我的解释语句中的派生表是什么？

Question

如何确定解释结果中列出的派生表？我导出的表似乎没有任何可以使用的键，我认为这意味着需要进行一些优化。

选择语句：

EXPLAIN SELECT * 
FROM vw_environment_deployment_statuses v
  WHERE asset_id=47;

查看SQL：

SELECT
  `ac`.`asset_id` AS `asset_id`,
  `es`.`environment_id` AS `environment_id`,
  `d1`.`deployed_date` AS `last_deployed_date`,
  `d1`.`revision` AS `last_deployed_revision`,
  `d2`.`deployed_date` AS `last_successful_deployed_date`,
  `d2`.`revision` AS `last_successful_deployed_revision`,
  `e`.`acronym` AS `environment`,
  `v3`.`count_is_failed` AS `last_deployed_is_failed`
FROM (((((((((`asset_configurations` `ac`
  JOIN `vw_deployments` `d1`
    ON ((`ac`.`id` = `d1`.`asset_configuration_id`)))
  JOIN `vw_deployments` `d2`
    ON ((`ac`.`id` = `d2`.`asset_configuration_id`)))
  JOIN `servers` `s`
    ON ((`ac`.`server_id` = `s`.`id`)))
  JOIN `environments_servers` `es`
    ON ((`s`.`id` = `es`.`server_id`)))
  JOIN `environments` `e`
    ON ((`es`.`environment_id` = `e`.`id`)))
  JOIN `vw_last_environment_deployment_statuses` `v1`
    ON (((`ac`.`asset_id` = `v1`.`asset_id`) AND (`es`.`environment_id` = `v1`.`environment_id`) AND (`d1`.`deployed_date` = `v1`.`deployed_date`))))
  JOIN `vw_last_successful_environment_deployment_statuses` `v2`
    ON (((`ac`.`asset_id` = `v2`.`asset_id`) AND (`es`.`environment_id` = `v2`.`environment_id`) AND (`d2`.`deployed_date` = `v2`.`deployed_date`))))
  JOIN `vw_environment_fail_count` `v3`
    ON (((`ac`.`asset_id` = `v3`.`asset_id`) AND (`v3`.`environment_id` = `e`.`id`))))
  JOIN `domains` `dom`
    ON ((`ac`.`domain_id` = `dom`.`id`)))

解释声明：

1   PRIMARY ac  ref PRIMARY,FK_asset_configurations_assets_id,FK_asset_configurations_domains_id,FK_asset_configurations_servers_id FK_asset_configurations_assets_id   4   const   15  
1   PRIMARY s   eq_ref  PRIMARY PRIMARY 4   aps_cmdb.ac.server_id   1   Using index
1   PRIMARY es  ref PRIMARY,FK_environments_servers_servers_id2 FK_environments_servers_servers_id2 4   aps_cmdb.ac.server_id   1   Using index
1   PRIMARY dom eq_ref  PRIMARY PRIMARY 4   aps_cmdb.ac.domain_id   1   Using index
1   PRIMARY dep ref FK_deployments_asset_configurations_id  FK_deployments_asset_configurations_id  4   aps_cmdb.ac.id  5   
1   PRIMARY dep ref FK_deployments_asset_configurations_id  FK_deployments_asset_configurations_id  4   aps_cmdb.dep.asset_configuration_id 5   Using where

我在这里很好..。下面的派生表我不知道它们由什么组成。

1   PRIMARY <derived9>  ALL (null)  (null)  (null)  (null)  148 Using where; Using join buffer
1   PRIMARY <derived7>  ALL (null)  (null)  (null)  (null)  148 Using where; Using join buffer
1   PRIMARY <derived4>  ALL (null)  (null)  (null)  (null)  150 Using where; Using join buffer
1   PRIMARY e   eq_ref  PRIMARY PRIMARY 4   v3.environment_id   1   Using where
9   DERIVED <derived15> ALL (null)  (null)  (null)  (null)  195 Using temporary; Using filesort
9   DERIVED ac  eq_ref  PRIMARY,FK_asset_configurations_assets_id,FK_asset_configurations_domains_id,FK_asset_configurations_servers_id PRIMARY 4   lsd.asset_configuration_id  1   Using where
9   DERIVED es  ref PRIMARY,FK_environments_servers_servers_id2 FK_environments_servers_servers_id2 4   aps_cmdb.ac.server_id   1   Using index
9   DERIVED d   eq_ref  PRIMARY PRIMARY 4   aps_cmdb.ac.domain_id   1   Using index
9   DERIVED Asset   eq_ref  PRIMARY PRIMARY 4   aps_cmdb.ac.asset_id    1   Using index
9   DERIVED e   eq_ref  PRIMARY PRIMARY 4   aps_cmdb.es.environment_id  1   Using index
9   DERIVED s   eq_ref  PRIMARY PRIMARY 4   aps_cmdb.es.server_id   1   Using where; Using index
9   DERIVED dep ref FK_deployments_asset_configurations_id  FK_deployments_asset_configurations_id  4   aps_cmdb.ac.id  5   Using where
9   DERIVED dep ref FK_deployments_asset_configurations_id  FK_deployments_asset_configurations_id  4   aps_cmdb.dep.asset_configuration_id 5   Using where
9   DERIVED <derived12> ALL (null)  (null)  (null)  (null)  197 Using where; Using join buffer
15  DERIVED ac  index   PRIMARY UK_asset_configurations 777 (null)  229 Using where; Using index; Using temporary; Using filesort
15  DERIVED dep ref FK_deployments_asset_configurations_id  FK_deployments_asset_configurations_id  4   aps_cmdb.ac.id  5   Using where
12  DERIVED ac  index   PRIMARY UK_asset_configurations 777 (null)  229 Using where; Using index; Using temporary; Using filesort
12  DERIVED dep ref FK_deployments_asset_configurations_id  FK_deployments_asset_configurations_id  4   aps_cmdb.ac.id  5   
7   DERIVED ac  ALL PRIMARY,FK_asset_configurations_servers_id  (null)  (null)  (null)  229 Using where; Using temporary; Using filesort
7   DERIVED es  ref FK_environments_servers_servers_id2 FK_environments_servers_servers_id2 4   aps_cmdb.ac.server_id   1   Using index
7   DERIVED s   eq_ref  PRIMARY PRIMARY 4   aps_cmdb.ac.server_id   1   Using index
7   DERIVED dep ref FK_deployments_asset_configurations_id  FK_deployments_asset_configurations_id  4   aps_cmdb.ac.id  5   Using where
4   DERIVED ac  ALL PRIMARY,FK_asset_configurations_servers_id  (null)  (null)  (null)  229 Using where; Using temporary; Using filesort
4   DERIVED es  ref FK_environments_servers_servers_id2 FK_environments_servers_servers_id2 4   aps_cmdb.ac.server_id   1   Using index
4   DERIVED s   eq_ref  PRIMARY PRIMARY 4   aps_cmdb.ac.server_id   1   Using index
4   DERIVED dep ref FK_deployments_asset_configurations_id  FK_deployments_asset_configurations_id  4   aps_cmdb.ac.id  5   

Answer 1

神圣的嵌套子查询和偏执。阿!

派生表是为使查询工作而创建的临时表。它们可以解释如下：

SELECT
    foo.horse
FROM
    (SELECT horse from bar) as foo

其中foo是一个派生表。在服务器上执行查询时，这些表通常会变成临时表。在你的例子中，它们并不是那么明确。这可能是因为您正在查询视图中的视图，而主只知道它有多深。

派生表很好，因为它们允许您在将数据连接到另一个表、视图或派生表之前从表(或视图)中选择数据。他们有一个不利的一面，但他们没有索引。派生表上的联接更昂贵，因为您失去了对索引的控制。如果您的数据很小，或者您在嵌套())设计中很小心，那么一切都会好起来的。

最后，而且不相关，我相信你的妄想是多余的。我相信如果您删除了它们，您的查询将更加可读性更强。