Interesting MSR paper. With regards to Orca, we did take a look at it. It's an OLAP optimizer written in C++ (and very lightly commented and documented, unfortunately), and we wanted a Go-based optimizer that was more integrated with our codebase, as well as targeted at OLTP scenarios (as example Orca supports parallel search, which makes sense in OLAP, but not so much in OLTP). There is quite a bit of overlap, though, and we read their excellent paper and consulted the code from time to time (I hereby invoke a curse on Hungarian notation, which makes reading their code quite painful). We also suspect that Greenplum did not open source some key bits of the optimizer code, such as their search prioritization algorithms.