database/postgres
1
0
Fork 0
mirror of https://github.com/postgres/postgres.git synced 2025-07-28 23:42:10 +03:00
Code Activity

Allow I/O conversion casts to be applied to or from any type that is a member

Browse Source 
of the STRING type category, thereby opening up the mechanism for user-defined
types.  This is mainly for the benefit of citext, though; there aren't likely
to be a lot of types that are all general-purpose character strings.
Per discussion with David Wheeler.
This commit is contained in:
Tom Lane
2008-07-30 21:23:17 +00:00
parent 7df49cef72
commit c8572986ad
4 changed files with 43 additions and 32 deletions
Download Patch File Download Diff File Expand all files Collapse all files

32
doc/src/sgml/ref/create_cast.sgml
View File

@ -1,4 +1,4 @@
<!-- $PostgreSQL: pgsql/doc/src/sgml/ref/create_cast.sgml,v 1.28 2008/07/12 16:20:06 tgl Exp $ -->
<!-- $PostgreSQL: pgsql/doc/src/sgml/ref/create_cast.sgml,v 1.29 2008/07/30 21:23:17 tgl Exp $ -->
<refentry id="SQL-CREATECAST">
<refmeta>
@ -26,7 +26,7 @@ CREATE CAST (<replaceable>sourcetype</replaceable> AS <replaceable>targettype</r
[ AS ASSIGNMENT | AS IMPLICIT ]
</synopsis>
</refsynopsisdiv>
<refsect1 id="sql-createcast-description">
<title>Description</title>
@ -131,6 +131,18 @@ SELECT CAST ( 2 AS numeric ) + 4.0;
to <type>int4</>, are best made explicit-only.
</para>
<note>
<para>
Sometimes it is necessary for usability or standards-compliance reasons
to provide multiple implicit casts among a set of types, resulting in
ambiguity that cannot be avoided as above. The parser has a fallback
heuristic based on <firstterm>type categories</> and <firstterm>preferred
types</> that can help to provide desired behavior in such cases. See
<xref linkend="sql-createtype" endterm="sql-createtype-title"> for
more information.
</para>
</note>
<para>
To be able to create a cast, you must own the source or the target
data type. To create a binary-coercible cast, you must be superuser.
@ -181,8 +193,8 @@ SELECT CAST ( 2 AS numeric ) + 4.0;
<listitem>
<para>
Indicates that the source type and the target type are binary
coercible, so no function is required to perform the cast.
Indicates that the source type is binary-coercible to the target type,
so no function is required to perform the cast.
</para>
</listitem>
</varlistentry>
@ -218,7 +230,7 @@ SELECT CAST ( 2 AS numeric ) + 4.0;
if there is none. The third argument,
if present, must be type <type>boolean</>; it receives <literal>true</>
if the cast is an explicit cast, <literal>false</> otherwise.
(Bizarrely, the SQL spec demands different behaviors for explicit and
(Bizarrely, the SQL standard demands different behaviors for explicit and
implicit casts in some cases. This argument is supplied for functions
that must implement such casts. It is not recommended that you design
your own data types so that this matters.)
@ -271,7 +283,8 @@ SELECT CAST ( 2 AS numeric ) + 4.0;
<para>
It is normally not necessary to create casts between user-defined types
and the standard string types (<type>text</>, <type>varchar</>, and
<type>char(<replaceable>n</>)</type>). <productname>PostgreSQL</> will
<type>char(<replaceable>n</>)</type>, as well as user-defined types that
are defined to be in the string category). <productname>PostgreSQL</> will
automatically handle a cast to a string type by invoking the other
type's output function, or conversely handle a cast from a string type
by invoking the other type's input function. These
@ -340,16 +353,15 @@ SELECT CAST ( 2 AS numeric ) + 4.0;
<title>Examples</title>
<para>
To create a cast from type <type>bigint</type> to type
To create an assignment cast from type <type>bigint</type> to type
<type>int4</type> using the function <literal>int4(bigint)</literal>:
<programlisting>
CREATE CAST (bigint AS int4) WITH FUNCTION int4(bigint);
CREATE CAST (bigint AS int4) WITH FUNCTION int4(bigint) AS ASSIGNMENT;
</programlisting>
(This cast is already predefined in the system.)
</para>
</refsect1>
<refsect1 id="sql-createcast-compat">
<title>Compatibility</title>
@ -358,7 +370,7 @@ CREATE CAST (bigint AS int4) WITH FUNCTION int4(bigint);
<acronym>SQL</acronym> standard,
except that SQL does not make provisions for binary-coercible
types or extra arguments to implementation functions.
<literal>AS IMPLICIT</> is a <productname>PostgreSQL</productname>
<literal>AS IMPLICIT</> is a <productname>PostgreSQL</productname>
extension, too.
</para>
</refsect1>