Question

我想优化以下查询，不使用子查询来获取最大值：

select c.ida2a2 from table1 m, table2 c 
where c.ida3a5 = m.ida2a2 
and (c.createstampa2 < (select max(cc.createstampa2) 
                        from table2 cc where cc.ida3a5 = c.ida3a5));

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Answer 1

这可能是编写查询的更有效方式：

select c.ida2a2
from table1 m join
     (select c.*, MAX(createstampa2) over (partition by ida3a5) as maxcs
      from table2 c 
     ) c
     on c.ida3a5 = m.ida2a2
where c.createstampa2 < maxcs

我非常确定Oracle正确地优化了这一点（在连接之前过滤行）。如果你想更清楚：

select c.ida2a2
from table1 m join
     (select c.*
      from (select c.*, MAX(createstampa2) over (partition by ida3a5) as maxcs
            from table2 c 
           ) c
      where c.createstamp2 < c.maxcs
     ) c
     on c.ida3a5 = m.ida2a2

具有条件的查询优化

1 个答案: