For same reasons, i.e. a need for precision, I find it hard to accept
the idea of mixing positive and negative units in the same interval.
The plus or minus sign should be outside of the interval.
The interval data type is really useful. I see no reason to restrict its
usefulness with an arbitrary constraint. Date arithmetic is treacherous
and INTERVAL is a lifesaver.
Forcing a global sign on the interval would break interval arithmetic.
How would you compute '1 month'::interval - '1 week'::interval ?
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