Question

我正在运行这样的查询：

SELECT DISTINCT `tableA`.`field1`,
                `tableA`.`filed2` AS field2Alias,
                `tableA`.`field3`,
                `tableB`.`field4` AS field4Alias,
                `tableA`.`field6` AS field6Alias
FROM (`tableC`)
RIGHT JOIN `tableA` ON `tableC`.`idfield` = `tableA`.`idfield`
JOIN `tableB` ON `tableB`.`idfield` = `tableA`.`idfield`
AND tableA.field2 IN
  (SELECT field2
   FROM tableA
   GROUP BY tableA. HAVING count(*)>1)
ORDER BY tableA.field2

这是为了找到所有重复的条目，但现在它需要花费大量的时间来执行。有任何优化建议吗？

Answer 1

您似乎正试图在field2 TableA上查找所有重复项。第一步是将in子查询移动到from子句：

SELECT DISTINCT a.`field1`, a.`filed2` AS field2Alias,
       a.`field3`, b.`field4` AS field4Alias, a.`field6` AS field6Alias 
FROM tableA a left join
     tableC c
     on c.`idfield` = a`.`idfield` join
     `tableB` b
     ON b.`idfield` = a.`idfield` join
     (SELECT field2 
      FROM tableA
      group by field2 
      having count(*) > 1
     ) asum
     on asum.field2 = a.field2
ORDER BY tableA.field2

可能还有其他优化措施，但很难说。你的问题“找到重复”和你的查询“加入一堆表并过滤它们”并不完全匹配。了解哪些表具有哪些索引和唯一/主键也很有用。

优化mysql查询以查找所有重复条目

1 个答案: