Redis GET vs. SQL SELECT

Question

I am pretty new to NoSQL, but I always liked the idea of it. I took a look at Redis, and got a few questions about the best ways of storing and recieving multiple hashes.

Assuming the following scenario:

Store a list of objects (redis 'Hashes') and select them by their timestamp.

To archive this in SQL, it would require one table and two simple queries (INSERT & SELECT).

Trying to do this in Redis, I ended up creating the following structure:

1. Key object:$id (hash) containing the object
2. Key index:timestamp:$id (sorted set) score equals timestamp and value includes id

While I can live with the additional maintenance work of two keys instead of one table (SQL), I am curious about the process of selecting multiple objects:

ZRANGEBYSCORE index:timestamp:$id timestampStart timestampEnd

This returns an array of all IDs which got created between timestampStart and timestampEnd. To get the object itself I am requesting every single one by:

GET object:$id 

• Is this the right way of doing it?
• In comparison with an SQL Database: Is it still appreciably faster or might it even become slower caused by the high number of GETs?

Answer 1

A ZRANGEBYSCORE costs O(log(N) + M) where N=|items in your set| and M=|items you're selecting|. So, doing the ZRANGEBYSCORE and then M GET operations is just O(long(N)+M+M) = O(log(N)+M) and would at most be twice as slow. The network back and forth could have been a major slow down, but since each of your gets is an independent operation, you can just pipeline them. You can also put the whole thing in a Lua script and just have one back and forth, which would be the most optimal. I'd say with 99% certainty this would be faster than doing the same thing in SQL.

Also, if this is a very frequent operation for you, you can get even more speed up by just storing the entire object in your sorted set instead of just the id. You'd have key = object encoded as json, score = timestamp. This would save you O(M) on your operation in terms of not needing to do any GETs.

Whether or not this is a good way of doing things really depends on your use case. How much speed do you really need, and how important are other features of a traditional database to you? Remember, Redis is much more just datastructures accessible by clients than a traditional database, and it must store everything in RAM. To know whether it's the right thing for you, we'd need more information.