If you're looking for more #opensource OLAP solutions, there are alternatives to Pinot as Apache Druid, Clickhouse and Apache Kylin. I made a list specific to that below, but also other categories as Cloud Data Warehouses, Data Virtualisations and Serviced Cloud and Analytics. In case of interest: https://www.sspaeti.com/blog/olap-whats-coming-next/#olap-technologies