Re: Something for the TODO list: deprecating abstime and friends

> On Jul 17, 2017, at 3:12 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> Mark Dilger <hornschnorter(at)gmail(dot)com> writes:
>>> On Jul 17, 2017, at 2:14 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>>> Well, if you or somebody is willing to do the legwork, I'd be on board
>>> with a plan that says that every 68 years we redefine the origin of
>>> abstime. I imagine it could be done so that currently-stored abstime
>>> values retain their present meaning as long as they're not too old.
>>> For example the initial change would toss abstimes before 1970 overboard,
>>> repurposing that range of values as being 2038-2106, but values between
>>> 1970 and 2038 still mean the same as they do today. If anybody still
>>> cares in circa 2085, we toss 1970-2038 overboard and move the origin
>>> again, lather rinse repeat.
>
>> Assuming other members of the community would not object to such
>> a plan, I'd be willing to step up to that plate.
>
> So, thinking about how that would actually work ... the thing to do in
> order to preserve existing on-disk values is to alternate between signed
> and unsigned interpretations of abstimes. That is, right now, abstime
> is signed with origin 1970. The conversion I'm arguing we should make
> real soon now is to unsigned with origin 1970. If the project lives
> so long, in circa 70 years we'd switch it to signed with origin 2106.
> Yadda yadda.
>
> Now, this should mostly work conveniently, except that we have
> +/-infinity (NOEND_ABSTIME/NOSTART_ABSTIME) to deal with. Those
> are nicely positioned at the ends of the signed range so that
> abstime_cmp_internal() doesn't need to treat them specially.
> In an unsigned world they'd be smack in the middle of the range.
> We could certainly teach abstime_cmp_internal to special-case them
> and deliver logically correct results, but there's a bigger problem:
> those will correspond to two seconds in January 2038 that will need
> to be representable when the time comes. Maybe we can make that
> work by defining the values past 2038 as being two seconds less than
> you might otherwise expect, but I bet it's gonna be messy. It might
> be saner to just desupport +/-infinity for abstime.
>
> The same goes for INVALID_ABSTIME, which doesn't have any clear
> use-case that I know of, and is certainly not documented.

I use the abstime type in some catalog tables, and if I allowed those
fields to have something equivalent to NULL, which I do not, I would need
INVALID_ABSTIME, since NULL is not allowed in the constant header of a
catalog table.

I wonder if old versions of postgres had catalog tables with abstime used
in this way? Other than that, I can't think of a reason to use INVALID_ABSTIME.

mark