1 <!-- doc/src/sgml/ref/create_cast.sgml -->
3 <refentry id="SQL-CREATECAST">
5 <refentrytitle>CREATE CAST</refentrytitle>
6 <manvolnum>7</manvolnum>
7 <refmiscinfo>SQL - Language Statements</refmiscinfo>
11 <refname>CREATE CAST</refname>
12 <refpurpose>define a new cast</refpurpose>
15 <indexterm zone="sql-createcast">
16 <primary>CREATE CAST</primary>
21 CREATE CAST (<replaceable>source_type</replaceable> AS <replaceable>target_type</replaceable>)
22 WITH FUNCTION <replaceable>function_name</replaceable> (<replaceable>argument_type</replaceable> [, ...])
23 [ AS ASSIGNMENT | AS IMPLICIT ]
25 CREATE CAST (<replaceable>source_type</replaceable> AS <replaceable>target_type</replaceable>)
27 [ AS ASSIGNMENT | AS IMPLICIT ]
29 CREATE CAST (<replaceable>source_type</replaceable> AS <replaceable>target_type</replaceable>)
31 [ AS ASSIGNMENT | AS IMPLICIT ]
35 <refsect1 id="sql-createcast-description">
36 <title>Description</title>
39 <command>CREATE CAST</command> defines a new cast. A cast
40 specifies how to perform a conversion between
41 two data types. For example,
43 SELECT CAST(42 AS float8);
45 converts the integer constant 42 to type <type>float8</type> by
46 invoking a previously specified function, in this case
47 <literal>float8(int4)</>. (If no suitable cast has been defined, the
52 Two types can be <firstterm>binary coercible</firstterm>, which
53 means that the conversion can be performed <quote>for free</quote>
54 without invoking any function. This requires that corresponding
55 values use the same internal representation. For instance, the
56 types <type>text</type> and <type>varchar</type> are binary
57 coercible both ways. Binary coercibility is not necessarily a
58 symmetric relationship. For example, the cast
59 from <type>xml</type> to <type>text</type> can be performed for
60 free in the present implementation, but the reverse direction
61 requires a function that performs at least a syntax check. (Two
62 types that are binary coercible both ways are also referred to as
67 You can define a cast as an <firstterm>I/O conversion cast</> by using
68 the <literal>WITH INOUT</literal> syntax. An I/O conversion cast is
69 performed by invoking the output function of the source data type, and
70 passing the resulting string to the input function of the target data type.
71 In many common cases, this feature avoids the need to write a separate
72 cast function for conversion. An I/O conversion cast acts the same as
73 a regular function-based cast; only the implementation is different.
77 By default, a cast can be invoked only by an explicit cast request,
78 that is an explicit <literal>CAST(<replaceable>x</> AS
79 <replaceable>typename</>)</literal> or
80 <replaceable>x</><literal>::</><replaceable>typename</>
85 If the cast is marked <literal>AS ASSIGNMENT</> then it can be invoked
86 implicitly when assigning a value to a column of the target data type.
87 For example, supposing that <literal>foo.f1</literal> is a column of
88 type <type>text</type>, then:
90 INSERT INTO foo (f1) VALUES (42);
92 will be allowed if the cast from type <type>integer</type> to type
93 <type>text</type> is marked <literal>AS ASSIGNMENT</>, otherwise not.
94 (We generally use the term <firstterm>assignment
95 cast</firstterm> to describe this kind of cast.)
99 If the cast is marked <literal>AS IMPLICIT</> then it can be invoked
100 implicitly in any context, whether assignment or internally in an
101 expression. (We generally use the term <firstterm>implicit
102 cast</firstterm> to describe this kind of cast.)
103 For example, consider this query:
107 The parser initially marks the constants as being of type <type>integer</>
108 and <type>numeric</> respectively. There is no <type>integer</>
109 <literal>+</> <type>numeric</> operator in the system catalogs,
110 but there is a <type>numeric</> <literal>+</> <type>numeric</> operator.
111 The query will therefore succeed if a cast from <type>integer</> to
112 <type>numeric</> is available and is marked <literal>AS IMPLICIT</> —
113 which in fact it is. The parser will apply the implicit cast and resolve
114 the query as if it had been written
116 SELECT CAST ( 2 AS numeric ) + 4.0;
121 Now, the catalogs also provide a cast from <type>numeric</> to
122 <type>integer</>. If that cast were marked <literal>AS IMPLICIT</> —
123 which it is not — then the parser would be faced with choosing
124 between the above interpretation and the alternative of casting the
125 <type>numeric</> constant to <type>integer</> and applying the
126 <type>integer</> <literal>+</> <type>integer</> operator. Lacking any
127 knowledge of which choice to prefer, it would give up and declare the
128 query ambiguous. The fact that only one of the two casts is
129 implicit is the way in which we teach the parser to prefer resolution
130 of a mixed <type>numeric</>-and-<type>integer</> expression as
131 <type>numeric</>; there is no built-in knowledge about that.
135 It is wise to be conservative about marking casts as implicit. An
136 overabundance of implicit casting paths can cause
137 <productname>PostgreSQL</productname> to choose surprising
138 interpretations of commands, or to be unable to resolve commands at
139 all because there are multiple possible interpretations. A good
140 rule of thumb is to make a cast implicitly invokable only for
141 information-preserving transformations between types in the same
142 general type category. For example, the cast from <type>int2</type> to
143 <type>int4</type> can reasonably be implicit, but the cast from
144 <type>float8</type> to <type>int4</type> should probably be
145 assignment-only. Cross-type-category casts, such as <type>text</>
146 to <type>int4</>, are best made explicit-only.
151 Sometimes it is necessary for usability or standards-compliance reasons
152 to provide multiple implicit casts among a set of types, resulting in
153 ambiguity that cannot be avoided as above. The parser has a fallback
154 heuristic based on <firstterm>type categories</> and <firstterm>preferred
155 types</> that can help to provide desired behavior in such cases. See
156 <xref linkend="sql-createtype"> for
162 To be able to create a cast, you must own the source or the target data type
163 and have <literal>USAGE</literal> privilege on the other type. To create a
164 binary-coercible cast, you must be superuser. (This restriction is made
165 because an erroneous binary-coercible cast conversion can easily crash the
171 <title>Parameters</title>
175 <term><replaceable>source_type</replaceable></term>
179 The name of the source data type of the cast.
185 <term><replaceable>target_type</replaceable></term>
189 The name of the target data type of the cast.
195 <term><replaceable>function_name</replaceable>(<replaceable>argument_type</replaceable> [, ...])</term>
199 The function used to perform the cast. The function name can
200 be schema-qualified. If it is not, the function will be looked
201 up in the schema search path. The function's result data type must
202 match the target type of the cast. Its arguments are discussed below.
208 <term><literal>WITHOUT FUNCTION</literal></term>
212 Indicates that the source type is binary-coercible to the target type,
213 so no function is required to perform the cast.
219 <term><literal>WITH INOUT</literal></term>
223 Indicates that the cast is an I/O conversion cast, performed by
224 invoking the output function of the source data type, and passing the
225 resulting string to the input function of the target data type.
231 <term><literal>AS ASSIGNMENT</literal></term>
235 Indicates that the cast can be invoked implicitly in assignment
242 <term><literal>AS IMPLICIT</literal></term>
246 Indicates that the cast can be invoked implicitly in any context.
253 Cast implementation functions can have one to three arguments.
254 The first argument type must be identical to or binary-coercible from
255 the cast's source type. The second argument,
256 if present, must be type <type>integer</>; it receives the type
257 modifier associated with the destination type, or <literal>-1</>
258 if there is none. The third argument,
259 if present, must be type <type>boolean</>; it receives <literal>true</>
260 if the cast is an explicit cast, <literal>false</> otherwise.
261 (Bizarrely, the SQL standard demands different behaviors for explicit and
262 implicit casts in some cases. This argument is supplied for functions
263 that must implement such casts. It is not recommended that you design
264 your own data types so that this matters.)
268 The return type of a cast function must be identical to or
269 binary-coercible to the cast's target type.
273 Ordinarily a cast must have different source and target data types.
274 However, it is allowed to declare a cast with identical source and
275 target types if it has a cast implementation function with more than one
276 argument. This is used to represent type-specific length coercion
277 functions in the system catalogs. The named function is used to
278 coerce a value of the type to the type modifier value given by its
283 When a cast has different source and
284 target types and a function that takes more than one argument, it
285 supports converting from one type to another and applying a length
286 coercion in a single step. When no such entry is available, coercion
287 to a type that uses a type modifier involves two cast steps, one to
288 convert between data types and a second to apply the modifier.
292 A cast to or from a domain type currently has no effect. Casting
293 to or from a domain uses the casts associated with its underlying type.
298 <refsect1 id="sql-createcast-notes">
302 Use <xref linkend="sql-dropcast"> to remove user-defined casts.
306 Remember that if you want to be able to convert types both ways you
307 need to declare casts both ways explicitly.
310 <indexterm zone="sql-createcast">
311 <primary>cast</primary>
312 <secondary>I/O conversion</secondary>
316 It is normally not necessary to create casts between user-defined types
317 and the standard string types (<type>text</>, <type>varchar</>, and
318 <type>char(<replaceable>n</>)</type>, as well as user-defined types that
319 are defined to be in the string category). <productname>PostgreSQL</>
320 provides automatic I/O conversion casts for that. The automatic casts to
321 string types are treated as assignment casts, while the automatic casts
322 from string types are
323 explicit-only. You can override this behavior by declaring your own
324 cast to replace an automatic cast, but usually the only reason to
325 do so is if you want the conversion to be more easily invokable than the
326 standard assignment-only or explicit-only setting. Another possible
327 reason is that you want the conversion to behave differently from the
328 type's I/O function; but that is sufficiently surprising that you
329 should think twice about whether it's a good idea. (A small number of
330 the built-in types do indeed have different behaviors for conversions,
331 mostly because of requirements of the SQL standard.)
335 Prior to <productname>PostgreSQL</> 7.3, every function that had
336 the same name as a data type, returned that data type, and took one
337 argument of a different type was automatically a cast function.
338 This convention has been abandoned in face of the introduction of
339 schemas and to be able to represent binary-coercible casts in the
340 system catalogs. The built-in cast functions still follow this naming
341 scheme, but they have to be shown as casts in the system catalog
342 <structname>pg_cast</> as well.
346 While not required, it is recommended that you continue to follow this old
347 convention of naming cast implementation functions after the target data
348 type. Many users are used to being able to cast data types using a
349 function-style notation, that is
350 <replaceable>typename</>(<replaceable>x</>). This notation is in fact
351 nothing more nor less than a call of the cast implementation function; it
352 is not specially treated as a cast. If your conversion functions are not
353 named to support this convention then you will have surprised users.
354 Since <productname>PostgreSQL</> allows overloading of the same function
355 name with different argument types, there is no difficulty in having
356 multiple conversion functions from different types that all use the
362 Actually the preceding paragraph is an oversimplification: there are
363 two cases in which a function-call construct will be treated as a cast
364 request without having matched it to an actual function.
365 If a function call <replaceable>name</>(<replaceable>x</>) does not
366 exactly match any existing function, but <replaceable>name</> is the name
367 of a data type and <structname>pg_cast</> provides a binary-coercible cast
368 to this type from the type of <replaceable>x</>, then the call will be
369 construed as a binary-coercible cast. This exception is made so that
370 binary-coercible casts can be invoked using functional syntax, even
371 though they lack any function. Likewise, if there is no
372 <structname>pg_cast</> entry but the cast would be to or from a string
373 type, the call will be construed as an I/O conversion cast. This
374 exception allows I/O conversion casts to be invoked using functional
381 There is also an exception to the exception: I/O conversion casts from
382 composite types to string types cannot be invoked using functional
383 syntax, but must be written in explicit cast syntax (either
384 <literal>CAST</> or <literal>::</> notation). This exception was added
385 because after the introduction of automatically-provided I/O conversion
386 casts, it was found too easy to accidentally invoke such a cast when
387 a function or column reference was intended.
393 <refsect1 id="sql-createcast-examples">
394 <title>Examples</title>
397 To create an assignment cast from type <type>bigint</type> to type
398 <type>int4</type> using the function <literal>int4(bigint)</literal>:
400 CREATE CAST (bigint AS int4) WITH FUNCTION int4(bigint) AS ASSIGNMENT;
402 (This cast is already predefined in the system.)
406 <refsect1 id="sql-createcast-compat">
407 <title>Compatibility</title>
410 The <command>CREATE CAST</command> command conforms to the
411 <acronym>SQL</acronym> standard,
412 except that SQL does not make provisions for binary-coercible
413 types or extra arguments to implementation functions.
414 <literal>AS IMPLICIT</> is a <productname>PostgreSQL</productname>
420 <refsect1 id="sql-createcast-seealso">
421 <title>See Also</title>
424 <xref linkend="sql-createfunction">,
425 <xref linkend="sql-createtype">,
426 <xref linkend="sql-dropcast">