It's using a temporary cache that re-uses the latest data from the sheet (checking if it was updated) so that's why the subsequent queries should be much faster than the initial one.
Now about the headers, it's indeed an issue that I need to figure out, as there are some sheets that have weird structures and it would be nice to find an easy UXsy way to preview the sheet data and say "select from this range" or "these are the headers".
The extensibility system also lets you show a modal dialog, which we use for previewing API calls to third party HTTP servers -- sounds like a similar thing could work well for you, too.
You could also look at supporting named ranges if you don't already, so people can refer to a range as `FinanceData` instead of `Sheet 1!A3:F90`
Indeed. Defining the UX when the only UI you have is a SQL query is a bit challenging.
Very impressive project indeed and solves a real problem a lot of people and companies have. I wish you luck.
Thanks for the comment!!