    | index_groups_users_on_group_id_and_user_id    | index_groups_users_on_group_id_and_user_id | 5       | 05707d890df9347c.edit_groups.id            | 1    | "Using index"                                        |
| 1  | SIMPLE      | view_users          | eq_ref | PRIMARY                                       | PRIMARY                                    | 4       | 05707d890df9347c.view_groups_users.user_id | 1    | "Using index"                                        |
| 1  | SIMPLE      | edit_users          | eq_ref | PRIMARY                                       | PRIMARY                                    | 4       | 05707d890df9347c.edit_groups_users.user_id | 1    | "Using index"                                        |
+----+-------------+---------------------+--------+-----------------------------------------------+--------------------------------------------+---------+--------------------------------------------+------+------------------------------------------------------+

CREATE TABLE `component_instances` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `visible` int(11) DEFAULT '1',
  `instance_id` int(11) DEFAULT NULL,
  `deleted_on` date DEFAULT NULL,
  `instance_type` varchar(255) DEFAULT NULL,
  `component_id` int(11) DEFAULT NULL,
  `deleted_root_item` int(11) DEFAULT NULL,
  `locked_until` datetime DEFAULT NULL,
  `theme_id` int(11) DEFAULT NULL,
  `position` int(11) DEFAULT NULL,
  `ancestry` varchar(255) DEFAULT NULL,
  `ancestry_depth` int(11) DEFAULT '0',
  `cached_name` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `index_component_instances_on_ancestry` (`ancestry`)
) ENGINE=InnoDB AUTO_INCREMENT=121 DEFAULT CHARSET=utf8

CREATE TABLE `groups` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(255) NOT NULL DEFAULT '',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8

CREATE TABLE `groups_users` (
  `group_id` int(11) DEFAULT NULL,
  `user_id` int(11) DEFAULT NULL,
  KEY `index_groups_users_on_group_id_and_user_id` (`group_id`,`user_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

CREATE TABLE `permissions` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `component_instance_id` int(11) DEFAULT NULL,
  `group_id` int(11) DEFAULT NULL,
  `view` tinyint(1) DEFAULT '0',
  `edit` tinyint(1) DEFAULT '0',
  PRIMARY KEY (`id`),
  KEY `edit_permissions_index` (`edit`,`group_id`,`component_instance_id`),
  KEY `view_permissions_index` (`view`,`group_id`,`component_instance_id`)
) ENGINE=InnoDB AUTO_INCREMENT=28 DEFAULT CHARSET=utf8

CREATE TABLE `users` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `real_name` varchar(255) DEFAULT NULL,
  `username` varchar(255) NOT NULL DEFAULT '',
  `email` varchar(255) NOT NULL DEFAULT '',
  `crypted_password` varchar(255) DEFAULT NULL,
  `administrator` int(11) NOT NULL DEFAULT '0',
  `password_salt` varchar(255) DEFAULT NULL,
  `remember_token_expires` datetime DEFAULT NULL,
  `persistence_token` varchar(255) DEFAULT NULL,
  `disabled` tinyint(1) DEFAULT NULL,
  `time_zone` varchar(255) DEFAULT NULL,
  `login_count` int(11) DEFAULT NULL,
  `failed_login_count` int(11) DEFAULT NULL,
  `last_request_at` datetime DEFAULT NULL,
  `current_login_at` datetime DEFAULT NULL,
  `last_login_at` datetime DEFAULT NULL,
  `current_login_ip` varchar(255) DEFAULT NULL,
  `last_login_ip` varchar(255) DEFAULT NULL,
  `perishable_token` varchar(255) NOT NULL DEFAULT '',
  PRIMARY KEY (`id`),
  UNIQUE KEY `index_users_on_username` (`username`),
  KEY `index_users_on_perishable_token` (`perishable_token`)
) ENGINE=InnoDB AUTO_INCREMENT=12 DEFAULT CHARSET=utf8

ORDER BY来自ancestry gem，但如果有更好的方法，我很乐意将其作为拉取请求提交给他们。

Answer 1

如果我们没有您的表格结构和索引，那么几乎不可能优化您的查询。使用EXPLAIN语句是查询优化的必要部分。

如果没有上述信息，我可以对您的问题发表评论的是，您的ORDER BY部分可以从某些优化中获益。在条件中使用任何函数或语句将始终导致灾难。同样使用ORDER BY中的可空字段也会导致问题。也许最简单的方法是在表中添加一个新字段，保存0和1而不是当前的CASE语句。

不要忘记，如果记录数量相当大，那么在/ group by的条件/顺序中的任何字段上都有索引。

<强> [UPDATE]

您的查询非常简单。 EXPLAIN的结果显示，唯一适合作为索引候选者的部分是：

CREATE INDEX inx4 ON permissions (`component_instance_id`, `group_id`, `edit`, `view`);

EXPLAIN的第二行显示您的查询中没有表permissions的索引。这是因为MySQL在使用索引时有几条规则：

每个表（子）查询中只能使用每个表的一个索引。
只有在查询中提及其所有字段时才能使用任何索引（如条件/顺序中的/ group by）。

考虑到您的查询以及提及表permissions的所有四个字段这一事实，您需要对所有这四个字段进行索引，否则它将无用。

然而ORDER BY可以从我之前提到的修正案中受益。

Answer 2

首先放置

NULL（可以使用COALESCE将NULL替换为其他内容，而不是使用其他排序列。第二件事是减少连接，因为最后两个是在我们连接的id上。

SELECT
   component_instances.*,
   GROUP_CONCAT(DISTINCT view_groups.id) AS view_group_ids,
   GROUP_CONCAT(DISTINCT edit_groups.id) AS edit_group_ids,
   GROUP_CONCAT(DISTINCT view_groups_users.user_id) AS view_user_ids,
   GROUP_CONCAT(DISTINCT edit_groups_users.user_id) AS edit_user_ids
FROM
   `component_instances`
   LEFT OUTER JOIN permissions
      ON permissions.component_instance_id = component_instances.id
   LEFT OUTER JOIN groups view_groups
      ON view_groups.id = permissions.group_id AND permissions.view = 1
   LEFT OUTER JOIN groups edit_groups
      ON edit_groups.id = permissions.group_id AND permissions.edit = 1
   LEFT OUTER JOIN groups_users view_groups_users
      ON view_groups_users.group_id = view_groups.id
   LEFT OUTER JOIN groups_users edit_groups_users
      ON edit_groups_users.group_id = edit_groups.id
GROUP BY
   component_instances.id
ORDER BY
   component_instances.ancestry, -- MySQL was sorting the NULL values already correctly
   position
;

使GROUP_CONCAT查询更有效

2 个答案: