Creating a seconday Index?
hhimanshu opened this issue · comments
Hey @dain
I have a usecase where I wanted a BiDirectional Map, where I can search on both key and value (they both are unique). Currently, I store them twice in 2 different databases
String key = // generated from somewhere
String hashedKey = // encrypt key and save
mapDb.put(key, hashedKey)
decryptMapDb(hashKey, key)
Our key and hashedKey both are unique
I was reading on leveldb google groups, where someone recommended to create secondary index. The link is here
What you have is a kind of secondary index, so you may be all set. In the linked discussion, they are referring to a design where you would store both directions in the same table, using a prefix to distinguish between the two. You might have something like this:
prefixes: a2b, b2a
key structure:
a2b:keyForDirectionA:KeyForDirectionB
with a value of new byte[0].
The advantage to putting both 'keys' in the the table key is that you can now support indexes that are not unique. Of course, you have to parse out the values you are interested in.
@lwhite1 that is exactly what I was going to suggest.
@lwhite1, @dain , my apologies but I am confused. Let us say my two indexes are
keyToHashKey and hashKeyToKey
How am I storing them?
keyToHashKey:key:hashedKey
hashKeyToKey:hashedKey:key
Is that what you were saying?
If yes, then I am not saving on space at all, right? so what I do currently is better, isn't it?
As I said, what you're doing is fine. This approach is not intended to save space, but to enable non-unique secondary indexes, which you don't need.