Question

假设我有一个查找表mylookup（lkid，lkname，...）

然后我还有其他两个表：

mytab(id, parentid, name, lookname, ...)
yourtab(id, parentid, ...)  --id, parentid coming from mytab

然后我有一个更新尝试获取lkid和它的父lkid，sql像：

 update yourtab set columnx = 
    case 
    when (select lkid from mylookup a join mytab b on a.lkname = b.lkname where b.id = c.parentid ) > 3
        and (select lkid from mylookup a join mytab b on a.lkname = b.lkname where b.id = c.parentid )  >
            (select lkid from mylookup where lkname = c.lkname )
    then 1
    else 0
    end
    from yourtab c

这个sql性能不好。（从mylookup中选择lkid，在a.lkname = b.lkname上加入mytab b，其中b.id = c.parentid）为yourtb中的每一行调用2次。

如何重写sql以提高此案例的性能？

Answer 1

也许是这样的......

未经测试需要一个带有一些示例数据的SQLFiddle才能真正理解内部工作原理。

Update yourtab set columnx = 
case when  not null (
  SELECT lkid 
  FROM mylookup a 
  INNER join mytab b 
    on a.lkname = b.lkname 
  WHERE b.id = c.parentid 
    and a.lkid > 3 
    and a.lkname >  c.lkname) then 1 
  else 0 
 end 
FROM yourtab c

Answer 2

update yourtab
   set columnx = 1
  FROM yourtab c
  join mylookup a 
    on a.lkname > c.lkname 
   and a.lkid > 3
  join mytab b 
    on a.lkname = b.lkname
   and b.id = c.parentid  
 where columnx <> 1


update yourtab
   set columnx = 0
  FROM yourtab c
  left join mylookup a 
    on a.lkname > c.lkname 
   and a.lkid > 3
  left join mytab b 
    on a.lkname = b.lkname
   and b.id = c.parentid  
 where columnx <> 0
   and ( a.lkname is null or b.id is null )

两个语句，但如果值已经正确，则会保存锁定

如何提高更新语句的性能？

2 个答案: