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Add a note to the docs describing NaN's equality and ordering behavior.

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Per recent -hackers thread, this is noteworthy because Postgres behaves
differently from most implementations of NaN, including IEEE754.
This commit is contained in:
Neil Conway 2007-01-14 22:37:59 +00:00
parent cb8906b354
commit 7021d6f6c8
1 changed files with 44 additions and 2 deletions
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46
doc/src/sgml/datatype.sgml
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@ -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