Next month, I'll be hosting a discussion of Melanie Plageman's talk, Intro to Postgres Planner, given at PGCon 2019. You can sign up using this form. To be clear, the talk is not an introduction to how the planner works from a user perspective, but rather how to hack on it and try to make it better and perhaps get your improvements committed to PostgreSQL. If you're interested, please join us. I anticipate that both Melanie and I will be present for the discussions.