[cevian]

In practice this doesn't come up a lot. Say you have a hypertable with measurement(time, device_id, value) and a device table with (device_id, device_manufacturer, device_type). Timescale fully support a foreign-key from the measurement table into the devices table. This is a common usage. A FK from another table which references a measurement row is not supported, but is also uncommon. To see why note that a part of the primary-key of the measurement table is time and so conceptually the only type of table that would want a FK into it is also a time-based table, and so the only real usage is a 1-to-1 relation. That is also uncommon and can be gotten-around with normalization.