I recently opened https://github.com/apache/pinot/issues/18740 and https://github.com/apache/pinot/pull/18741, adding the scaffolding and the first use of Cost-Based Optimizer (CBO) into MSE. Feel free to take a look and review it!

is there a doc? or do you prefer comments directly in the issue?

There is no doc. Comments on the PR or issue ar efine