clob转化为string的orcal函数

CREATE OR REPLACE FUNCTION blob_to_clob (blob_in IN BLOB) RETURN CLOB
   AS
    v_clob    CLOB;
    v_varchar VARCHAR2(400000);
    v_start PLS_INTEGER := 1;
    v_buffer PLS_INTEGER := 400000;
    tmp_num number;
    BEGIN
        DBMS_LOB.CREATETEMPORARY(v_clob, TRUE);
        tmp_num := CEIL(DBMS_LOB.GETLENGTH(blob_in) / v_buffer);
        if tmp_num > 0 then --防止传入的BLOB为NULL或长度为0时引发错误
          FOR i IN 1..tmp_num
          LOOP
            v_varchar := UTL_RAW.CAST_TO_VARCHAR2(DBMS_LOB.SUBSTR(blob_in, v_buffer, v_start));
            DBMS_LOB.WRITEAPPEND(v_clob, LENGTH(v_varchar), v_varchar);
             v_start := v_start + v_buffer;
          END LOOP;
        end if;
        RETURN v_clob;
    END blob_to_clob;