On Mon, 25 Oct 2004, Tom Lane wrote:
> There are two classes of intervals. One class, called year-month
> intervals, has an express or implied datetime precision that in-
> cludes no fields other than YEAR and MONTH, though not both are
> required. The other class, called day-time intervals, has an ex-
> press or implied interval precision that can include any fields
> other than YEAR or MONTH.
>
> AFAICS the reason for this rule is that they expect all Y/M intervals to
> be comparable (which they are) and they also expect all D/H/M/S intervals
> to be comparable, which you can only do by assuming that 1 D == 24 H.
I said I was not going to send any more mails, but here we go again :-)
The standard restrict the hour field to the interval 0-23, so there can
never be any compare between for example '1 day 1 hour' and '25 hours'.
This means that one can not add two intervals together to get a bigger
one but that it would still work to do timestamp+interval+interval.
> It seems to me though that we can store days separately and do interval
> comparisons with the assumption 1 D == 24 H, and be perfectly
> SQL-compatible as far as that goes, and still make good use of the
> separate day info when adding to a timestamptz that has a DST-aware
> timezone. In a non-DST-aware timezone the addition will act the same as
> if we weren't distinguishing days from h/m/s. Therefore, an application
> using only the spec-defined features (ie, only fixed-numeric-offset
> timezones) will see no deviation from the spec behavior.
I agree with this.
--
/Dennis Björklund