Have a look at ClickHouse for OLAP workloads: https://clickhouse.yandex/ It's a recently open sourced database by Yandex. It powers their web analytics backend. CloudFlare is already using it for their DNS analytics.
Do you know if it is an in-memory solution or can it work with data that doesn't fit into RAM?
Yes, in fact, it's specifically designed for data that doesn't fit in RAM. Their storage engine optimizes data locality for fast retrieval from non-flash storage.
See: https://clickhouse.yandex/presentations/data_at_scale/
Their docs also talk about it a lot.