Ask HN: Best database engine for complex queries

What is the best database engine for executing complex queries which perform well? Planning to migrate these out of oracle, but not able to find a suitable alternative that supports scale out, performance and low latency queries. Any suggestions would be helpful. Current implementation has stuff like self join. I can remodel data to denormalize etc but not sure which database technology to target to. Cassandra seems optimized for fast writes, can’t support multiple clustering keys for performance reasons. Postgres would be similar in comparison to oracle for scalability. What else?