SQL1：
CDMBAS.BAS_PAPPVACTINFO表的PA_REQUNO ， PA_ORDER 两个字段上存在复合索引，但是下面SQL却使用了全表扫描，即使使用了index的hint也不行。想想可能是什么原因？（3300万记录的表，全表扫描执行一次19秒；问题解决后使用索引只需1.7毫秒）：
SELECT B2.* FROM
( SELECT B1.*, ROWNUM rnum
FROM ( SELECT
/* index(t1 IDX_BAS_PAPPVACTINFO_0001) */
T1.PA_CLINAME AS T1__PA_CLINAME ,
......
T1.PA_OPTIME AS T1__PA_OPTIME
FROM CDMBAS.BAS_PAPPVACTINFO T1
WHERE 1=1
ORDER BY PA_REQUNO DESC , PA_ORDER DESC
) B1
WHERE ROWNUM <= 21
) B2
WHERE rnum > 0 ;
SQL2：
下面SQL从写法上看看该如何优化一下？
SELECT *
FROM (
SELECT oracle_table.* ,ROWNUM AS startrownum
FROM (
SELECT ......
FROM mocha_oa_intramsg_message t1
INNER JOIN (
SELECT C_MESSAGE_ID,max(C_ID) AS C_ID
FROM mocha_oa_intramsg_message
GROUP BY C_MESSAGE_ID
) t2
ON t1.C_ID = t2.C_ID
WHERE (C_DEL_STATUS = 0 OR C_DEL_STATUS = 1) AND
C_FROM_ID = : 1 AND : 2 <= C_SEND_TIME AND : 3 >= C_SEND_TIME
ORDER BY C_SEND_TIME DESC
) oracle_table
WHERE ROWNUM <= 14
)
WHERE startrownum > 0;
