如何正确的使用or展开来改写SQL查询

select * from sys_user 
where user_code = 'zhangyong' 
or user_code in 
(select grp_code 
from sys_grp 
where sys_grp.user_code = 'zhangyong')


Execution Plan
----------------------------------------------------------
0  SELECT STATEMENT Optimizer=RULE
1  0   FILTER
2  1     TABLE ACCESS (FULL) OF 'SYS_USER'
3  1     INDEX (UNIQUE SCAN) OF 'PK_SYS_GRP' (UNIQUE)


Statistics
----------------------------------------------------------
14  recursive calls
4  db block gets
30590 consistent gets
0  physical reads
0  redo size
1723  bytes sent via SQL*Net to client
425  bytes received via SQL*Net from client
2  SQL*Net roundtrips to/from client
0  sorts (memory)
0  sorts (disk)
3  rows processed

SQL> select count(*) from sys_grp;
COUNT(*)----------25130
SQL> select count(*) from sys_user;
COUNT(*)
----------
15190