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Re-add type conversion examples.

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Bruce Momjian 2003-03-20 16:17:32 +00:00
parent 2e6f97560a
commit da1534d952
1 changed files with 52 additions and 1 deletions
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53
doc/src/sgml/typeconv.sgml
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@ -1,5 +1,5 @@
<!-- <!--
$Header: /cvsroot/pgsql/doc/src/sgml/typeconv.sgml,v 1.28 2003/03/19 21:20:52 momjian Exp $ $Header: /cvsroot/pgsql/doc/src/sgml/typeconv.sgml,v 1.29 2003/03/20 16:17:32 momjian Exp $
--> -->
<chapter Id="typeconv"> <chapter Id="typeconv">
@ -403,6 +403,57 @@ type to resolve the unknown literals to.
</para> </para>
</example> </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>
<sect1 id="typeconv-func"> <sect1 id="typeconv-func">