oracle的select_insert超慢

insert into e_rel_customer(
CUSTOM_ID  
,REL_CUSTOM_ID  
,REL_ID  
,CUSTOM_NAME  
,idtype_id  
)
select
 fun_getcustid(CUSTOMERID) as CUSTOMERID
,fun_getcustid(RELATIVEID) as RELATIVEID
,RELATIONSHIP
, CUSTOMERNAME
,CERTTYPE
from xd_CUSTOMER_RELATIVE;
function :
fun_getcustid 为 
  select nvl(MFCUSTOMERID,'') into refstr from SOR_XD_CUSTOMER_INFO where MFCUSTOMERID is not null and CUSTOMERID=trim(var) and rownum=1;
表xd_CUSTOMER_RELATIVE 有108029条记录
表SOR_XD_CUSTOMER_INFO 73047条记录
这个语句执行下来要半个小时，为什么啊
在此列上建索引，看看效率:MFCUSTOMERID，CUSTOMERID
最少返回数据应该是 108029 行
试试这个，function执行次数太多了，大概这个意思

SQL code
insert into e_rel_customer(
CUSTOM_ID  
,REL_CUSTOM_ID  
,REL_ID  
,CUSTOM_NAME  
,idtype_id  
)
SELECT T1.MFCUSTOMERID AS CUSTOMERID,
       T2.MFCUSTOMERID AS RELATIVEID,
       RELATIONSHIP,
       CUSTOMERNAME,
       CERTTYPE
  FROM XD_CUSTOMER_RELATIVE,
  LEFT JOIN (SELECT NVL(MAX(MFCUSTOMERID), '') MFCUSTOMERID, CUSTOMERID
               FROM SOR_XD_CUSTOMER_INFO
              WHERE MFCUSTOMERID IS NOT NULL
              GROUP BY CUSTOMERID) T1
       ON T1.CUSTOMERID = XD_CUSTOMER_RELATIVE.CUSTOMERID
  LEFT JOIN (SELECT NVL(MAX(MFCUSTOMERID), '') MFCUSTOMERID, CUSTOMERID
               FROM SOR_XD_CUSTOMER_INFO
              WHERE MFCUSTOMERID IS NOT NULL
              GROUP BY CUSTOMERID) T2
       ON T2.CUSTOMERID = XD_CUSTOMER_RELATIVE.RELATIVEID