Re: Missing documentation for uses of extract(field from interval)

I have added documentation for EXTRACT(DAY) usage with intervals.  We
had something for MONTH, but for some reason nothing for DAY.  Applied
patch attached.

---------------------------------------------------------------------------

Daniel Grace wrote:
> (Note: I'm not subscribed to pgsql-docs, so please CC me on any
> replies.  Thanks!)
>
> I pulled up the documentation today to see if I could get just the
> number of days in an interval using EXTRACT.  The answer provided was
> vague: While you can use EXTRACT(field from interval) (shown in table
> 9-27), section 9.9.1 only lists:
>
> day
>
>     The day (of the month) field (1 - 31)
>
>     SELECT EXTRACT(DAY FROM TIMESTAMP '2001-02-16 20:38:40');
>     Result: 16
>
>
>
> It turns out that EXTRACT(day FROM interval) does work as expected --
> so perhaps the documentation should be updated to something like:
>
> day
>
>     The day (of the month) field (1 - 31), or the number of days in the interval
>
>     SELECT EXTRACT(DAY FROM TIMESTAMP '2001-02-16 20:38:40');
>     Result: 16
>
>     SELECT EXTRACT(DAY FROM INTERVAL '42 days 21:00:00');
>     Result: 42
>
> Likewise for month, year, century and maybe some others.  (hour,
> minute, and *second{s?} probably don't need updated text).
>
> There also should probably be some warning in here about EXTRACT on
> intervals however.  Namely cases like this:
>
> SELECT EXTRACT(YEAR FROM INTERVAL '400 days')  --> Result: 0
> (because the interval is '400 days' not '1 year 35 days')
> SELECT EXTRACT(DAY FROM INTERVAL '1 year')  --> Result: 0    (because
> the interval is '1 year' not '365 days')
> SELECT EXTRACT(DAY FROM INTERVAL '1 month')  --> Result: 0
>
> Though, this works:
> SELECT EXTRACT(CENTURY FROM INTERVAL '99 years')  --> Result: 0
> SELECT EXTRACT(CENTURY FROM INTERVAL '100 years')  --> Result: 1
> --
> Daniel Grace
> AGE, LLC
> System Administrator and Software Developer
>
> --
> Sent via pgsql-docs mailing list (pgsql-docs@postgresql.org)
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--
  Bruce Momjian  <bruce@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 4635cf2..c392109
*** a/doc/src/sgml/func.sgml
--- b/doc/src/sgml/func.sgml
*************** SELECT EXTRACT(CENTURY FROM TIMESTAMP '2
*** 6605,6617 ****
        <term><literal>day</literal></term>
        <listitem>
         <para>
!         The day (of the month) field (1 - 31)
         </para>

  <screen>
  SELECT EXTRACT(DAY FROM TIMESTAMP '2001-02-16 20:38:40');
  <lineannotation>Result: </lineannotation><computeroutput>16</computeroutput>
  </screen>
        </listitem>
       </varlistentry>

--- 6605,6624 ----
        <term><literal>day</literal></term>
        <listitem>
         <para>
!         For <type>timestamp</type> values, the day (of the month) field
!         (1 - 31) ; for <type>interval</type> values, the number of days
         </para>

  <screen>
  SELECT EXTRACT(DAY FROM TIMESTAMP '2001-02-16 20:38:40');
  <lineannotation>Result: </lineannotation><computeroutput>16</computeroutput>
+
+ SELECT EXTRACT(DAY FROM INTERVAL '40 days 1 minute');
+ <lineannotation>Result: </lineannotation><computeroutput>40</computeroutput>
  </screen>
+
+
+
        </listitem>
       </varlistentry>

*************** SELECT EXTRACT(MINUTE FROM TIMESTAMP '20
*** 6827,6833 ****
        <listitem>
         <para>
          For <type>timestamp</type> values, the number of the month
!         within the year (1 - 12) ; for <type>interval</type> values
          the number of months, modulo 12 (0 - 11)
         </para>

--- 6834,6840 ----
        <listitem>
         <para>
          For <type>timestamp</type> values, the number of the month
!         within the year (1 - 12) ; for <type>interval</type> values,
          the number of months, modulo 12 (0 - 11)
         </para>