if you use star tree index and say you have a time column, if you wanted to do different aggregations based on time, does that mean that the time column also has to be part of the indexes for the star tree? say user_id , click_count, serve_time then I should do user_id, serve_time and then SUM(click_count) as part as the aggregation

In the default config it is already added: https://docs.pinot.apache.org/basics/indexing/star-tree-index#default-index-generation-configuration

Short answer - yes time should be per of the index