From 7021d6f6c831132ee351c424df039f054d44de7d Mon Sep 17 00:00:00 2001
From: Neil Conway <neilc@samurai.com>
Date: Sun, 14 Jan 2007 22:37:59 +0000
Subject: [PATCH] Add a note to the docs describing NaN's equality and ordering
behavior. Per recent -hackers thread, this is noteworthy because Postgres
behaves differently from most implementations of NaN, including IEEE754.
---
doc/src/sgml/datatype.sgml | 46 ++++++++++++++++++++++++++++++++++++--
1 file changed, 44 insertions(+), 2 deletions(-)
diff --git a/doc/src/sgml/datatype.sgml b/doc/src/sgml/datatype.sgml
index 34af82629e..269998a3dd 100644
--- a/doc/src/sgml/datatype.sgml
+++ b/doc/src/sgml/datatype.sgml
@@ -1,4 +1,4 @@
-<!-- $PostgreSQL: pgsql/doc/src/sgml/datatype.sgml,v 1.183 2006/12/22 22:09:31 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/datatype.sgml,v 1.184 2007/01/14 22:37:59 neilc Exp $ -->
<chapter id="datatype">
<title id="datatype-title">Data Types</title>
@@ -438,10 +438,14 @@
<sect2 id="datatype-numeric-decimal">
<title>Arbitrary Precision Numbers</title>
- <indexterm zone="datatype-numeric-decimal">
+ <indexterm>
<primary>numeric (data type)</primary>
</indexterm>
+ <indexterm>
+ <primary>arbitrary precision numbers</primary>
+ </indexterm>
+
<indexterm>
<primary>decimal</primary>
<see>numeric</see>
@@ -515,6 +519,16 @@ NUMERIC
plus eight bytes overhead.
</para>
+ <indexterm>
+ <primary>NaN</primary>
+ <see>not a number</see>
+ </indexterm>
+
+ <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
@@ -525,6 +539,18 @@ NUMERIC
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>
@@ -613,6 +639,11 @@ NUMERIC
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:
@@ -631,6 +662,17 @@ NUMERIC
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
--
2.50.0