DB2 Alternate to EXISTS函数

Question

我在DB2上运行的应用程序中有以下查询：

SELECT COD.POST_CD,CLS.CLASS,COD2.STATUS_CD
FROM DC01.POSTAL_CODES COD
INNER JOIN DC02.STATUS_CODES COD2
ON COD.ORDER=COD2.ORDER
INNER JOIN DC02.VALID_ORDERS ORD
ON ORD.ORDER=COD.ORDER
WHERE
( 
    ( EXISTS (SELECT 1 FROM DC00.PROCESS_ORDER PRD
          WHERE PRD.ORDER=COD.ORDER
              AND PRD.IDNUM=COD.IDNUM
        )
    ) OR
    ( EXISTS (SELECT 1 FROM DC00.PENDING_ORDER PND
          WHERE PND.ORDER=COD.ORDER
              AND PND.IDNUM=COD.IDNUM
        )
    )
)
AND EXISTS (SELECT 1 FROM DC00.CUSTOM_ORDER CRD 
           WHERE CRD.ORDER=COD.ORDER
                  )
;

当我们更改为UDB (LUW v9.5)时，我们将收到以下警告：

IWAQ0003W SQL warnings were found
SQLState=01602 Performance of this complex query might be sub-optimal.  
Reason code: "3".. SQLCODE=437, SQLSTATE=01602, DRIVER=4.13.111

我知道这个警告是由EXISTS ()或EXISTS语句引起的。但我不确定有没有其他方法可以写这个查询来替换。如果它是AND，我可以进行内连接，但是我不能改变这个条件，因为它是OR。有没有人建议用更好的方法替换这些EXISTS语句？

Answer 1

SELECT COD.POST_CD,CLS.CLASS,COD2.STATUS_CD
FROM DC01.POSTAL_CODES COD
INNER JOIN DC02.STATUS_CODES COD2
ON COD.ORDER=COD2.ORDER
INNER JOIN DC02.VALID_ORDERS ORD
ON ORD.ORDER=COD.ORDER
WHERE
( 
     EXISTS SELECT 1 FROM 
              (SELECT ORDER,IDNUM FROM DC00.PROCESS_ORDER PRD UNION 
               SELECT ORDER,IDNUM FROM DC00.PENDING_ORDER PND) PD
          WHERE PD.ORDER=COD.ORDER
              AND PD.IDNUM=COD.IDNUM
)
AND EXISTS (SELECT 1 FROM DC00.CUSTOM_ORDER CRD 
           WHERE CRD.ORDER=COD.ORDER
              )
;