Rownum / sorting / query stops working / friday afternoon nightmate

Question

Greetings,
Let's say I have a perfectly working and very simple sql statement. This little thing turned my friday to nightmare. And I hope one of you can help me. :)
SELECT 1
FROM my_tablel l
WHERE l.first_id = :b1
AND l.open_flag = 'Y'
AND NVL (l.flag_id, 'N') = 'N'
AND l.flag_id IN (SELECT flag2_id
FROM my_second_table
WHERE flag3_id = :b2)
If I add this row:
AND ROWNUM = 1;
stops working.
If I would use some agg, or sorting ie. "SELECT COUNT (*)" instead of "SELECT 1" , query works once again.
This (similar)small problem caused many problems in our system, and began after applying a patch (for years it was fine). Worse enough, I can not change the SQL. I really wonder if anyone can say, what may be the reason.
Thanks a lot.
CHEERS.
Edited by: 855652 on 29.Nis.2011 08:29

Answer 1

While I am trying to eat the keyboard (yeah, that feeling many of you know very well), I decide to write down even simpler form of the question.
SELECT 1
FROM my_tablel l
WHERE l.first_id = :b1
AND l.open_flag = 'Y'
AND NVL (l.flag_id, 'N') = 'N'
AND l.flag_id IN (SELECT flag2_id
FROM my_second_table
WHERE flag3_id = :b2)
Works..
SELECT 1
FROM my_tablel l
WHERE l.first_id = :b1
AND l.open_flag = 'Y'
AND NVL (l.flag_id, 'N') = 'N'
AND l.flag_id IN (SELECT flag2_id
FROM my_second_table
WHERE flag3_id = :b2)
AND ROWNUM = 1
Goes into infinite loop
SELECT count(*)
FROM my_tablel l
WHERE l.first_id = :b1
AND l.open_flag = 'Y'
AND NVL (l.flag_id, 'N') = 'N'
AND l.flag_id IN (SELECT flag2_id
FROM my_second_table
WHERE flag3_id = :b2)
AND ROWNUM = 1
Works again.