I used to get this complicated, and a lot more, with SQL, but that was years ago now. Still, this is an intriguing question.
The row_number() function you are using is a part of a whole class of "analytic" or "windowing" functions. A few years back Derby had only minimal support, but if that has been fleshed out now then those functions might be a way to remove the distinct and maybe even the self join.
Also, I believe derby supports the SELECT TOP … N syntax. Will that work to get the first 25 rows for you?
Top will help for most cases, we will still need to pull a few pages of results the more pages the user goes into the result set, but that may well be better than the cost of row_number().
There is a possibility that removing the a0 table in the distinct select might cause the query to execute the distinct not using a sort operation. (ie using a hash).
A quick search: 1) reminded me that derby uses the OLAP term for these functions; 2) suggests that the support is still mostly missing.
I hope your other ideas give better results!
select distinct a.rid, ROW_NUMBER() OVER () AS R from cn_css a where a.cid = ? and a.v = ?