SQL代碼性能

Question

我有一個SQL查詢需要很長時間才能執行。

它是這樣的

select 
    columns 
from 
    tab1 
where 
    tab1.id in (select col from tab2 where conditions) --32000 rows 
    or 
    tab1.id in (select col from tab3 where conditions) ---14000 rows 
    or 
    tab1.id in (select col from tab4 where conditions) --6000 rows 

有什麼辦法，我可以在這裏提高性能？

我試過使用EXISTS()，但這並沒有幫助。

Answer 1

對於優化具有子查詢in的查詢，Oracle應該非常好。你最好的選擇是增加索引。但是，您的查詢不足以提供特定索引。您需要明確說明where條款。

Answer 2

選項1：

select 
    columns 
from 
    tab1 
where 
    tab1.id in (select col from tab2 where conditions --32000 rows 
       union all 
       select col from tab3 where conditions ---14000 rows 
       union all  
       select col from tab4 where conditions --6000 rows 
       ); 

選項2：

select 
    columns 
from 
    tab1 
inner join (select distinct col 
      from (select col from tab2 where conditions --32000 rows 
       union all 
       select col from tab3 where conditions ---14000 rows 
       union all  
       select col from tab4 where conditions --6000 rows 
       ) 
      ) x 
    on tab1.id = x.col; 

選項3：

select 
    columns 
from 
    tab1 
where 
    exists (select col from tab2 where conditions --??? rows 
      where col = tab1.id 
       union all 
       select col from tab3 where conditions ---??? rows 
       where col = tab1.id 
       union all  
       select col from tab4 where conditions --??? rows 
       where col = tab1.id 
       );