Oracle 优化效率

1、rownum

(1)

较慢
SELECT ywlsdm, qybm, spzt, czsj, czrydm FROM GH_hs 
WHERE ywlsdm in (SELECT ywlsdm FROM GH_hs WHERE spzt = ‘2‘)
较快
SELECT ywlsdm, qybm, spzt, czsj, czrydm FROM GH_hs 
WHERE ywlsdm in (SELECT ywlsdm FROM GH_hs WHERE spzt = ‘2‘ and rownum <= (SELECT COUNT(*) FROM GH_hs hs WHERE EXISTS (SELECT ywlsdm FROM GH_hs WHERE spzt = ‘2‘)))

2、with as 临时表

select * from (
with 
qybm as (
select gx.qybm
          from (select a.ghdm
                   from gh_ghdm a
                  start with a.ghdm = ‘1‘
                 connect by prior a.ghdm = a.sjghdm) gh
         inner join gh_qyghgx gx on gh.ghdm = gx.tjghdm
         
),
yg as (
   select * from gh_jc_yg t where t.yglx = ‘0‘
)
select yg.* from qybm , yg where qybm.qybm = yg.qyghdm 
)