oracle pagination query performance issue

The following query for paginating say 2000 records is having performace issues.
for initial pages its fine.As the page number passed increases, time for execution increases.
I need some way to skip already traveres records or some alternative way to query reults speedily.
Say i am showing 20 records per page in total of 50 pages

and i want records from 980 to 1000.

How can i spped this query up?

    SELECT b.*, rownum r
          SELECT AA,BB,CC,DD
  FROM table tab  WHERE AA= 'testdata1'
                AND B = 'testdata2'
                AND C >= 'date1' AND B < 'date2' -- C is date column
                AND D  BETWEEN 1 AND 1000000  -- amount column
                ORDER BY C DESC -- Date column C is indexed
    ) b
    WHERE rownum < ((Pagenum * Pagesize) + 1 )
WHERE r >= (((Pagenum-1) * Pagesize) + 1)
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slightwv (䄆 Netminder) Commented:
Try something like this:
select whatever from (
select whatever, row_number() over(order by some_column) rn
from some_table
where rn between pagenum*pagesize and (pagenum+1)*pagesize

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Geert GOracle dbaCommented:
upgrade to oracle 12
a new feature of oracle 12 is an extended clause for limiting the rows

or use an advanced grid component which has the pagination built in
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