I’ve been casually trying to knock into the black box of MySQL’s internals with no success yet. I have queries that do GROUP BYs across multiple unindexed colum

I’ve been casually trying to knock into the black box of MySQL’s internals with no success yet. I have queries that do GROUP BYs across multiple unindexed columns in frequently large tables, so it ends up with temp tables on disk and filesort. Can someone point to the source code for processing this kind of GROUP BY situation?