SQL语句优化问题执行速度太慢，如何优化

需要执行一段sql语句但是效率太低能不能有办法优化子查询太多了
SELECT ROWNUM RANKING, RES.*
FROM (SELECT STEPMEDAL.USER_ID,
SUM(STEPMEDAL.MEDALS) MEDALS,
REALNAME,
USER_IMG,
START_DATE,
END_DATE,
NVL(TOTAL.TOTAL_STEP_NUM, 0) TOTAL_STEP_NUM
FROM (SELECT INFO.USER_ID,
REALNAME,
START_DATE,
END_DATE,
USER_IMG,
NVL(MEDALS, 0) MEDALS
FROM (SELECT USERINFO.USER_ID,
EXT.REALNAME,
NVL(EXT.USER_IMG,0) USER_IMG,
TO_CHAR(ACTI.START_DATE, 'YYYY-MM-DD') START_DATE,
TO_CHAR(ACTI.END_DATE, 'YYYY-MM-DD') END_DATE
FROM JC_USER_EXT EXT,
JC_USER USERINFO,
JC_USER USERDEPART,
WALK_ACTIVITY_MAIN ACTI,
WALK_ACTIVITY_COMPANY_REL REL,
WALK_ACTIVITY_COMPANY_REL WACOM
WHERE USERDEPART.USER_ID = 12519
AND REL.COMPANY_ID = USERDEPART.DEPART_ID
AND WACOM.WA_ID = REL.WA_ID
AND USERINFO.DEPART_ID = WACOM.COMPANY_ID
AND EXT.USER_ID = USERINFO.USER_ID
AND ACTI.STATE = 1
AND ACTI.STATUS = 1
AND EXT.JOB_TYPE = 1
AND ACTI.WA_ID = WACOM.WA_ID) INFO
LEFT OUTER JOIN (SELECT USERINFO.USER_ID,
CASE
WHEN AVG(STEP.STEP_NUM) > 1800 THEN
1
ELSE
0
END MEDALS
FROM JC_USER USERINFO
LEFT OUTER JOIN WALK_USER_STEP STEP
ON USERINFO.USER_ID =
STEP.USER_ID, JC_USER
USERDEPART,
WALK_ACTIVITY_MAIN ACTI,
WALK_ACTIVITY_COMPANY_REL REL,
WALK_USER_STEP STEPDATE,
WALK_ACTIVITY_COMPANY_REL
WACOM
WHERE USERDEPART.USER_ID = 12519
AND REL.COMPANY_ID =
USERDEPART.DEPART_ID
AND WACOM.WA_ID = REL.WA_ID
AND USERINFO.DEPART_ID =
WACOM.COMPANY_ID
AND ACTI.STATE = 1
AND ACTI.STATUS = 1
AND ACTI.WA_ID = WACOM.WA_ID
AND STEP.STEP_ID =
STEPDATE.STEP_ID
AND TO_CHAR(STEPDATE.STEP_DATE,
'YYYY/MM') <>
TO_CHAR(SYSDATE, 'YYYY/MM')
AND STEPDATE.STEP_DATE BETWEEN
ACTI.START_DATE AND
ACTI.END_DATE
GROUP BY USERINFO.USER_ID,
TO_CHAR(STEPDATE.STEP_DATE,
'YYYY/MM')) STEPRES
ON INFO.USER_ID = STEPRES.USER_ID) STEPMEDAL
LEFT OUTER JOIN (SELECT STEPDATE.USER_ID,
SUM(STEPDATE.STEP_NUM) TOTAL_STEP_NUM
FROM JC_USER USERINFO
LEFT OUTER JOIN WALK_USER_STEP STEP
ON USERINFO.USER_ID = STEP.USER_ID,
JC_USER USERDEPART,
WALK_ACTIVITY_MAIN ACTI,
WALK_ACTIVITY_COMPANY_REL REL,
WALK_USER_STEP STEPDATE,
WALK_ACTIVITY_COMPANY_REL WACOM
WHERE USERDEPART.USER_ID = 12519
AND REL.COMPANY_ID =
USERDEPART.DEPART_ID
AND ACTI.WA_ID = WACOM.WA_ID
AND WACOM.WA_ID = REL.WA_ID
AND USERINFO.DEPART_ID =
WACOM.COMPANY_ID
AND ACTI.STATE = 1
AND ACTI.STATUS = 1
AND STEP.STEP_ID = STEPDATE.STEP_ID
AND STEPDATE.STEP_DATE BETWEEN
ACTI.START_DATE AND ACTI.END_DATE
GROUP BY STEPDATE.USER_ID) TOTAL
ON STEPMEDAL.USER_ID = TOTAL.USER_ID
GROUP BY STEPMEDAL.USER_ID,
REALNAME,
USER_IMG,
START_DATE,
END_DATE,
TOTAL.TOTAL_STEP_NUM
ORDER BY TOTAL_STEP_NUM DESC) RES