Re: "interval hour to minute" or "interval day to minute"

On Sun, Apr 17, 2011 at 04:55:51PM +0100, Jack Douglas wrote:
> I discovered the 'fields' option of 'interval', but i can't figure out
> from the docs how it is supposed to work. Are "hour to minute" and "day
> to minute" really the same thing? And if not, in what circumstances are
> they treated differently?

As of version 8.4, they behave identically. The code has this comment, some
form of which probably belongs in the documentation:

/*
* Our interpretation of intervals with a limited set of fields is
* that fields to the right of the last one specified are zeroed out,
* but those to the left of it remain valid. Thus for example there
* is no operational difference between INTERVAL YEAR TO MONTH and
* INTERVAL MONTH. In some cases we could meaningfully enforce that
* higher-order fields are zero; for example INTERVAL DAY could reject
* nonzero "month" field. However that seems a bit pointless when we
* can't do it consistently. (We cannot enforce a range limit on the
* highest expected field, since we do not have any equivalent of
* SQL's <interval leading field precision>.)
*
* Note: before PG 8.4 we interpreted a limited set of fields as
* actually causing a "modulo" operation on a given value, potentially
* losing high-order as well as low-order information. But there is
* no support for such behavior in the standard, and it seems fairly
* undesirable on data consistency grounds anyway. Now we only
* perform truncation or rounding of low-order fields.
*/