connect by问题

[noteasytoregister]

select sys_connect_by_path(column_name, ',') -- select *
from user_tab_columns 
where table_name='EMP'
connect by prior column_id+1=column_id
start with column_id=1

这段SQL一直在Executing...，几分钟都未完成，象是出现了死循环，请问哪里错了？

[opps_zhou]

select sys_connect_by_path(column_name, ',')
  from user_tab_columns 
 where connect_by_isleaf = 1
connect by prior column_id = column_id - 1 and prior table_name = table_name
start with table_name='EMP' and column_id = 1;

[noteasytoregister]

[Quote=引用 1 楼 opps_zhou 的回复:]

SQL code

select sys_connect_by_path(column_name, ',')
from user_tab_columns
where connect_by_isleaf = 1
connect by prior column_id = column_id - 1 and prior table_name = table_name
start with t……
[/Quote]
我的写法中table_name='EMP'应该最后才判断的，所以慢了。
我发现下面的写法更快些：

select sys_connect_by_path(column_name, ',') -- select *
from (select * from user_tab_columns 
where table_name='EMP')
connect by prior column_id+1=column_id
start with column_id=1