How do I perform a custom sort of secondary duplicates?
If you have a secondary database with sorted duplicates configured, you may wish to sort the duplicates according to some other field in the primary record. Let’s say your secondary key is F1 and you have another field in your primary record, F2, that you wish to use for ordering duplicates. You would like to use F1 as your secondary key, with duplicates ordered by F2. In Berkeley DB, the “data” for a secondary database is the primary key. When duplicates are allowed in a secondary, the duplicate comparison function simply compares those primary key values. Therefore, a duplicate comparison function cannot be used to sort by F2, since the primary record is not available to the comparison function. The purpose of key and duplicate comparison functions in Berkeley DB is to allow sorting values in some way other than simple byte-by-byte comparison. In general it is not intended to provide a way to order keys or duplicates using record data that is not present in the key or duplicate entry
