Most have some sort of caching logic but you can't rely on it to do what you want. Always benchmark, and never assume that just because you're paying for something or that it's the industry standard, it does what you want. I've seen a number of devs completely surprised by the throughput & latency numbers they get from their databases because they assume everything is going to be cached in RAM and the database is really hitting disk for some obscure reason on every query.