can I add partitioning to a table that is already existing/ingesting data? and can partitioning work by itself without the groupReplicas? would it speed up queries?

yes, existing segments wont benefit but new segments will use that information to speed up queries

thank you! that would explain why old segments still seem to be slow

Yes, this is true for any kind of indexing @Luis Fernandez. Availability of metadata/indexing is per segment, and query execution makes use of it as long as it is available.

I was about to say its not true 🙂 but I think there are multiple parts to your question

Ah yes, there are multiple parts indeed. What I described in my previous response was a case where older segments did not have the desired index, and newer ones did. In that case, new segments will use the indexes. However, like @Kishore G mentioned, you can invoke segment reload api to rebuild indexes for existing segments as well.

in the UI that would be “Reload All Segments” right?

Partitioning does not really fall under indexing.. it can only be done during ingestion as of now