几年前，Oracle为分层查询引入了一个可选的NOCYCLE参数。然而，除了避免在检测到循环时抛出错误之外，它到底做了什么还不清楚。我在网上看到的大多数材料都是错误的或误导性的。例如，权威的Oracle-Base说：
CONNECT BY NOCYCLE子句告诉数据库不要遍历循环层次结构。（错！）
official documentation可能是故意的，模棱两可：
NOCYCLE参数指示Oracle数据库从查询中返回行，即使数据中存在CONNECT BY循环。
确实如此，但这并没有说明当查询运行到循环中时，它何时甚至是否会停止跟踪循环。
事实是，在某些情况下，当使用CONNECT BY NOCYCLE遍历循环图时，Oracle将进入循环，但在完成它们之前停止，返回终止响应。然而，在其他情况下，甲骨文将永远愉快地在圈子里运行。（事实上，上面的Oracle文章提到了可能发生无限循环的情况，尽管不是在同一个上下文中。

是否有一个规定的方式为一个分层查询与NOCYCLE子句的行为方面的循环？

ETA：下面是一个可以在http://sqlfiddle.com中尝试的明显无限循环的工作示例，它是从this post借用的。（我没有提供固定链接，因为脚本在运行一次后就会损坏，所以每次都必须启动一个新的。
设置：

CREATE TABLE edges (tail CHAR(1), head CHAR(1));

INSERT INTO edges values('A','B');
INSERT INTO edges values('A','C');
INSERT INTO edges values('A','D');
INSERT INTO edges values('A','E');
INSERT INTO edges values('A','F');
INSERT INTO edges values('A','G');
INSERT INTO edges values('A','H');
INSERT INTO edges values('A','I');
INSERT INTO edges values('A','J');
INSERT INTO edges values('B','K');
INSERT INTO edges values('B','L');
INSERT INTO edges values('B','C');
INSERT INTO edges values('B','E');
INSERT INTO edges values('B','M');
INSERT INTO edges values('B','N');
INSERT INTO edges values('B','O');
INSERT INTO edges values('B','J');
INSERT INTO edges values('C','K');
INSERT INTO edges values('C','N');
INSERT INTO edges values('C','J');
INSERT INTO edges values('D','K');
INSERT INTO edges values('D','B');
INSERT INTO edges values('D','L');
INSERT INTO edges values('D','C');
INSERT INTO edges values('D','E');
INSERT INTO edges values('D','F');
INSERT INTO edges values('D','G');
INSERT INTO edges values('D','M');
INSERT INTO edges values('D','P');
INSERT INTO edges values('D','Q');
INSERT INTO edges values('D','N');
INSERT INTO edges values('D','R');
INSERT INTO edges values('D','O');
INSERT INTO edges values('D','J');
INSERT INTO edges values('E','K');
INSERT INTO edges values('E','L');
INSERT INTO edges values('E','S');
INSERT INTO edges values('E','M');
INSERT INTO edges values('E','N');
INSERT INTO edges values('E','O');
INSERT INTO edges values('E','J');
INSERT INTO edges values('F','K');
INSERT INTO edges values('F','B');
INSERT INTO edges values('F','L');
INSERT INTO edges values('F','C');
INSERT INTO edges values('F','G');
INSERT INTO edges values('F','S');
INSERT INTO edges values('F','M');
INSERT INTO edges values('F','N');
INSERT INTO edges values('E','O');
INSERT INTO edges values('E','J');
INSERT INTO edges values('G','K');
INSERT INTO edges values('G','B');
INSERT INTO edges values('G','L');
INSERT INTO edges values('G','E');
INSERT INTO edges values('G','M');
INSERT INTO edges values('G','N');
INSERT INTO edges values('G','O');
INSERT INTO edges values('G','J');
INSERT INTO edges values('H','K');
INSERT INTO edges values('H','B');
INSERT INTO edges values('H','L');
INSERT INTO edges values('H','C');
INSERT INTO edges values('H','D');
INSERT INTO edges values('H','E');
INSERT INTO edges values('H','F');
INSERT INTO edges values('H','G');
INSERT INTO edges values('H','M');
INSERT INTO edges values('H','P');
INSERT INTO edges values('H','Q');
INSERT INTO edges values('H','R');
INSERT INTO edges values('H','O');
INSERT INTO edges values('H','J');
INSERT INTO edges values('I','K');
INSERT INTO edges values('I','B');
INSERT INTO edges values('I','L');
INSERT INTO edges values('I','C');
INSERT INTO edges values('I','D');
INSERT INTO edges values('I','E');
INSERT INTO edges values('I','F');
INSERT INTO edges values('I','G');
INSERT INTO edges values('I','H');
INSERT INTO edges values('I','M');
INSERT INTO edges values('I','P');
INSERT INTO edges values('I','Q');
INSERT INTO edges values('I','N');
INSERT INTO edges values('I','R');
INSERT INTO edges values('I','O');
INSERT INTO edges values('I','J');
INSERT INTO edges values('J','K');
INSERT INTO edges values('J','L');
INSERT INTO edges values('J','N');
INSERT INTO edges values('J','O');
INSERT INTO edges values('K','N');
INSERT INTO edges values('K','J');
INSERT INTO edges values('L','K');
INSERT INTO edges values('L','N');
INSERT INTO edges values('L','J');
INSERT INTO edges values('M','K');
INSERT INTO edges values('M','B');
INSERT INTO edges values('M','L');
INSERT INTO edges values('M','C');
INSERT INTO edges values('M','E');
INSERT INTO edges values('M','F');
INSERT INTO edges values('M','G');
INSERT INTO edges values('M','N');
INSERT INTO edges values('M','O');
INSERT INTO edges values('M','J');
INSERT INTO edges values('M','F');
INSERT INTO edges values('N','J');
INSERT INTO edges values('O','K');
INSERT INTO edges values('O','L');
INSERT INTO edges values('O','N');
INSERT INTO edges values('O','J');
INSERT INTO edges values('P','K');
INSERT INTO edges values('P','A');
INSERT INTO edges values('P','B');
INSERT INTO edges values('P','L');
INSERT INTO edges values('P','T');
INSERT INTO edges values('P','C');
INSERT INTO edges values('P','D');
INSERT INTO edges values('P','E');
INSERT INTO edges values('P','F');
INSERT INTO edges values('P','G');
INSERT INTO edges values('P','H');
INSERT INTO edges values('P','I');
INSERT INTO edges values('P','N');
INSERT INTO edges values('P','R');
INSERT INTO edges values('P','O');
INSERT INTO edges values('P','J');
INSERT INTO edges values('Q','H');
INSERT INTO edges values('Q','I');
INSERT INTO edges values('Q','D');
INSERT INTO edges values('Q','E');
INSERT INTO edges values('Q','F');
INSERT INTO edges values('Q','G');
INSERT INTO edges values('Q','N');
INSERT INTO edges values('Q','L');
INSERT INTO edges values('Q','K');
INSERT INTO edges values('Q','R');
INSERT INTO edges values('Q','T');
INSERT INTO edges values('Q','A');
INSERT INTO edges values('Q','C');
INSERT INTO edges values('Q','B');
INSERT INTO edges values('Q','O');
INSERT INTO edges values('Q','J');
INSERT INTO edges values('R','K');
INSERT INTO edges values('R','B');
INSERT INTO edges values('R','L');
INSERT INTO edges values('R','C');
INSERT INTO edges values('R','G');
INSERT INTO edges values('R','M');
INSERT INTO edges values('R','N');
INSERT INTO edges values('R','O');
INSERT INTO edges values('R','J');
INSERT INTO edges values('S','K');
INSERT INTO edges values('S','B');
INSERT INTO edges values('S','L');
INSERT INTO edges values('S','C');
INSERT INTO edges values('S','E');
INSERT INTO edges values('S','F');
INSERT INTO edges values('S','G');
INSERT INTO edges values('S','N');
INSERT INTO edges values('S','O');
INSERT INTO edges values('S','J');
INSERT INTO edges values('T','K');
INSERT INTO edges values('T','A');
INSERT INTO edges values('T','B');
INSERT INTO edges values('T','L');
INSERT INTO edges values('T','C');
INSERT INTO edges values('T','D');
INSERT INTO edges values('T','E');
INSERT INTO edges values('T','F');
INSERT INTO edges values('T','G');
INSERT INTO edges values('T','H');
INSERT INTO edges values('T','I');
INSERT INTO edges values('T','M');
INSERT INTO edges values('T','P');
INSERT INTO edges values('T','Q');
INSERT INTO edges values('T','R');
INSERT INTO edges values('T','O');
INSERT INTO edges values('T','J');

脚本：

select tail, head 
from edges
connect by nocycle
   prior head = tail
start with head = 'A';