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| -rw-r--r-- | doc/src/sgml/typeconv.sgml | 53 |
1 files changed, 52 insertions, 1 deletions
diff --git a/doc/src/sgml/typeconv.sgml b/doc/src/sgml/typeconv.sgml index f17e2e5c9a3..3aaffdc05dc 100644 --- a/doc/src/sgml/typeconv.sgml +++ b/doc/src/sgml/typeconv.sgml @@ -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"> @@ -403,6 +403,57 @@ type to resolve the unknown literals to. </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"> |