oracle 分页过程超慢。直接SQL N快，咋回事

oracle 分页过程超慢。直接SQL N快，怎么回事
包
create or replace package DB_Oper is
  type ref_DataSet IS REF CURSOR;
  function GetCount(mTableName varchar2, mTerm varchar2) return number;
procedure return_DataSet(
  mTableName in varchar2, --表名
  mTerm in varchar2, --条件
  mPageSize in number, --每页显示记录数
  mPageIndex in number, --当前页
  mOrderField in varchar2, --排序字段
  mOrderStyle in number, --排序方式 0 升序 1 降序
  mTotalRecords out number, --总记录数
  mDateSet out ref_DataSet --记录
  );
end DB_Oper;

包体：
create or replace package body DB_Oper is

  ----- 获取记录数 ------
  function GetCount(mTableName varchar2, mTerm varchar2) return number is
  i number;
  vSql varchar2(1000);
  begin
  i := 0;
  vSql := 'select count(*) from ' || mTableName ;

  if length(mTerm) > 0 then
  vSql := vSql || ' where ' || mTerm;
  end if;

  execute IMMEDIATE vSql into i;

  return(i);
  end;

  ----- 数据分页 ----------
  procedure return_DataSet(
  mTableName in varchar2, --表名
  mTerm in varchar2, --条件
  mPageSize in number, --每页显示记录数
  mPageIndex in number, --当前页
  mOrderField in varchar2, --排序字段
  mOrderStyle in number, --排序方式 0 升序 1 降序
  mTotalRecords out number, --总记录数
  mDateSet out ref_DataSet --记录集
  ) is
  begin
  declare
  Invalid_Input Exception;
  vSql varchar2(1000);
  Start_page number;
  End_page number;
  n_PageIndex number;
  n_PageSize number;
  --mTotalRecords number;
  begin
  mTotalRecords := GetCount(trim(mTableName), trim(mTerm));
  vSql := 'select * from ' || mTableName;
  if length(trim(mTerm)) > 0 then
  vSql := vSql || ' where ' || trim(mTerm) ;
  end if;
  if length(trim(mOrderField)) > 0 then
  begin
  vSql := vSql || ' order by ' || trim(mOrderField);
  if morderStyle > 0 then
  vSql := vSql || ' desc ';
  end if;
  end;
  end if;

  --
  n_PageSize := mPageSize;
  if mPageSize <= 0 then
  n_PageSize := 10;
  end if;
  n_PageIndex := mPageIndex;
  if mPageIndex <= 0 then
  n_PageIndex := 1;
  end if;
  if mTotalRecords > 0 and n_PageIndex > 1 then
  begin
  --如果n_pageindex大于实际的页数，则取实际页数
  if n_PageIndex > round((mTotalRecords / n_PageSize) + 0.5) then
  n_PageIndex := round((mTotalRecords / n_PageSize) + 0.5);
  end if;
  end;
  end if;
  Start_page := (n_PageIndex - 1) * n_PageSize + 1;
  End_page := n_PageIndex * n_PageSize;
  vSql := 'SELECT * FROM (SELECT A.*, rownum r FROM ( ' || vSql || ' ) A WHERE rownum <= ' || End_page || ' ) B WHERE r >= '|| Start_page;

  Open mDateSet for vSql;

  exception
  When Invalid_Input Then
  open mDateSet for select null from dual;
  end;
  end return_DataSet;
end DB_Oper;

------解决方案--------------------
是第一次吗？还是一直都是