I wrote:
> Either way, though, the WW weeks don't line up with the D weeks,
> and we're not likely to make them do so.
> So I think an acceptable version of this feature has to involve
> defining at least one new format code and maybe as many as three,
> to produce year, week and day values that agree on whichever
> definition of "a week" you want to use, and then to_date has to
> enforce that input uses matching year/week/day field types,
> very much like it already does for ISO versus Gregorian dates.
A different line of thought could be to accept the current to_char()
behavior for WW and D, and go ahead and teach to_date() to invert that.
That is, take YYYY plus WW as specifying a seven-day interval, and then
D chooses the matching day within that interval. This would still have
the property you complained about originally that WW-plus-D don't form
a monotonically increasing sequence, but I think that ship has sailed.
regards, tom lane