postgresql/doc/src/sgml/ref/create_cast.sgml

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<refentry id="SQL-CREATECAST">
 <refmeta>
  <refentrytitle id="SQL-CREATECAST-TITLE">CREATE CAST</refentrytitle>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

 <refnamediv>
  <refname>CREATE CAST</refname>
  <refpurpose>define a new cast</refpurpose>
 </refnamediv>

 <indexterm zone="sql-createcast">
  <primary>CREATE CAST</primary>
 </indexterm>

 <refsynopsisdiv>
<synopsis>
CREATE CAST (<replaceable>sourcetype</replaceable> AS <replaceable>targettype</replaceable>)
    WITH FUNCTION <replaceable>funcname</replaceable> (<replaceable>argtype</replaceable>)
    [ AS ASSIGNMENT | AS IMPLICIT ]

CREATE CAST (<replaceable>sourcetype</replaceable> AS <replaceable>targettype</replaceable>)
    WITHOUT FUNCTION
    [ AS ASSIGNMENT | AS IMPLICIT ]
</synopsis>
 </refsynopsisdiv>
  
 <refsect1 id="sql-createcast-description">
  <title>Description</title>

  <para>
   <command>CREATE CAST</command> defines a new cast.  A cast
   specifies how to perform a conversion between
   two data types.  For example,
<programlisting>
SELECT CAST(42 AS text);
</programlisting>
   converts the integer constant 42 to type <type>text</type> by
   invoking a previously specified function, in this case
   <literal>text(int4)</>. (If no suitable cast has been defined, the
   conversion fails.)
  </para>

  <para>
   Two types may be <firstterm>binary compatible</firstterm>, which
   means that they can be converted into one another <quote>for
   free</quote> without invoking any function.  This requires that
   corresponding values use the same internal representation.  For
   instance, the types <type>text</type> and <type>varchar</type> are
   binary compatible.
  </para>

  <para>
   By default, a cast can be invoked only by an explicit cast request,
   that is an explicit <literal>CAST(<replaceable>x</> AS
   <replaceable>typename</>)</literal>,
   <replaceable>x</><literal>::</><replaceable>typename</>, or
   <replaceable>typename</>(<replaceable>x</>) construct.
  </para>

  <para>
   If the cast is marked <literal>AS ASSIGNMENT</> then it can be invoked
   implicitly when assigning a value to a column of the target data type.
   For example, supposing that <literal>foo.f1</literal> is a column of
   type <type>text</type>, then
<programlisting>
INSERT INTO foo (f1) VALUES (42);
</programlisting>
   will be allowed if the cast from type <type>integer</type> to type
   <type>text</type> is marked <literal>AS ASSIGNMENT</>, otherwise
   not.
   (We generally use the term <firstterm>assignment
   cast</firstterm> to describe this kind of cast.)
  </para>

  <para>
   If the cast is marked <literal>AS IMPLICIT</> then it can be invoked
   implicitly in any context, whether assignment or internally in an
   expression.  For example, since <literal>||</> takes <type>text</>
   operands,
<programlisting>
SELECT 'The time is ' || now();
</programlisting>
   will be allowed only if the cast from type <type>timestamp</> to
   <type>text</type> is marked <literal>AS IMPLICIT</>.  Otherwise it
   will be necessary to write the cast explicitly, for example
<programlisting>
SELECT 'The time is ' || CAST(now() AS text);
</programlisting>
   (We generally use the term <firstterm>implicit
   cast</firstterm> to describe this kind of cast.)
  </para>

  <para>
   It is wise to be conservative about marking casts as implicit.  An
   overabundance of implicit casting paths can cause
   <productname>PostgreSQL</productname> to choose surprising
   interpretations of commands, or to be unable to resolve commands at
   all because there are multiple possible interpretations.  A good
   rule of thumb is to make a cast implicitly invokable only for
   information-preserving transformations between types in the same
   general type category.  For example, the cast from <type>int2</type> to
   <type>int4</type> can reasonably be implicit, but the cast from
   <type>float8</type> to <type>int4</type> should probably be
   assignment-only.  Cross-type-category casts, such as <type>text</>
   to <type>int4</>, are best made explicit-only.
  </para>

  <para>
   To be able to create a cast, you must own the source or the target
   data type.  To create a binary-compatible cast, you must be superuser.
   (This restriction is made because an erroneous binary-compatible cast
   conversion can easily crash the server.)
  </para>
 </refsect1>

 <refsect1>
  <title>Parameters</title>

   <variablelist>
    <varlistentry>
     <term><replaceable>sourcetype</replaceable></term>

     <listitem>
      <para>
       The name of the source data type of the cast.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><replaceable>targettype</replaceable></term>

     <listitem>
      <para>
       The name of the target data type of the cast.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><replaceable>funcname</replaceable>(<replaceable>argtype</replaceable>)</term>

     <listitem>
      <para>
       The function used to perform the cast.  The function name may
       be schema-qualified.  If it is not, the function will be looked
       up in the schema search path.  The argument type must be
       identical to the source type and the result data type must
       match the target type of the cast.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>WITHOUT FUNCTION</literal></term>

     <listitem>
      <para>
       Indicates that the source type and the target type are binary
       compatible, so no function is required to perform the cast.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>AS ASSIGNMENT</literal></term>

     <listitem>
      <para>
       Indicates that the cast may be invoked implicitly in assignment
       contexts.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>AS IMPLICIT</literal></term>

     <listitem>
      <para>
       Indicates that the cast may be invoked implicitly in any context.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>

 </refsect1>

 <refsect1 id="sql-createcast-notes">
  <title>Notes</title>

  <para>
   Use <command>DROP CAST</command> to remove user-defined casts.
  </para>

  <para>
   Remember that if you want to be able to convert types both ways you
   need to declare casts both ways explicitly.
  </para>

  <para>
   Prior to <productname>PostgreSQL</> 7.3, every function that had
   the same name as a data type, returned that data type, and took one
   argument of a different type was automatically a cast function.
   This convention has been abandoned in face of the introduction of
   schemas and to be able to represent binary compatible casts in the
   system catalogs.  (The built-in cast functions still follow this naming
   scheme, but they have to be shown as casts in the system catalog <literal>pg_cast</>
   now.)
  </para>
 </refsect1>


 <refsect1 id="sql-createcast-examples">
  <title>Examples</title>

  <para>
   To create a cast from type <type>text</type> to type
   <type>int4</type> using the function <literal>int4(text)</literal>:
<programlisting>
CREATE CAST (text AS int4) WITH FUNCTION int4(text);
</programlisting>
   (This cast is already predefined in the system.)
  </para>
 </refsect1>

 
 <refsect1 id="sql-createcast-compat">
  <title>Compatibility</title>

  <para>
   The <command>CREATE CAST</command> command conforms to SQL99,
   except that SQL99 does not make provisions for binary-compatible
   types.  <literal>AS IMPLICIT</> is a <productname>PostgreSQL</productname>
   extension, too.
  </para>
 </refsect1>


 <refsect1 id="sql-createcast-seealso">
  <title>See Also</title>

  <para>
   <xref linkend="sql-createfunction" endterm="sql-createfunction-title">,
   <xref linkend="sql-createtype" endterm="sql-createtype-title">,
   <xref linkend="sql-dropcast" endterm="sql-dropcast-title">
  </para>
 </refsect1>

</refentry>

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