I think this came up again recently and somebody pointed out that the
correct definition isn't as obvious as all that. The components of
an interval can have different signs, so should abs('-1 day 1 hour') be
'1 day -1 hour' or '1 day 1 hour'? Or what about corner cases like
'1 day -25 hours'?
I agree with Sam. The absolute value of a negative interval should be
equidistant from zero, not the removal of negative signs. So abs('-1 day
1 hour') should be ('1 day -1 hour'). I don't think your corner case is
any different. So his function and operator should be perfectly valid.
But there is some ambiguity around the length of a month. So INTERVAL '1
month - 30 days' = INTERVAL '0 days' = INTERVAL '-1 month +30 days'.
But when added to a date, it makes no change for months with 30 days,
adds 1 day for months with 31 days and subtracts 2 days for February.
Scott Bailey
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