create_cast.sgml

来自「postgresql8.3.4源码,开源数据库」· SGML 代码 · 共 367 行

<!-- $PostgreSQL: pgsql/doc/src/sgml/ref/create_cast.sgml,v 1.26 2007/06/05 21:31:04 tgl Exp $ -->

<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>argtypes</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 float8);
</programlisting>
   converts the integer constant 42 to type <type>float8</type> by
   invoking a previously specified function, in this case
   <literal>float8(int4)</>. (If no suitable cast has been defined, the
   conversion fails.)
  </para>

  <para>
   Two types can 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> or
   <replaceable>x</><literal>::</><replaceable>typename</>
   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.  (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</>
   and <type>numeric</> respectively.  There is no <type>integer</>
   <literal>+</> <type>numeric</> operator in the system catalogs,
   but there is a <type>numeric</> <literal>+</> <type>numeric</> operator.
   The query will therefore succeed if a cast from <type>integer</> to
   <type>numeric</> is available and is marked <literal>AS IMPLICIT</> &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</> to
   <type>integer</>.  If that cast were marked <literal>AS IMPLICIT</> &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</> constant to <type>integer</> and applying the
   <type>integer</> <literal>+</> <type>integer</> 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</>-and-<type>integer</> expression as
   <type>numeric</>; 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</>
   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>argtypes</replaceable>)</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.
      </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 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 the cast's source type.
   The second argument,
   if present, must be type <type>integer</>; it receives the type
   modifier associated with the destination type, or <literal>-1</>
   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
   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>
   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
   represents 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
   convert between data types and a second to apply the modifier.
  </para>

 </refsect1>

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

  <para>
   Use <xref linkend="sql-dropcast"
   endterm="sql-dropcast-title"> 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>varchar</>, and
   <type>char(<replaceable>n</>)</type>).  <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
   automatically-provided casts are known as <firstterm>I/O conversion
   casts</>.  I/O conversion casts to string types are treated as
   assignment casts, while I/O conversion casts from string types are
   explicit-only.  You can override this behavior by declaring your own
   cast to replace an I/O conversion 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>
   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
   <structname>pg_cast</> as well.
  </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>x</>).  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</> 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>x</>) does not
    exactly match any existing function, but <replaceable>name</> is the name
    of a data type and <structname>pg_cast</> provides a binary-compatible cast
    to this type from the type of <replaceable>x</>, then the call will be
    construed as a binary-compatible cast.  This exception is made so that
    binary-compatible casts can be invoked using functional syntax, even
    though they lack any function.  Likewise, if there is no
    <structname>pg_cast</> 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>
 </refsect1>


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

  <para>
   To create a 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);
</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-compatible
   types or extra arguments to implementation functions.
   <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>