doc/src/sgml/ref/create_cast.sgml

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doc/src/sgml/ref/create_cast.sgml
PostgreSQL documentation
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<refentry id="sql-createcast">
 <indexterm zone="sql-createcast">
  <primary>CREATE CAST</primary>
 </indexterm>

 <refmeta>
  <refentrytitle>CREATE CAST</refentrytitle>
  <manvolnum>7</manvolnum>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

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

 <refsynopsisdiv>
<synopsis>
CREATE CAST (<replaceable>source_type</replaceable> AS <replaceable>target_type</replaceable>)
    WITH FUNCTION <replaceable>function_name</replaceable> [ (<replaceable>argument_type</replaceable> [, ...]) ]
    [ AS ASSIGNMENT | AS IMPLICIT ]

CREATE CAST (<replaceable>source_type</replaceable> AS <replaceable>target_type</replaceable>)
    WITHOUT FUNCTION
    [ AS ASSIGNMENT | AS IMPLICIT ]

CREATE CAST (<replaceable>source_type</replaceable> AS <replaceable>target_type</replaceable>)
    WITH INOUT
    [ 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 float8);
</programlisting>
   converts the integer constant 42 to type <type>float8</type> by
   invoking a previously specified function, in this case
   <literal>float8(int4)</literal>. (If no suitable cast has been defined, the
   conversion fails.)
  </para>

  <para>
   Two types can be <firstterm>binary coercible</firstterm>, which
   means that the conversion can be performed <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
   coercible both ways.  Binary coercibility is not necessarily a
   symmetric relationship.  For example, the cast
   from <type>xml</type> to <type>text</type> can be performed for
   free in the present implementation, but the reverse direction
   requires a function that performs at least a syntax check.  (Two
   types that are binary coercible both ways are also referred to as
   binary compatible.)
  </para>

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

  <para>
   If the cast is marked <literal>AS ASSIGNMENT</literal> 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</literal>, 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</literal> then it can be invoked
   implicitly in any context, whether assignment or internally in an
   expression.  (We generally use the term <firstterm>implicit
   cast</firstterm> to describe this kind of cast.)
   For example, consider this query:
<programlisting>
SELECT 2 + 4.0;
</programlisting>
   The parser initially marks the constants as being of type <type>integer</type>
   and <type>numeric</type> respectively.  There is no <type>integer</type>
   <literal>+</literal> <type>numeric</type> operator in the system catalogs,
   but there is a <type>numeric</type> <literal>+</literal> <type>numeric</type> operator.
   The query will therefore succeed if a cast from <type>integer</type> to
   <type>numeric</type> is available and is marked <literal>AS IMPLICIT</literal> &mdash;
   which in fact it is.  The parser will apply the implicit cast and resolve
   the query as if it had been written
<programlisting>
SELECT CAST ( 2 AS numeric ) + 4.0;
</programlisting>
  </para>

  <para>
   Now, the catalogs also provide a cast from <type>numeric</type> to
   <type>integer</type>.  If that cast were marked <literal>AS IMPLICIT</literal> &mdash;
   which it is not &mdash; then the parser would be faced with choosing
   between the above interpretation and the alternative of casting the
   <type>numeric</type> constant to <type>integer</type> and applying the
   <type>integer</type> <literal>+</literal> <type>integer</type> operator.  Lacking any
   knowledge of which choice to prefer, it would give up and declare the
   query ambiguous.  The fact that only one of the two casts is
   implicit is the way in which we teach the parser to prefer resolution
   of a mixed <type>numeric</type>-and-<type>integer</type> expression as
   <type>numeric</type>; there is no built-in knowledge about that.
  </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</type>
   to <type>int4</type>, 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</firstterm> and <firstterm>preferred
    types</firstterm> that can help to provide desired behavior in such cases.  See
    <xref linkend="sql-createtype"/> for
    more information.
   </para>
  </note>

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

 <refsect1>
  <title>Parameters</title>

   <variablelist>
    <varlistentry>
     <term><replaceable>source_type</replaceable></term>

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

    <varlistentry>
     <term><replaceable>target_type</replaceable></term>

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

    <varlistentry>
     <term><literal><replaceable>function_name</replaceable>[(<replaceable>argument_type</replaceable> [, ...])]</literal></term>

     <listitem>
      <para>
       The function used to perform the cast.  The function name can
       be schema-qualified.  If it is not, the function will be looked
       up in the schema search path.  The function's result data type must
       match the target type of the cast.   Its arguments are discussed below.
       If no argument list is specified, the function name must be unique in
       its schema.
      </para>
     </listitem>
    </varlistentry>

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

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

    <varlistentry>
     <term><literal>WITH INOUT</literal></term>

     <listitem>
      <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
       resulting string to the input function of the target data type.
      </para>
     </listitem>
    </varlistentry>

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

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

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

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

  <para>
   Cast implementation functions can have one to three arguments.
   The first argument type must be identical to or binary-coercible from
   the cast's source type.  The second argument,
   if present, must be type <type>integer</type>; it receives the type
   modifier associated with the destination type, or <literal>-1</literal>
   if there is none.  The third argument,
   if present, must be type <type>boolean</type>; it receives <literal>true</literal>
   if the cast is an explicit cast, <literal>false</literal> otherwise.
   (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.)
  </para>

  <para>
   The return type of a cast function must be identical to or
   binary-coercible to the cast's target type.
  </para>

  <para>
   Ordinarily a cast must have different source and target data types.
   However, it is allowed to declare a cast with identical source and
   target types if it has a cast implementation function with more than one
   argument.  This is used to represent type-specific length coercion
   functions in the system catalogs.  The named function is used to
   coerce a value of the type to the type modifier value given by its
   second argument.
  </para>

  <para>
   When a cast has different source and
   target types and a function that takes more than one argument, it
   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 cast steps, one to
   convert between data types and a second to apply the modifier.
  </para>

  <para>
   A cast to or from a domain type currently has no effect.  Casting
   to or from a domain uses the casts associated with its underlying type.
  </para>

 </refsect1>

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

  <para>
   Use <link linkend="sql-dropcast"><command>DROP CAST</command></link> 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>

 <indexterm zone="sql-createcast">
  <primary>cast</primary>
  <secondary>I/O conversion</secondary>
 </indexterm>

  <para>
   It is normally not necessary to create casts between user-defined types
   and the standard string types (<type>text</type>, <type>varchar</type>, and
   <type>char(<replaceable>n</replaceable>)</type>, as well as user-defined types that
   are defined to be in the string category).  <productname>PostgreSQL</productname>
   provides automatic I/O conversion casts for that. The automatic casts to
   string types are treated as assignment casts, while the automatic casts
   from string types are
   explicit-only.  You can override this behavior by declaring your own
   cast to replace an automatic cast, but usually the only reason to
   do so is if you want the conversion to be more easily invokable than the
   standard assignment-only or explicit-only setting.  Another possible
   reason is that you want the conversion to behave differently from the
   type's I/O function; but that is sufficiently surprising that you
   should think twice about whether it's a good idea.  (A small number of
   the built-in types do indeed have different behaviors for conversions,
   mostly because of requirements of the SQL standard.)
  </para>

  <para>
   While not required, it is recommended that you continue to follow this old
   convention of naming cast implementation functions after the target data
   type.  Many users are used to being able to cast data types using a
   function-style notation, that is
   <replaceable>typename</replaceable>(<replaceable>x</replaceable>).  This notation is in fact
   nothing more nor less than a call of the cast implementation function; it
   is not specially treated as a cast.  If your conversion functions are not
   named to support this convention then you will have surprised users.
   Since <productname>PostgreSQL</productname> allows overloading of the same function
   name with different argument types, there is no difficulty in having
   multiple conversion functions from different types that all use the
   target type's name.
  </para>

  <note>
   <para>
    Actually the preceding paragraph is an oversimplification: there are
    two cases in which a function-call construct will be treated as a cast
    request without having matched it to an actual function.
    If a function call <replaceable>name</replaceable>(<replaceable>x</replaceable>) does not
    exactly match any existing function, but <replaceable>name</replaceable> is the name
    of a data type and <structname>pg_cast</structname> provides a binary-coercible cast
    to this type from the type of <replaceable>x</replaceable>, then the call will be
    construed as a binary-coercible cast.  This exception is made so that
    binary-coercible casts can be invoked using functional syntax, even
    though they lack any function.  Likewise, if there is no
    <structname>pg_cast</structname> entry but the cast would be to or from a string
    type, the call will be construed as an I/O conversion cast.  This
    exception allows I/O conversion casts to be invoked using functional
    syntax.
   </para>
  </note>

  <note>
   <para>
    There is also an exception to the exception: 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</literal> or <literal>::</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>


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

  <para>
   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) AS ASSIGNMENT;
</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 the
   <acronym>SQL</acronym> standard,
   except that SQL does not make provisions for binary-coercible
   types or extra arguments to implementation functions.
   <literal>AS IMPLICIT</literal> is a <productname>PostgreSQL</productname>
   extension, too.
  </para>
 </refsect1>


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

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

</refentry>