如何优化这个在 ORACLE 中运行大约 4 秒的 SQL 语句？我想查询和使用更少的时间

Question

The following sql in my project runs about 4 seconds.TB_Order has a full table scan. There is an index on the state field of TB_Order.The data volume of the table is 20 million, and the data volume of E and P is less than 50.This SQL can be optimized.and I have use UNION ALL,is it right?Do you have a better sql?

--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

Not a best approach,eliminated OR and forced INDEX Scan,Try explain plan without HINT to see if index on TB_ORDER.STATE is utilized. Add the hint only if index is not used. Also recommend run statistics on TB_ORDER table.

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

You may use IN to replace with UNION.The sql will query the table only one time.

  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