How to Handle Multi Value Indexes

Question

Hi all,
i'm a newbye and i've a class attribute which is a String Array, and i want to create a secondary index on this attibute.
I'm wondering which is the best solution to achieve this.

Answer 1

For new users we recommend the DPL. It was created for ease of use.
However, the DPL doesn't support arrays as key fields. So you would have to concatenate the strings with a delimiter and use a single String field, or if the number of strings is fixed use a composite key with a separate String field for each element. Some references are:
http://www.oracle.com/technology/documentation/berkeley-db/je/java/com/sleepycat/persist/model/SecondaryKey.html
http://www.oracle.com/technology/documentation/berkeley-db/je/java/com/sleepycat/persist/model/KeyField.html
If you use the Base API or the Collections API you can use a tuple binding to create a secondary key DatabaseEntry from a String array. For example, you can call TupleOutput.writeString for each element.
http://www.oracle.com/technology/documentation/berkeley-db/je/java/com/sleepycat/bind/tuple/TupleBinding.html
What sort order for your key are you expecting? I assume you want the keys sorted by each string in array element order. That means you should NOT write the length of the array at the beginning of the key, since that will cause the sort order to be primarily based on the array length. Instead you can write an empty string as a terminator, or use TupleInput.available to test for whether you've read all strings in the key. The latter is recommended.
I assumed above that the array length is variable. If it is fixed length, you don't need a terminator.
If you don't care about key sort order, consider using a hash of the String array as the key. The hash field could be a single key field, which would work with the DPL.
I'm giving you a lot of general info. If you can be more specific about what you're doing, I can give more specific advice. I suggest that you start with one of the APIs and try doing what you want. Then you can post specifics about the questions you run into.
--mark