If it's going about RDBMS then PostgreSQL definitely worth attention. It's most advanced in regard to functionality and query optimization open source DB. Another relational DB which worth attention is the YugabyteDB. It makes an attempt to be compatible with PostgreSQL but unlike it, it's a distributed, highly scalable DB.
Among non-SQL solutions I think Cassandra and Apache Ignite are most interesting and useful.
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If it's going about RDBMS then PostgreSQL definitely worth attention. It's most advanced in regard to functionality and query optimization open source DB. Another relational DB which worth attention is the YugabyteDB. It makes an attempt to be compatible with PostgreSQL but unlike it, it's a distributed, highly scalable DB.
Among non-SQL solutions I think Cassandra and Apache Ignite are most interesting and useful.