I somehow missed this post from Flickr team describing their use of (app enforced) capped sorted sets in Redis as sort of a reduced optimized secondary index for MySQL:
[…] the bottleneck was not in generating the list of photos for your most
recently active contact, it was just in finding who your most recently
active contact was (specifically if you have thousands or tens of thousands
of contacts). What if, instead of fully denormalizing, we just maintain a
list of your recently active contacts? That would allow us to optimize the
slow query, much like a native MySQL index would; instead of needing to look
through a list of 20,000 contacts to see which one has uploaded a photo
recently, we only need to look at your most recent 5 or 10 (regardless of
your total contacts count)!
This is the first time I’m encountaring this approach where a NoSQL database is used not to provide directly the final data (usually in a denormalized format), but rather to optimize the access to the master of data. Basically this is a metadata layer optimizer. Neat!
Original title and link: Using Redis to Optimize MySQL Queries ( ©myNoSQL)