Does any sort of query cache exist in Pinot ?

No, there isn't one right now. For realtime cases, the data is changing in realtime, so caching the query result might not make sense.

Yes in the case of realtime table the case would be invalidated very often In case of offline tables, I guess the best way is to add an external cache at the REST API layer ? And invalidate it whenever new segments are pushed

Added to FAQ: https://docs.pinot.apache.org/basics/getting-started/frequent-questions/query-faq#does-pinot-cache-query-results

Looks great and clear, thanks @Mayank!