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diff --git a/doc/src/sgml/html/sql-createcast.html b/doc/src/sgml/html/sql-createcast.html new file mode 100644 index 0000000..9d2938f --- /dev/null +++ b/doc/src/sgml/html/sql-createcast.html @@ -0,0 +1,256 @@ +<?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 15.4 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 /></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 15.4 Documentation">Home</a></td><td width="40%" align="right" valign="top"> CREATE COLLATION</td></tr></table></div></body></html>
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