Question

有人可以优化这个mysql查询

SELECT submittedform.*, inspectors.first_name, inspectors.last_name
  FROM (
       SELECT `dinsp`,`departure`,`arrival`,'cabin' as type FROM cabinets 
       UNION
       SELECT `dinsp`,`departure`,`arrival`,'cockpit' as type FROM cockpits
       ORDER BY `date_of_inspection` ASC
       ) AS submittedform 
       INNER JOIN inspectors ON inspectors.id = submittedform.dinsp

我不想依赖嵌套查询，或者在这种情况下是否正常？还建议我一个cakephp解决方案，但表格无关。

Answer 1

您可以尝试：

SELECT sf.`dinsp`, sf.`departure`, sf.`arrival`, sf.`type`, i.`first_name`, i.`last_name`
FROM
    `inspectors` AS i INNER JOIN (
    SELECT `dinsp`, `departure`, `arrival`, `date_of_inspection`, 'cabin' AS `type`
    FROM `cabinets`
    UNION ALL
    SELECT `dinsp`, `departure`, `arrival`, `date_of_inspection`, 'cockpit' AS `type`
    FROM `cockpits`
) AS sf ON sf.`dinsp` = i.`id`
ORDER BY sf.`date_of_inspection`

UNION ALL不会检查重复项。始终将ORDER BY子句放在外部查询中以确保正确排序。

最好避免使用UNION，因为它不允许查询优化器使用您在dinsp和date_of_inspection上可能拥有的任何索引。但这意味着要改变架构。

Answer 2

UNION子查询的替代方法是将主查询分为两部分，其中包含UNION：

SELECT c.dinsp, c.departure, d.arrival, 'cabin'   AS type, i.first_name, i.last_name
  FROM cabinets AS c JOIN inspectors AS i ON i.id = c.dinsp
SELECT c.dinsp, c.departure, d.arrival, 'cockpit' AS type, i.first_name, i.last_name
  FROM cockpits AS c JOIN inspectors AS i ON i.id = c.dinsp

目前尚不清楚这会产生明显不同的表现。如果有的话，它会更糟，因为它涉及Inspectors表的两次扫描，但这可能不是很大，所以它可能并不重要。您的UNION子查询减去ORDER BY可能与此一样好或稍好一些。您在非选定字段上的ORDER BY在内部查询中存在问题;并且需要在我提议的UNION中小心处理（可能通过选择额外的列）。

SELECT c.dinsp, c.date_of_inspection, c.departure, d.arrival, 'cabin'   AS type,
       i.first_name, i.last_name
  FROM cabinets AS c JOIN inspectors AS i ON i.id = c.dinsp
SELECT c.dinsp, c.date_of_inspection, c.departure, d.arrival, 'cockpit' AS type,
       i.first_name, i.last_name
  FROM cockpits AS c JOIN inspectors AS i ON i.id = c.dinsp
 ORDER BY date_of_inspection;

优化这个mysql查询

2 个答案: