Baoding-FenShuaJiang
/
Green_GoodERP18_FSJ_Standard


			
				
					
						
						
							
							<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>CREATE CAST</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets V1.79.1" /><link rel="prev" href="sql-createaggregate.html" title="CREATE AGGREGATE" /><link rel="next" href="sql-createcollation.html" title="CREATE COLLATION" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">CREATE CAST</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="sql-createaggregate.html" title="CREATE AGGREGATE">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="sql-commands.html" title="SQL Commands">Up</a></td><th width="60%" align="center">SQL Commands</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 12.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="sql-createcollation.html" title="CREATE COLLATION">Next</a></td></tr></table><hr></hr></div><div class="refentry" id="SQL-CREATECAST"><div class="titlepage"></div><a id="id-1.9.3.58.1" class="indexterm"></a><div class="refnamediv"><h2><span class="refentrytitle">CREATE CAST</span></h2><p>CREATE CAST — define a new cast</p></div><div class="refsynopsisdiv"><h2>Synopsis</h2><pre class="synopsis">
CREATE CAST (<em class="replaceable"><code>source_type</code></em> AS <em class="replaceable"><code>target_type</code></em>)
    WITH FUNCTION <em class="replaceable"><code>function_name</code></em> [ (<em class="replaceable"><code>argument_type</code></em> [, ...]) ]
    [ AS ASSIGNMENT | AS IMPLICIT ]

CREATE CAST (<em class="replaceable"><code>source_type</code></em> AS <em class="replaceable"><code>target_type</code></em>)
    WITHOUT FUNCTION
    [ AS ASSIGNMENT | AS IMPLICIT ]

CREATE CAST (<em class="replaceable"><code>source_type</code></em> AS <em class="replaceable"><code>target_type</code></em>)
    WITH INOUT
    [ AS ASSIGNMENT | AS IMPLICIT ]
</pre></div><div class="refsect1" id="SQL-CREATECAST-DESCRIPTION"><h2>Description</h2><p>
   <code class="command">CREATE CAST</code> defines a new cast.  A cast
   specifies how to perform a conversion between
   two data types.  For example,
</p><pre class="programlisting">
SELECT CAST(42 AS float8);
</pre><p>
   converts the integer constant 42 to type <code class="type">float8</code> by
   invoking a previously specified function, in this case
   <code class="literal">float8(int4)</code>. (If no suitable cast has been defined, the
   conversion fails.)
  </p><p>
   Two types can be <em class="firstterm">binary coercible</em>, which
   means that the conversion can be performed <span class="quote">“<span class="quote">for free</span>”</span>
   without invoking any function.  This requires that corresponding
   values use the same internal representation.  For instance, the
   types <code class="type">text</code> and <code class="type">varchar</code> are binary
   coercible both ways.  Binary coercibility is not necessarily a
   symmetric relationship.  For example, the cast
   from <code class="type">xml</code> to <code class="type">text</code> 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.)
  </p><p>
   You can define a cast as an <em class="firstterm">I/O conversion cast</em> by using
   the <code class="literal">WITH INOUT</code> 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.
  </p><p>
   By default, a cast can be invoked only by an explicit cast request,
   that is an explicit <code class="literal">CAST(<em class="replaceable"><code>x</code></em> AS
   <em class="replaceable"><code>typename</code></em>)</code> or
   <em class="replaceable"><code>x</code></em><code class="literal">::</code><em class="replaceable"><code>typename</code></em>
   construct.
  </p><p>
   If the cast is marked <code class="literal">AS ASSIGNMENT</code> then it can be invoked
   implicitly when assigning a value to a column of the target data type.
   For example, supposing that <code class="literal">foo.f1</code> is a column of
   type <code class="type">text</code>, then:
</p><pre class="programlisting">
INSERT INTO foo (f1) VALUES (42);
</pre><p>
   will be allowed if the cast from type <code class="type">integer</code> to type
   <code class="type">text</code> is marked <code class="literal">AS ASSIGNMENT</code>, otherwise not.
   (We generally use the term <em class="firstterm">assignment
   cast</em> to describe this kind of cast.)
  </p><p>
   If the cast is marked <code class="literal">AS IMPLICIT</code> then it can be invoked
   implicitly in any context, whether assignment or internally in an
   expression.  (We generally use the term <em class="firstterm">implicit
   cast</em> to describe this kind of cast.)
   For example, consider this query:
</p><pre class="programlisting">
SELECT 2 + 4.0;
</pre><p>
   The parser initially marks the constants as being of type <code class="type">integer</code>
   and <code class="type">numeric</code> respectively.  There is no <code class="type">integer</code>
   <code class="literal">+</code> <code class="type">numeric</code> operator in the system catalogs,
   but there is a <code class="type">numeric</code> <code class="literal">+</code> <code class="type">numeric</code> operator.
   The query will therefore succeed if a cast from <code class="type">integer</code> to
   <code class="type">numeric</code> is available and is marked <code class="literal">AS IMPLICIT</code> —
   which in fact it is.  The parser will apply the implicit cast and resolve
   the query as if it had been written
</p><pre class="programlisting">
SELECT CAST ( 2 AS numeric ) + 4.0;
</pre><p>
  </p><p>
   Now, the catalogs also provide a cast from <code class="type">numeric</code> to
   <code class="type">integer</code>.  If that cast were marked <code class="literal">AS IMPLICIT</code> —
   which it is not — then the parser would be faced with choosing
   between the above interpretation and the alternative of casting the
   <code class="type">numeric</code> constant to <code class="type">integer</code> and applying the
   <code class="type">integer</code> <code class="literal">+</code> <code class="type">integer</code> 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 <code class="type">numeric</code>-and-<code class="type">integer</code> expression as
   <code class="type">numeric</code>; there is no built-in knowledge about that.
  </p><p>
   It is wise to be conservative about marking casts as implicit.  An
   overabundance of implicit casting paths can cause
   <span class="productname">PostgreSQL</span> 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 <code class="type">int2</code> to
   <code class="type">int4</code> can reasonably be implicit, but the cast from
   <code class="type">float8</code> to <code class="type">int4</code> should probably be
   assignment-only.  Cross-type-category casts, such as <code class="type">text</code>
   to <code class="type">int4</code>, are best made explicit-only.
  </p><div class="note"><h3 class="title">Note</h3><p>
    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 <em class="firstterm">type categories</em> and <em class="firstterm">preferred
    types</em> that can help to provide desired behavior in such cases.  See
    <a class="xref" href="sql-createtype.html" title="CREATE TYPE"><span class="refentrytitle">CREATE TYPE</span></a> for
    more information.
   </p></div><p>
   To be able to create a cast, you must own the source or the target data type
   and have <code class="literal">USAGE</code> 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.)
  </p></div><div class="refsect1" id="id-1.9.3.58.6"><h2>Parameters</h2><div class="variablelist"><dl class="variablelist"><dt><span class="term"><em class="replaceable"><code>source_type</code></em></span></dt><dd><p>
       The name of the source data type of the cast.
      </p></dd><dt><span class="term"><em class="replaceable"><code>target_type</code></em></span></dt><dd><p>
       The name of the target data type of the cast.
      </p></dd><dt><span class="term"><code class="literal"><em class="replaceable"><code>function_name</code></em>[(<em class="replaceable"><code>argument_type</code></em> [, ...])]</code></span></dt><dd><p>
       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.
      </p></dd><dt><span class="term"><code class="literal">WITHOUT FUNCTION</code></span></dt><dd><p>
       Indicates that the source type is binary-coercible to the target type,
       so no function is required to perform the cast.
      </p></dd><dt><span class="term"><code class="literal">WITH INOUT</code></span></dt><dd><p>
       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.
      </p></dd><dt><span class="term"><code class="literal">AS ASSIGNMENT</code></span></dt><dd><p>
       Indicates that the cast can be invoked implicitly in assignment
       contexts.
      </p></dd><dt><span class="term"><code class="literal">AS IMPLICIT</code></span></dt><dd><p>
       Indicates that the cast can be invoked implicitly in any context.
      </p></dd></dl></div><p>
   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 <code class="type">integer</code>; it receives the type
   modifier associated with the destination type, or <code class="literal">-1</code>
   if there is none.  The third argument,
   if present, must be type <code class="type">boolean</code>; it receives <code class="literal">true</code>
   if the cast is an explicit cast, <code class="literal">false</code> 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.)
  </p><p>
   The return type of a cast function must be identical to or
   binary-coercible to the cast's target type.
  </p><p>
   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.
  </p><p>
   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.
  </p><p>
   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.
  </p></div><div class="refsect1" id="SQL-CREATECAST-NOTES"><h2>Notes</h2><p>
   Use <a class="xref" href="sql-dropcast.html" title="DROP CAST"><span class="refentrytitle">DROP CAST</span></a> to remove user-defined casts.
  </p><p>
   Remember that if you want to be able to convert types both ways you
   need to declare casts both ways explicitly.
  </p><a id="id-1.9.3.58.7.4" class="indexterm"></a><p>
   It is normally not necessary to create casts between user-defined types
   and the standard string types (<code class="type">text</code>, <code class="type">varchar</code>, and
   <code class="type">char(<em class="replaceable"><code>n</code></em>)</code>, as well as user-defined types that
   are defined to be in the string category).  <span class="productname">PostgreSQL</span>
   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.)
  </p><p>
   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
   <em class="replaceable"><code>typename</code></em>(<em class="replaceable"><code>x</code></em>).  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 <span class="productname">PostgreSQL</span> 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.
  </p><div class="note"><h3 class="title">Note</h3><p>
    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 <em class="replaceable"><code>name</code></em>(<em class="replaceable"><code>x</code></em>) does not
    exactly match any existing function, but <em class="replaceable"><code>name</code></em> is the name
    of a data type and <code class="structname">pg_cast</code> provides a binary-coercible cast
    to this type from the type of <em class="replaceable"><code>x</code></em>, 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
    <code class="structname">pg_cast</code> 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.
   </p></div><div class="note"><h3 class="title">Note</h3><p>
    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
    <code class="literal">CAST</code> or <code class="literal">::</code> 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.
   </p></div></div><div class="refsect1" id="SQL-CREATECAST-EXAMPLES"><h2>Examples</h2><p>
   To create an assignment cast from type <code class="type">bigint</code> to type
   <code class="type">int4</code> using the function <code class="literal">int4(bigint)</code>:
</p><pre class="programlisting">
CREATE CAST (bigint AS int4) WITH FUNCTION int4(bigint) AS ASSIGNMENT;
</pre><p>
   (This cast is already predefined in the system.)
  </p></div><div class="refsect1" id="SQL-CREATECAST-COMPAT"><h2>Compatibility</h2><p>
   The <code class="command">CREATE CAST</code> command conforms to the
   <acronym class="acronym">SQL</acronym> standard,
   except that SQL does not make provisions for binary-coercible
   types or extra arguments to implementation functions.
   <code class="literal">AS IMPLICIT</code> is a <span class="productname">PostgreSQL</span>
   extension, too.
  </p></div><div class="refsect1" id="SQL-CREATECAST-SEEALSO"><h2>See Also</h2><p>
   <a class="xref" href="sql-createfunction.html" title="CREATE FUNCTION"><span class="refentrytitle">CREATE FUNCTION</span></a>,
   <a class="xref" href="sql-createtype.html" title="CREATE TYPE"><span class="refentrytitle">CREATE TYPE</span></a>,
   <a class="xref" href="sql-dropcast.html" title="DROP CAST"><span class="refentrytitle">DROP CAST</span></a>
  </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="sql-createaggregate.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="sql-commands.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="sql-createcollation.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">CREATE AGGREGATE </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> CREATE COLLATION</td></tr></table></div></body></html>