What are some NewSql relational scalable databases?

NewSql, a scalable relational category includes Clustrix, VoltDB, MemSQL,Pivotal's GemFire XD, SAP HANA, NuoDB, and Trafodion as well as CockroachDB, Altibase, TransLattice, JustOneDB, Datomic, InfiniSQL, FairCom. (Sources: wikipedia; Data Platform Map 2015).

To add some DBs I know which may in fact fit the bill: Apache Phoenix is a relational DB on top of HBase. FoundationDB, RethinkDB should be general purpose too.

Apache Hive is currently MapReduce, so scalable but slow latency. There are variations of BigSql from IBM to do Apache Spark computations and improve latency. In fact you may find BigSql and NewSql used to mean about the same thing.

I made this list from Databases Landscape publishing / Data Patform Map, last one is late 2015 and you can download the pdf with a huge subway-like map of databases, and they have like 15 categories, one of which, New SQL.

So there are huge numbers of theoretical possibilities. BUT. Cockroach DB is in infancy, alpha state. MemSQL is open source but not really: you get to pay to actually have its power (I heard). I have not used the others, but I had hard times with Druid, which is an OLAP database (analytic, data warehouse). You seem to search an OLTP database, right?

If I were to purely guess the most stable ones which are potentially actually free, should be Clustrix, VoltDB, NuoDB.

Now it really depends how much is writes and how much reads, for instance. There are plenty solutions to scale the read site with both Postgres and Mysql. If you want big write throughput as well, or your data does not fit a node, yes, you need to test a lot of solutions out of the presented list.

Did you expect so many alternatives?:)

One more thing:) If you open popularity ranking of relational DBMS you will see exactly 109 relational databases. Out of those, I guess 20 are old-style single-node and all the other 90 are scalable ones: OLTP ones and OLAP ones. So, good luck picking, use case matching and testing them on your test cases and simulations! Let us know what you found, so we can learn how they behave in practice!