I have developed the attached doc patch to address your very clear
illustration that our documentation is lacking in this area.
---------------------------------------------------------------------------
On Tue, Dec 4, 2012 at 08:18:31PM +0100, Francisco Olarte (M) wrote:
> > Wow, that is a weird case. In the first test, we count the number of
> > days because it is less than a full month. In the second case, we call
> > it a full month, but then forget how long it is. Not sure how we could
> > improve this.
>
> I do not think this needs to be improved, the problem is given two
> dates you can substract them in three different ways,
>
> 1.- (year months)+(days)+(hours minutes seconds), which is what age
> does and is documented as such.
>
> folarte=# select age('2013-11-24 16:41:31','2012-10-23 15:56:10');
> age
> -----------------------------
> 1 year 1 mon 1 day 00:45:21
> (1 row)
>
> Which is apropiate for things like 'I'm xxx old'
>
> 2.- (days)+(hours-minutes-seconds), which is what substractint dates
> do ( or seems to do for me, as I've done:
>
> select timestamp '2013-11-23 16:41:31' - '2012-10-23 15:56:10';
> ?column?
> -------------------
> 396 days 00:45:21
>
> Which I can not find a use for, but there sure are and I'm doomed to
> find one soon.
>
> 3.- Exact duration ( I do this a lot at work as I need to calculate
> call durations ):
>
> folarte=# select extract(epoch from timestamp '2013-11-23 16:41:31') -
> extract(epoch from timestamp '2012-10-23 15:56:10');
> ?column?
> ----------
> 34217121
> (1 row)
>
> folarte=# select (extract(epoch from timestamp '2013-11-23 16:41:31')
> - extract(epoch from timestamp '2012-10-23 15:56:10')) * interval '1
> second';
> ?column?
> ------------
> 9504:45:21
> (1 row)
>
> The problem I see is intervals are really complicated and difficult to
> undestand, so it is at most a documentation problem ( people usually
> understimate the difficulty of working with them, I see this a lot at
> work ).
>
> Francisco Olarte.
--
Bruce Momjian <br...@momjian.us> http://momjian.us
EnterpriseDB http://enterprisedb.com
+ It's impossible for everything to be true. +
diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
new file mode 100644
index 425544a..a411f86
*** a/doc/src/sgml/func.sgml
--- b/doc/src/sgml/func.sgml
*************** SELECT SUBSTRING('XY1234Z', 'Y*?([0-9]{1
*** 6431,6437 ****
</entry>
<entry><type>interval</type></entry>
<entry>Subtract arguments, producing a <quote>symbolic</> result that
! uses years and months</entry>
<entry><literal>age(timestamp '2001-04-10', timestamp '1957-06-13')</literal></entry>
<entry><literal>43 years 9 mons 27 days</literal></entry>
</row>
--- 6431,6437 ----
</entry>
<entry><type>interval</type></entry>
<entry>Subtract arguments, producing a <quote>symbolic</> result that
! uses years and months</entry>, rather than just days
<entry><literal>age(timestamp '2001-04-10', timestamp '1957-06-13')</literal></entry>
<entry><literal>43 years 9 mons 27 days</literal></entry>
</row>
*************** SELECT (DATE '2001-10-30', DATE '2001-10
*** 6794,6799 ****
--- 6794,6829 ----
days</> because May has 31 days, while April has only 30.
</para>
+ <para>
+ Subtraction of dates and timestamps can also be complex. The most
+ accurate way to perform subtraction is to convert each value to a number
+ of seconds using <literal>EXTRACT(EPOCH FROM ...)</> and compute the
+ number of <emphasis>seconds</> between the two values. This will adjust
+ for the number of days in each month, timezone changes, and daylight
+ saving time adjustments. Operator subtraction of date or timestamp
+ values returns the number of days (24-hours) and hours/minutes/seconds
+ between the values, making the same adjustments. The <function>age</>
+ function returns years, months, days, and hours/minutes/seconds,
+ performing field-by-field subtraction and then adjusting for negative
+ field values. The following queries, produced with <literal>timezone
+ = 'US/Eastern'</> and including a daylight saving time change,
+ illustrates these issues:
+ </para>
+
+ <screen>
+ SELECT EXTRACT(EPOCH FROM timestamptz '2013-07-01 12:00:00') -
+ EXTRACT(EPOCH FROM timestamptz '2013-03-01 12:00:00');
+ <lineannotation>Result: </lineannotation><computeroutput>10537200</computeroutput>
+ SELECT (EXTRACT(EPOCH FROM timestamptz '2013-07-01 12:00:00') -
+ EXTRACT(EPOCH FROM timestamptz '2013-03-01 12:00:00'))
+ / 60 / 60 / 24;
+ <lineannotation>Result: </lineannotation><computeroutput>121.958333333333</computeroutput>
+ SELECT timestamptz '2013-07-01 12:00:00' - timestamptz '2013-03-01 12:00:00';
+ <lineannotation>Result: </lineannotation><computeroutput>121 days 23:00:00</computeroutput>
+ SELECT age(timestamptz '2013-07-01 12:00:00', timestamptz '2013-03-01 12:00:00');
+ <lineannotation>Result: </lineannotation><computeroutput>4 mons</computeroutput>
+ </screen>
+
<sect2 id="functions-datetime-extract">
<title><function>EXTRACT</function>, <function>date_part</function></title>
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