Can I configure the BTREE compare function at search time?

Hi, all,
I was implementing the 'order by' feature on top of BDB. The problem I met is the "reverse order", that means 'order by xxx desc'.
BDB provides cursor->get API that is used to do search, with flags of 'DB_SET_RANGE', 'DB_GET_BOTH_RANGE'. This is perfect when do 'order by xxx asc' search.
But when I do reverse search, it is complicated, because such as 'DB_SET_RANGE', they find the record that is equal or large than, not equal or less than, this situation becomes even more complicated when the records is duplicated(I need move the cursor forward and back, back and forword). My question is, is there a way to specify the Btree compare function at search time? Now I can only find a way to specify the compare function when the db is created. If there are options that can reverse the compare function at search time, also things done.
Thanks very much!
Regards,
Steve
Edited by: Steve Chu on Jun 4, 2009 4:42 PM

Hi,
It is not possible to change the btree comparison function after the database has been created. It needs to return consistent results at all times, or the internal btree structure will become corrupt.
I don't really understand the issue you are facing. Can't you do a DBC->get(DB_SET_RANGE) call, and then walk the cursor backwards? That is if the position returned from the DBC->get(DB_SET_RANGE) is not equal to the requested key, call DBC->get(DB_PREV) before iterating any further?
It is possible to control the behavior of the previous/next operations on cursors in relation to duplicate sets by using the correct flags. See the documentation for [DB_PREV_NODUP|http://www.oracle.com/technology/documentation/berkeley-db/db/api_c/dbc_get.html#DB_PREV_NODUP]. You can see the other related flags documented on the same page.
Regards,
Alex Gorrod, Oracle Berkeley DB

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