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<chapter Id="typeconv">

</para>

</example>

<example>

<title>Absolute-Value and Factorial Operator Type Resolution</title>

<para>

The <productname>PostgreSQL</productname> operator catalog has several

entries for the prefix operator <literal>@</>, all of which implement

absolute-value operations for various numeric data types. One of these

entries is for type <type>float8</type>, which is the preferred type in

the numeric category. Therefore, <productname>PostgreSQL</productname>

will use that entry when faced with a non-numeric input:

<screen>

SELECT @ '-4.5' AS "abs";

abs

-----

4.5

(1 row)

</screen>

Here the system has performed an implicit conversion from <type>text</type> to <type>float8</type>

before applying the chosen operator. We can verify that <type>float8</type> and

not some other type was used:

<screen>

SELECT @ '-4.5e500' AS "abs";

ERROR: Input '-4.5e500' is out of range for float8

</screen>

</para>

<para>

On the other hand, the postfix operator <literal>!</> (factorial)

is defined only for integer data types, not for <type>float8</type>. So, if we

try a similar case with <literal>!</>, we get:

<screen>

SELECT '20' ! AS "factorial";

ERROR: Unable to identify a postfix operator '!' for type 'text'

You may need to add parentheses or an explicit cast

</screen>

This happens because the system can't decide which of the several

possible <literal>!</> operators should be preferred. We can help

it out with an explicit cast:

<screen>

SELECT CAST('20' AS int8) ! AS "factorial";

factorial

---------------------

2432902008176640000

(1 row)

</screen>

</para>

</example>

</sect1>

<sect1 id="typeconv-func">