diff options
Diffstat (limited to 'doc/src')
| -rw-r--r-- | doc/src/sgml/datatype.sgml | 75 |
1 files changed, 59 insertions, 16 deletions
diff --git a/doc/src/sgml/datatype.sgml b/doc/src/sgml/datatype.sgml index 7027758d28d..50e370cae44 100644 --- a/doc/src/sgml/datatype.sgml +++ b/doc/src/sgml/datatype.sgml @@ -554,9 +554,9 @@ NUMERIC(<replaceable>precision</replaceable>) <programlisting> NUMERIC </programlisting> - without any precision or scale creates a column in which numeric - values of any precision and scale can be stored, up to the - implementation limit on precision. A column of this kind will + without any precision or scale creates an <quote>unconstrained + numeric</quote> column in which numeric values of any length can be + stored, up to the implementation limits. A column of this kind will not coerce input values to any particular scale, whereas <type>numeric</type> columns with a declared scale will coerce input values to that scale. (The <acronym>SQL</acronym> standard @@ -568,10 +568,10 @@ NUMERIC <note> <para> - The maximum allowed precision when explicitly specified in the - type declaration is 1000; <type>NUMERIC</type> without a specified - precision is subject to the limits described in <xref - linkend="datatype-numeric-table"/>. + The maximum precision that can be explicitly specified in + a <type>NUMERIC</type> type declaration is 1000. An + unconstrained <type>NUMERIC</type> column is subject to the limits + described in <xref linkend="datatype-numeric-table"/>. </para> </note> @@ -594,6 +594,11 @@ NUMERIC </para> <indexterm> + <primary>infinity</primary> + <secondary>numeric (data type)</secondary> + </indexterm> + + <indexterm> <primary>NaN</primary> <see>not a number</see> </indexterm> @@ -604,13 +609,44 @@ NUMERIC </indexterm> <para> - In addition to ordinary numeric values, the <type>numeric</type> - type allows the special value <literal>NaN</literal>, meaning - <quote>not-a-number</quote>. Any operation on <literal>NaN</literal> - yields another <literal>NaN</literal>. When writing this value - as a constant in an SQL command, you must put quotes around it, - for example <literal>UPDATE table SET x = 'NaN'</literal>. On input, - the string <literal>NaN</literal> is recognized in a case-insensitive manner. + In addition to ordinary numeric values, the <type>numeric</type> type + has several special values: +<literallayout> +<literal>Infinity</literal> +<literal>-Infinity</literal> +<literal>NaN</literal> +</literallayout> + These are adapted from the IEEE 754 standard, and represent + <quote>infinity</quote>, <quote>negative infinity</quote>, and + <quote>not-a-number</quote>, respectively. When writing these values + as constants in an SQL command, you must put quotes around them, + for example <literal>UPDATE table SET x = '-Infinity'</literal>. + On input, these strings are recognized in a case-insensitive manner. + The infinity values can alternatively be spelled <literal>inf</literal> + and <literal>-inf</literal>. + </para> + + <para> + The infinity values behave as per mathematical expectations. For + example, <literal>Infinity</literal> plus any finite value equals + <literal>Infinity</literal>, as does <literal>Infinity</literal> + plus <literal>Infinity</literal>; but <literal>Infinity</literal> + minus <literal>Infinity</literal> yields <literal>NaN</literal> (not a + number), because it has no well-defined interpretation. Note that an + infinity can only be stored in an unconstrained <type>numeric</type> + column, because it notionally exceeds any finite precision limit. + </para> + + <para> + The <literal>NaN</literal> (not a number) value is used to represent + undefined calculational results. In general, any operation with + a <literal>NaN</literal> input yields another <literal>NaN</literal>. + The only exception is when the operation's other inputs are such that + the same output would be obtained if the <literal>NaN</literal> were to + be replaced by any finite or infinite numeric value; then, that output + value is used for <literal>NaN</literal> too. (An example of this + principle is that <literal>NaN</literal> raised to the zero power + yields one.) </para> <note> @@ -782,8 +818,13 @@ FROM generate_series(-3.5, 3.5, 1) as x; </note> <indexterm> + <primary>infinity</primary> + <secondary>floating point</secondary> + </indexterm> + + <indexterm> <primary>not a number</primary> - <secondary>double precision</secondary> + <secondary>floating point</secondary> </indexterm> <para> @@ -800,11 +841,13 @@ FROM generate_series(-3.5, 3.5, 1) as x; as constants in an SQL command, you must put quotes around them, for example <literal>UPDATE table SET x = '-Infinity'</literal>. On input, these strings are recognized in a case-insensitive manner. + The infinity values can alternatively be spelled <literal>inf</literal> + and <literal>-inf</literal>. </para> <note> <para> - IEEE754 specifies that <literal>NaN</literal> should not compare equal + IEEE 754 specifies that <literal>NaN</literal> should not compare equal to any other floating-point value (including <literal>NaN</literal>). In order to allow floating-point values to be sorted and used in tree-based indexes, <productname>PostgreSQL</productname> treats |