diff options
Diffstat (limited to 'doc/src/sgml/ref/create_cast.sgml')
| -rw-r--r-- | doc/src/sgml/ref/create_cast.sgml | 27 |
1 files changed, 21 insertions, 6 deletions
diff --git a/doc/src/sgml/ref/create_cast.sgml b/doc/src/sgml/ref/create_cast.sgml index 5e74efdcacc..d1c540da8aa 100644 --- a/doc/src/sgml/ref/create_cast.sgml +++ b/doc/src/sgml/ref/create_cast.sgml @@ -38,7 +38,7 @@ CREATE CAST (<replaceable>source_type</replaceable> AS <replaceable>target_type< <para> <command>CREATE CAST</command> defines a new cast. A cast specifies how to perform a conversion between - two data types. For example: + two data types. For example, <programlisting> SELECT CAST(42 AS float8); </programlisting> @@ -64,10 +64,13 @@ SELECT CAST(42 AS float8); </para> <para> - You can define a cast as an <firstterm>I/O conversion cast</> using + You can define a cast as an <firstterm>I/O conversion cast</> by using the <literal>WITH INOUT</literal> syntax. An I/O conversion cast is performed by invoking the output function of the source data type, and - passing the result to the input function of the target data type. + passing the resulting string to the input function of the target data type. + In many common cases, this feature avoids the need to write a separate + cast function for conversion. An I/O conversion cast acts the same as + a regular function-based cast; only the implementation is different. </para> <para> @@ -218,7 +221,7 @@ SELECT CAST ( 2 AS numeric ) + 4.0; <para> Indicates that the cast is an I/O conversion cast, performed by invoking the output function of the source data type, and passing the - result to the input function of the target data type. + resulting string to the input function of the target data type. </para> </listitem> </varlistentry> @@ -278,9 +281,9 @@ SELECT CAST ( 2 AS numeric ) + 4.0; <para> When a cast has different source and target types and a function that takes more than one argument, it - represents converting from one type to another and applying a length + supports converting from one type to another and applying a length coercion in a single step. When no such entry is available, coercion - to a type that uses a type modifier involves two steps, one to + to a type that uses a type modifier involves two cast steps, one to convert between data types and a second to apply the modifier. </para> @@ -366,6 +369,18 @@ SELECT CAST ( 2 AS numeric ) + 4.0; syntax. </para> </note> + + <note> + <para> + There's an exception to the exception, too: I/O conversion casts from + composite types to string types cannot be invoked using functional + syntax, but must be written in explicit cast syntax (either + <literal>CAST</> or <literal>::</> notation). This exception was added + because after the introduction of automatically-provided I/O conversion + casts, it was found too easy to accidentally invoke such a cast when + a function or column reference was intended. + </para> + </note> </refsect1> |