On 28/12/11 10:43, Phil Sorber wrote:
On Tue, Dec 27, 2011 at 1:00 PM, Tom Lane<t...@sss.pgh.pa.us> wrote:
Phil Sorber<p...@omniti.com> writes:
My search foo failed me. Someone just pointed me to a similar
conversation from some months ago:
http://archives.postgresql.org/pgsql-hackers/2011-07/msg00677.php
I would propose that since we can't know the hour or minute of
infinity that we should return null for those. I think NaN would be
wrong because it is a real number, it's just unknown. If we can just
pass infinity through the function, I think we should.
The last thread ended with a request for somebody to think through
the behavior for *all* extract field types and make a coherent proposal.
I don't think you've really advanced the discussion yet.
I think I agree with the position that we shouldn't return 0 unless
the correct value actually is 0, but it's not clear to me whether
to use NULL or NaN to represent "indeterminate". Traditionally we
consider NULL to mean "unknown", but it seems like "what's the hour
of an infinite timestamp" is a subtly different sort of situation:
it's not unknown, we know perfectly well that it's indeterminate.
OTOH, choosing NaN would put a pretty significant dependence on
IEEE-float arithmetic into the external specification of timestamps,
and I find that a bit worrisome, even though IEEE float arithmetic
is nigh universal these days. So maybe splitting hairs like that
would be ill-advised. It probably depends also on what you expect
people to do with the result of extract() --- NULL would presumably
propagate through any additional calculation steps as-is, whereas
NaN might have less predictable behavior.
There was also some support for throwing an error in the previous
thread, though I can't say I like that answer myself.
regards, tom lane
It is my understanding that NULL would be for "unknown" or "undefined"
and NaN for "indeterminate" as well as some other cases like complex
numbers. I believe per the standard NaN explicitly includes
indeterminate forms. But I don't think extract(hour from
'infinity'::timestamp) is an indeterminate form
(http://en.wikipedia.org/wiki/Indeterminate_form). It is an
oscillating function similar to sin(x). Limit of sin(x) as x
approaches infinity is undefined. To me that points to NULL as the
appropriate value.
Also, like epoch, the expressions that involve year are not
oscillating. They are monotonic. the limit of extract(millennium from
'infinity'::timestamp) is infinity.
I'm not going to claim to be a mathematician, so I concede I might be
wrong with my thought process here.
Given the preceding is true, my proposal is the following for
extract() when passed an infinite timestamp:
1) Monotonic values (century, decade, epoch, isoyear, millennium and
year) we return 'infinity'::float8 signed appropriately.
2) Oscillating values (day, dow, doy, hour, isodow, microseconds,
milliseconds, minute, month, quarter, second and week) would return
NULL.
3) timezone, timezone_hour and timezone_minute are almost a separate
issue since timezone is separate from the value. So we should support
something like 'infinity-05'::timestamp with time zone. Then the
timezone stuff would just behave normally.
Currently it does this:
postgres=# select 'infinity+00'::timestamp with time zone;
timestamptz
-------------
infinity
(1 row)
postgres=# select 'infinity-05'::timestamp with time zone;
ERROR: invalid input syntax for type timestamp with time zone: "infinity-05"
LINE 1: select 'infinity-05'::timestamp with time zone;
^
Hmm...
Infinity is conceptually the 'maximum' value possible - or more
pr4ecisely: a value greater than any you can specify a concrete value
for in finite time.
So I think the appropriate value should be the maximum representational
possibility and should be the same regardless of time zone, plus any
operation such as adding or subtracting finite values should not change
it (arithmetic ops with another 'infinite' value should be either an
error or a NaN/Null). This is to consistent that with the notion of
infinity.
I would suggest that hh:mm:ss.ssss...
should be: 23:59:59.9999...
Cheers,
Gavin
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