查询速度太慢了！高手帮我优化一下吧！急！！！！！

 select
        * 
    from
        (select
            a.resultid,
            a.dataid,
            a.tiptext,
            b.title,
            b.content,
            b.issuetime,
            b.url,
            rownum as rn 
        from
            r_resultnews a 
        left join
            v_chn_news b 
                on a.dataid=b.dataid 
        where
            a.classid=2378 
            and siteid in (
                select
                    siteid 
                from
                    tg_eventstatic 
                where
                    datatype=1 
                    and eventclass=2378 
                    and positionid='北京'
            ) 
            and rownum>0
        ) 
    where
        rn>0 
        and rn<=30