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<?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 Vsnapshot" /><link rel="prev" href="sql-createaggregate.html" title="CREATE AGGREGATE" /><link rel="next" href="sql-createcollation.html" title="CREATE COLLATION" /></head><body id="docContent" class="container-fluid col-10"><div 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 15.5 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 /></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="link" href="sql-dropcast.html" title="DROP CAST"><code class="command">DROP CAST</code></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" title="CREATE AGGREGATE">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="sql-commands.html" title="SQL Commands">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="sql-createcollation.html" title="CREATE COLLATION">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" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="40%" align="right" valign="top"> CREATE COLLATION</td></tr></table></div></body></html>
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