Improve Performance on Inline view Query

Question

All,
I have a unique situation where I have to limit the number of rows based on group. so I created an Inline view and applied a limit on it.
ex:
SELECT col_A, col_B FROM ( SELECT col_a, count(*) FROM tab_a WHERE col_a = 'XXX' GROUP BY col_a) ROWNUM <-10.
but this design was rejected, because it seems to have a great impact on performance due to inline view.
Also I cant set a rowlimit directly on the query because GROUP BY clause is used in the select.
When the rownum is directly applied in the WHERE, first it limits the rows and then it makes a GROUP, so when user asks to retrieve 10 records, it may show less than 10 rows because of the grouping.
please help to find a alternative solution which helps to get the expected result set and same time without loosing performance

Answer 1

but this design was rejected, because it seems to have a great impact on performance due to inline view.What proof was offered that this query performed badly. There is nothing inherently non-performant about in-line views. I would demand to see some evidence that this query uses an unreasonable amount of system resource.
However, you could try this:
SELECT t.col1, avg(t.col2)
FROM   t1 t
WHERE  t.col1 IN ( SELECT DISTINCT x.col1
                   FROM   t1 x
                   WHERE  col3 = 'XXX'
                   AND    rownum <=10 )
GROUP  BY t.col1
/I assume your example is not derived from your real code (otherwise the WHERE clause would restrict the inline query to one returned row - problem solved!)
Cheers, APC