On Thu, 2007-01-11 at 21:04 -0500, Neil Conway wrote:
> Comments? I'll write up a doc patch, barring any objections.

I'll apply the attached doc patch to CVS tomorrow, barring any
objections.

-Neil

Index: doc/src/sgml/datatype.sgml
===================================================================
RCS file: /home/neilc/postgres/cvs_root/pgsql/doc/src/sgml/datatype.sgml,v
retrieving revision 1.183
diff -c -p -r1.183 datatype.sgml
*** doc/src/sgml/datatype.sgml	22 Dec 2006 22:09:31 -0000	1.183
--- doc/src/sgml/datatype.sgml	12 Jan 2007 21:52:22 -0000
***************
*** 438,447 ****
     <sect2 id="datatype-numeric-decimal">
      <title>Arbitrary Precision Numbers</title>
  
!     <indexterm zone="datatype-numeric-decimal">
       <primary>numeric (data type)</primary>
      </indexterm>
  
      <indexterm>
       <primary>decimal</primary>
       <see>numeric</see>
--- 438,451 ----
     <sect2 id="datatype-numeric-decimal">
      <title>Arbitrary Precision Numbers</title>
  
!     <indexterm>
       <primary>numeric (data type)</primary>
      </indexterm>
  
+    <indexterm>
+     <primary>arbitrary precision numbers</primary>
+    </indexterm>
+ 
      <indexterm>
       <primary>decimal</primary>
       <see>numeric</see>
*************** NUMERIC
*** 515,520 ****
--- 519,529 ----
       plus eight bytes overhead.
      </para>
  
+     <indexterm>
+      <primary>not a number</primary>
+      <secondary>numeric (data type)</secondary>
+     </indexterm>
+ 
      <para>
       In addition to ordinary numeric values, the <type>numeric</type>
       type allows the special value <literal>NaN</>, meaning 
*************** NUMERIC
*** 525,530 ****
--- 534,551 ----
       the string <literal>NaN</> is recognized in a case-insensitive manner.
      </para>
  
+     <note>
+      <para>
+       In most implementations of the <quote>not-a-number</> concept,
+       <literal>NaN</> is not considered equal to any other numeric
+       value (including <literal>NaN</>).  In order to allow
+       <type>numeric</> values to be sorted and used in tree-based
+       indexes, <productname>PostgreSQL</> treats <literal>NaN</>
+       values as equal, and greater than all non-<literal>NaN</>
+       values.
+      </para>
+     </note>
+ 
      <para>
       The types <type>decimal</type> and <type>numeric</type> are
       equivalent.  Both types are part of the <acronym>SQL</acronym>
*************** NUMERIC
*** 613,618 ****
--- 634,644 ----
       from zero will cause an underflow error.
      </para>
  
+     <indexterm>
+      <primary>not a number</primary>
+      <secondary>double precision</secondary>
+     </indexterm>
+ 
      <para>
       In addition to ordinary numeric values, the floating-point types
       have several special values:
*************** NUMERIC
*** 631,636 ****
--- 657,673 ----
       these strings are recognized in a case-insensitive manner.
      </para>
  
+     <note>
+      <para>
+       IEEE754 specifies that <literal>NaN</> should not compare equal
+       to any other floating-point value (including <literal>NaN</>).
+       In order to allow floating-point values to be sorted and used
+       in tree-based indexes, <productname>PostgreSQL</> treats
+       <literal>NaN</> values as equal, and greater than all
+       non-<literal>NaN</> values.
+      </para>
+     </note>
+ 
      <para>
       <productname>PostgreSQL</productname> also supports the SQL-standard
       notations <type>float</type> and
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