ORACLE 使用xmlagg获得clob输出问题

因为要输出xml字符串,用xmlagg 然后转成varchar2(4000)出现了超过4000个字符的情况,但是直接赋值给clob变量出错
求输出办法create or replace rp_XXX
(
v_input varchar2,
v_output clob
)
as
beginselect  xmlagg(
        xmlelement("aa",
                xmlattributes(t.a as "aa_1",t.b as "aa_2"....    )

                     )
        )
into v_output
from XXX as t.....end
以前是cast( as varchar(4000)) into v_output,没有问题,
求高手解答