We initially had a pull-based query engine in ClickHouse, but then migrated to a dataflow graph query engine: https://github.com/ClickHouse/ClickHouse/blob/mast

We initially had a pull-based query engine in ClickHouse, but then migrated to a dataflow graph query engine: https://github.com/ClickHouse/ClickHouse/blob/master/src/Processors/IProcessor.h#L31 https://github.com/ClickHouse/ClickHouse/blob/master/src/Pro... It allows decoupling of the control flow and the data flow. The movement of the data in the query pipeline is controlled explicitly. We did this migration a few years ago. Many database engines forked or influenced by ClickHouse still use pull-based query engines.