Performance query

Question

10gR2 (10.2.0.2)
when using the following query , when i increase then n value the query takes much longer time although explain doesnt change..
can someone help?
select * from
SELECT Event.ID,
Event.CATEGORY ,
Event.DDOMAIN ,
Event.NETWORK ,
Event.NODE ,
Event.ENTITY ,
Event.HELPURL ,
Event.GROUPNAME ,
Event.OWNERNAME,
Event.WEBNMS ,
Event.SEVERITY ,
Event.SOURCE ,
Event.TEXT ,
Event.TTIME ,
null PROPNAME,
null PROPVAL,
ROW_NUMBER() OVER(order by Event.ttime desc) N
FROM bbnnms_user.Event
WHERE (((((Event.CATEGORY = 'Alarms')))))
where N BETWEEN 10000175 AND 10000224

Answer 1

Oracle processes this in two steps.
First it calculates all the rows that conform to <= the maximum in your between (10000224 ). This rowset is then passed on to the next step of returning just the rows >= your minimum (10000175).
[ Oracle pushed *JUST* the predicate N <= 10000224 into the nested view ]
When you have a small maximum, oracle has very few rows to hold internally and process/sort - so its quick, when you increase this value it has a lot or rows to process (10000224 rows) - It becomes slow.
Even if you the rows you required are always at the top end around the 10000175,
reordering it to select * from (select * from (select * from .... ROW_NUMBER...) where N >= 10000175) where N <= 10000224
still would not help.
Oracle would still push the "N <= 10000224" into the inner nested view, still returning 10000224 rows.