I am mostly working full-time on DataFusion for the immediate future and would like to take the lead in coordinating the effort to add support for subqueries, working with other contributors to implement the many features that are required, including planning, optimizations, and execution.
I have started a design document [1] and I have created an epic issue in GitHub [2] to track the work since there are already many issues and discussions related to this. If you are interested in this work then please review the design document and add any feedback that you have. Thanks, Andy. [1] https://docs.google.com/document/d/1j5vHyva-T_5l3POnHSS0-r5TOp86sEkfSUcRLqJZaK4/edit?usp=sharing [2] https://github.com/apache/arrow-datafusion/issues/2248
