A columnar database stores the data of each column independently. This allows reading data from disk only for those columns that are used in any given query.
ClickHouse is an open-source project developed on GitHub. As customary, contribution instructions are published in CONTRIBUTING file in the root of the source code repository.
We can refer to systems like MapReduce as distributed computing systems in which the reduce operation is based on distributed sorting. The most common open-source solution in this class is Apache Hadoop.