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<chapter id="datatype">

<title id="datatype-title">Data Types</title>

<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>

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

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>

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:

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