如何优化这条在ORACLE中运行约4秒的SQL语句，我想查询并使用更少的时间。

Question

在我的项目中，下面的sql运行时间约为4秒.TB_Order有一个全表扫描。TB_Order的状态字段上有一个索引，表的数据量是2000万，E和P的数据量小于50，这个SQL可以优化，我用了UNION ALL，对不对，你有更好的SQL吗？

--The first version
SELECT T.ORDER_ID, T.PRIORITY, T.SUB_TASK
FROM TB_ORDER T
WHERE (T.STATE = 'E' OR (T.STATE = 'P' AND SYSDATE > T.EFFECTIVE_DATE))
AND (T.PRIORITY <= 100 OR T.PRIORITY IS NULL)
AND ROWNUM <= :1

--The second version
SELECT * FROM (
SELECT T.ORDER_ID, T.PRIORITY, T.SUB_TASK
FROM TB_ORDER T
WHERE (T.STATE = 'E')
AND (T.PRIORITY <= 100 OR T.PRIORITY IS NULL)
UINON ALL 
SELECT T.ORDER_ID, T.PRIORITY, T.SUB_TASK
FROM TB_ORDER T
WHERE (T.STATE = 'P' AND SYSDATE > T.EFFECTIVE_DATE)
AND (T.PRIORITY <= 100 OR T.PRIORITY IS NULL)
)
where ROWNUM <= :1

Answer 1

不是最好的办法，取消了OR和强制INDEX扫描，试试解释计划，不加HINT，看看TB_ORDER.STATE上的索引是否被利用。只有在没有使用索引的情况下才加提示。同时建议在TB_ORDER表上运行统计。

SELECT * FROM
(SELECT /*+ INDEX(T STATE_NDX) */ T.ORDER_ID, T.PRIORITY, T.SUB_TASK
FROM TB_ORDER T
WHERE T.STATE = 'E'                        
AND DECODE(T.PRIORITY,null,0,T.PRIORITY) <= 100
AND ROWNUM <= :1
UNION ALL
SELECT /*+ INDEX(T STATE_NDX) */ T.ORDER_ID, T.PRIORITY, T.SUB_TASK
FROM TB_ORDER T
WHERE T.STATE = 'P' 
AND SYSDATE > T.EFFECTIVE_DATE
AND DECODE(T.PRIORITY,null,0,T.PRIORITY) <= 100
AND ROWNUM <= :1)

Answer 2

可以用IN代替UNION.sql只查询一次表。

  SELECT T.ORDER_ID, T.PRIORITY, T.SUB_TASK
   FROM TB_ORDER T
  WHERE T.STATE IN ('E', 'P')
  AND (CASE
     WHEN (T.STATE = 'P' AND SYSDATE > T.EFFECTIVE_DATE) THEN
      1
     WHEN T.STATE = 'E' THEN
      1
     ELSE
      0
   END) = 1
 AND (T.PRIORITY <= 100 OR T.PRIORITY IS NULL)
 AND ROWNUM <= :1