Docs: improve descriptions of ISO week-numbering date features.

tglsfdc · tglsfdc · commit 5f8fe02a56ec · 2014-12-31T16:42:58.000-05:00
Use the phraseology "ISO 8601 week-numbering year" in place of just
"ISO year", and make related adjustments to other terminology.

The point of this change is that it seems some people see "ISO year"
and think "standard year", whereupon they're surprised when constructs
like to_char(..., "IYYY-MM-DD") produce nonsensical results.  Perhaps
hanging a few more adjectives on it will discourage them from jumping
to false conclusions.  I put in an explicit warning against that
specific usage, too, though the main point is to discourage people
who haven't read this far down the page.

In passing fix some nearby markup and terminology inconsistencies.
diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
@@ -4990,11 +4990,11 @@ SELECT SUBSTRING('XY1234Z', 'Y*?([0-9]{1,3})');
        </row>
        <row>
         <entry><literal>Y,YYY</literal></entry>
-        <entry>year (4 and more digits) with comma</entry>
+        <entry>year (4 or more digits) with comma</entry>
        </row>
        <row>
         <entry><literal>YYYY</literal></entry>
-        <entry>year (4 and more digits)</entry>
+        <entry>year (4 or more digits)</entry>
        </row>
        <row>
         <entry><literal>YYY</literal></entry>
@@ -5010,19 +5010,19 @@ SELECT SUBSTRING('XY1234Z', 'Y*?([0-9]{1,3})');
        </row>
        <row>
         <entry><literal>IYYY</literal></entry>
-        <entry>ISO year (4 and more digits)</entry>
+        <entry>ISO 8601 week-numbering year (4 or more digits)</entry>
        </row>
        <row>
         <entry><literal>IYY</literal></entry>
-        <entry>last 3 digits of ISO year</entry>
+        <entry>last 3 digits of ISO 8601 week-numbering year</entry>
        </row>
        <row>
         <entry><literal>IY</literal></entry>
-        <entry>last 2 digits of ISO year</entry>
+        <entry>last 2 digits of ISO 8601 week-numbering year</entry>
        </row>
        <row>
         <entry><literal>I</literal></entry>
-        <entry>last digit of ISO year</entry>
+        <entry>last digit of ISO 8601 week-numbering year</entry>
        </row>
        <row>
         <entry><literal>BC</literal>, <literal>bc</literal>,
@@ -5092,35 +5092,35 @@ SELECT SUBSTRING('XY1234Z', 'Y*?([0-9]{1,3})');
        </row>
        <row>
         <entry><literal>IDDD</literal></entry>
-        <entry>ISO day of year (001-371; day 1 of the year is Monday of the first ISO week.)</entry>
+        <entry>day of ISO 8601 week-numbering year (001-371; day 1 of the year is Monday of the first ISO week)</entry>
        </row>
        <row>
         <entry><literal>DD</literal></entry>
         <entry>day of month (01-31)</entry>
        </row>
        <row>
         <entry><literal>D</literal></entry>
-        <entry>day of the week, Sunday(<literal>1</>) to Saturday(<literal>7</>)</entry>
+        <entry>day of the week, Sunday (<literal>1</>) to Saturday (<literal>7</>)</entry>
        </row>
        <row>
         <entry><literal>ID</literal></entry>
-        <entry>ISO day of the week, Monday(<literal>1</>) to Sunday(<literal>7</>)</entry>
+        <entry>ISO 8601 day of the week, Monday (<literal>1</>) to Sunday (<literal>7</>)</entry>
        </row>
        <row>
         <entry><literal>W</literal></entry>
-        <entry>week of month (1-5) (The first week starts on the first day of the month.)</entry>
-       </row> 
+        <entry>week of month (1-5) (the first week starts on the first day of the month)</entry>
+       </row>
        <row>
         <entry><literal>WW</literal></entry>
-        <entry>week number of year (1-53) (The first week starts on the first day of the year.)</entry>
+        <entry>week number of year (1-53) (the first week starts on the first day of the year)</entry>
        </row>
        <row>
         <entry><literal>IW</literal></entry>
-        <entry>ISO week number of year (01 - 53; the first Thursday of the new year is in week 1.)</entry>
+        <entry>week number of ISO 8601 week-numbering year (01-53; the first Thursday of the year is in week 1)</entry>
        </row>
        <row>
         <entry><literal>CC</literal></entry>
-        <entry>century (2 digits) (The twenty-first century starts on 2001-01-01.)</entry>
+        <entry>century (2 digits) (the twenty-first century starts on 2001-01-01)</entry>
        </row>
        <row>
         <entry><literal>J</literal></entry>
@@ -5293,16 +5293,16 @@ SELECT SUBSTRING('XY1234Z', 'Y*?([0-9]{1,3})');
 
      <listitem>
       <para>
-       An ISO week date (as distinct from a Gregorian date) can be
-       specified to <function>to_timestamp</function> and
+       An ISO 8601 week-numbering date (as distinct from a Gregorian date)
+       can be specified to <function>to_timestamp</function> and
        <function>to_date</function> in one of two ways:
        <itemizedlist>
         <listitem>
          <para>
-          Year, week, and weekday:  for example <literal>to_date('2006-42-4',
-          'IYYY-IW-ID')</literal> returns the date
-          <literal>2006-10-19</literal>.  If you omit the weekday it
-          is assumed to be 1 (Monday).
+          Year, week number, and weekday:  for
+          example <literal>to_date('2006-42-4', 'IYYY-IW-ID')</literal>
+          returns the date <literal>2006-10-19</literal>.
+          If you omit the weekday it is assumed to be 1 (Monday).
          </para>
         </listitem>
         <listitem>
@@ -5314,13 +5314,25 @@ SELECT SUBSTRING('XY1234Z', 'Y*?([0-9]{1,3})');
        </itemizedlist>
       </para>
       <para>
-       Attempting to construct a date using a mixture of ISO week and
-       Gregorian date fields is nonsensical, and will cause an error.  In the
-       context of an ISO year, the concept of a <quote>month</> or <quote>day
-       of month</> has no meaning.  In the context of a Gregorian year, the
-       ISO week has no meaning.  Users should avoid mixing Gregorian and
-       ISO date specifications.
+       Attempting to enter a date using a mixture of ISO 8601 week-numbering
+       fields and Gregorian date fields is nonsensical, and will cause an
+       error.  In the context of an ISO 8601 week-numbering year, the
+       concept of a <quote>month</> or <quote>day of month</> has no
+       meaning.  In the context of a Gregorian year, the ISO week has no
+       meaning.
       </para>
+      <caution>
+       <para>
+        While <function>to_date</function> will reject a mixture of
+        Gregorian and ISO week-numbering date
+        fields, <function>to_char</function> will not, since output format
+        specifications like <literal>YYYY-MM-DD (IYYY-IDDD)</> can be
+        useful.  But avoid writing something like <literal>IYYY-MM-DD</>;
+        that would yield surprising results near the start of the year.
+        (See <xref linkend="functions-datetime-extract"> for more
+        information.)
+       </para>
+      </caution>
      </listitem>
 
      <listitem>
@@ -6324,8 +6336,8 @@ SELECT EXTRACT(DECADE FROM TIMESTAMP '2001-02-16 20:38:40');
       <term><literal>dow</literal></term>
       <listitem>
        <para>
-        The day of the week as Sunday(<literal>0</>) to
-        Saturday(<literal>6</>)
+        The day of the week as Sunday (<literal>0</>) to
+        Saturday (<literal>6</>)
        </para>
 
 <screen>
@@ -6405,8 +6417,8 @@ SELECT EXTRACT(HOUR FROM TIMESTAMP '2001-02-16 20:38:40');
       <term><literal>isodow</literal></term>
       <listitem>
        <para>
-        The day of the week as Monday(<literal>1</>) to
-        Sunday(<literal>7</>)
+        The day of the week as Monday (<literal>1</>) to
+        Sunday (<literal>7</>)
        </para>
 
 <screen>
@@ -6425,7 +6437,8 @@ SELECT EXTRACT(ISODOW FROM TIMESTAMP '2001-02-18 20:38:40');
       <term><literal>isoyear</literal></term>
       <listitem>
        <para>
-        The <acronym>ISO</acronym> 8601 year that the date falls in (not applicable to intervals)
+        The <acronym>ISO</acronym> 8601 week-numbering year that the date
+        falls in (not applicable to intervals)
        </para>
 
 <screen>
@@ -6436,7 +6449,11 @@ SELECT EXTRACT(ISOYEAR FROM DATE '2006-01-02');
 </screen>
 
        <para>
-        Each <acronym>ISO</acronym> year begins with the Monday of the week containing the 4th of January, so in early January or late December the <acronym>ISO</acronym> year may be different from the Gregorian year.  See the <literal>week</literal> field for more information.
+        Each <acronym>ISO</acronym> 8601 week-numbering year begins with the
+        Monday of the week containing the 4th of January, so in early
+        January or late December the <acronym>ISO</acronym> year may be
+        different from the Gregorian year.  See the <literal>week</literal>
+        field for more information.
        </para>
        <para>
         This field is not available in PostgreSQL releases prior to 8.3.
@@ -6600,14 +6617,14 @@ SELECT EXTRACT(SECOND FROM TIME '17:12:28.5');
       <term><literal>week</literal></term>
       <listitem>
        <para>
-        The number of the week of the year that the day is in.  By definition
-        (<acronym>ISO</acronym> 8601), weeks start on Mondays and the first
+        The number of the <acronym>ISO</acronym> 8601 week-numbering week of
+        the year.  By definition, ISO weeks start on Mondays and the first
         week of a year contains January 4 of that year.  In other words, the
         first Thursday of a year is in week 1 of that year.
        </para>
        <para>
-        In the ISO definition, it is possible for early-January dates to be
-        part of the 52nd or 53rd week of the previous year, and for
+        In the ISO week-numbering system, it is possible for early-January
+        dates to be part of the 52nd or 53rd week of the previous year, and for
         late-December dates to be part of the first week of the next year.
         For example, <literal>2005-01-01</> is part of the 53rd week of year
         2004, and <literal>2006-01-01</> is part of the 52nd week of year
