Can one retrieve only the Nth row from a table?

Hi,
Can anyone explain the execution order of the below query to fetch the nth row data.
SELECT * FROM t1 a
WHERE n = (SELECT COUNT(rowid)
FROM t1 b
WHERE a.rowid >= b.rowid);
Thanks,
Satya.

>
If you're interested in a more efficient way to get the Nth row of the table, in order by ROWID:
>
Perhaps you should change that to 'tremendously more efficient'?
Ran some tests of the two using a table that contained 50+ thousand records created from ALL_OBJECTS
CREATE TABLE EMP_ALL_OBJECTS AS SELECT * FROM ALL_OBJECTSThen ran your query (modified for the new table) for r_num = 10000. It returned immediately and used 925 consistent gets.
Ran the original query (modified for the new table) for '10000 = '. It took 3 minutes and used 24189252 consistent gets.
Thought maybe it was a fluke so ran the orginal query for '1 = '. It took the same 3 minutes and used 24189253 consistent gets.
Maybe someone else could try to reproduce these results?
Here is the plan for Frank's query
>
Execution Plan
Plan hash value: 3171438729
| Id | Operation | Name | Rows | Bytes |TempSpc| Cos
t (%CPU)| Time |
| 0 | SELECT STATEMENT | | 47057 | 7858K| | 19
93 (1)| 00:00:24 |
|* 1 | VIEW | | 47057 | 7858K| | 19
93 (1)| 00:00:24 |
|* 2 | WINDOW SORT PUSHED RANK| | 47057 | 7812K| 9664K| 19
93 (1)| 00:00:24 |
| 3 | TABLE ACCESS FULL | ALL_OBJECTS_EMP | 47057 | 7812K| | 2
39 (1)| 00:00:03 |
Predicate Information (identified by operation id):
1 - filter("R_NUM"=10000)
2 - filter(ROW_NUMBER() OVER ( ORDER BY ROWID)<=10000)
Note
- dynamic sampling used for this statement (level=2)
Statistics
0 recursive calls
0 db block gets
866 consistent gets
0 physical reads
0 redo size
1281 bytes sent via SQL*Net to client
407 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
1 rows processed
SQL>
>
And the plan for the original query
>
Execution Plan
Plan hash value: 308981268
| Id | Operation | Name | Rows | Bytes | Cost (
%CPU)| Time |
| 0 | SELECT STATEMENT | | 471 | 80070 | 10M
(1)| 35:14:03 |
|* 1 | FILTER | | | |
| |
| 2 | TABLE ACCESS FULL | ALL_OBJECTS_EMP | 47057 | 7812K| 239
(1)| 00:00:03 |
| 3 | SORT AGGREGATE | | 1 | 12 |
| |
|* 4 | TABLE ACCESS BY ROWID RANGE| ALL_OBJECTS_EMP | 2353 | 28236 | 238
(0)| 00:00:03 |
Predicate Information (identified by operation id):
1 - filter( (SELECT COUNT(ROWID) FROM "ALL_OBJECTS_EMP" "B" WHERE "B".ROWID<=
:B1)=1)
4 - access("B".ROWID<=:B1)
Note
- dynamic sampling used for this statement (level=2)
Statistics
7 recursive calls
0 db block gets
24189253 consistent gets
0 physical reads
0 redo size
1218 bytes sent via SQL*Net to client
408 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL>
>
Looks like the original query is doing a probe of the entire 2nd table for each row of the 1st table - almost a CARTESIAN result. It takes the same time no matter which record you are looking for.

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