I'm the product owner of TiFlash. Yes. We used ClickHouse as the compute engine for TiFlash. The project started as a modification of ClickHouse (more or less it still is). It was like "pushdown query to an actual OLAP database" style 2 years ago. But later on we made a lot tighter integration (raft instead of binlog sync from TP or data-replication for TiFlash itself, implemented same type system, txn, online ddl, Coprocessor interface as TiKV and etc) to make it more "transparent" for query layer.

We will have more detail explained recently.

It will be open sourced in a year or two. For us we need to make the code open-source ready instead of just turn on github settings.