FLUX is InfluxDB's new query & manipulation language. It's not at all like SQL (unlike the previous query language) - but is more powerful, testable & development friendly
QuestDB is fast to both injest and query data. It does this by mmaping your data into partitioned files and uses SIMD for aggregations. For databases that don't fit in memory this is the fastest way to store and query columnar data given how modern operating systems work.
KairosDB primary storage backend is Apache Cassandra. Cassandra is one of the biggest player in NoSQL world, supported by Datastax and used by large companies such as Netflix.
Extending KairosDB is very easy.
Software is based on Guice modules and all parts can be extended using modules: aggregations, query engines, webservice APIs.