1CREATE CAST(7) PostgreSQL 9.2.24 Documentation CREATE CAST(7)
2
6 CREATE_CAST - define a new cast
7
9 CREATE CAST (source_type AS target_type)
10 WITH FUNCTION function_name (argument_type [, ...])
11 [ AS ASSIGNMENT | AS IMPLICIT ]
12 CREATE CAST (source_type AS target_type)
14 WITHOUT FUNCTION
15 [ AS ASSIGNMENT | AS IMPLICIT ]
16 CREATE CAST (source_type AS target_type)
18 WITH INOUT
19 [ AS ASSIGNMENT | AS IMPLICIT ]
20
22 CREATE CAST defines a new cast. A cast specifies how to perform a
23 conversion between two data types. For example,
24 SELECT CAST(42 AS float8);
26 converts the integer constant 42 to type float8 by invoking a
28 previously specified function, in this case float8(int4). (If no
29 suitable cast has been defined, the conversion fails.)
30 Two types can be binary coercible, which means that the conversion can
32 be performed “for free” without invoking any function. This requires
33 that corresponding values use the same internal representation. For
34 instance, the types text and varchar are binary coercible both ways.
35 Binary coercibility is not necessarily a symmetric relationship. For
36 example, the cast from xml to text can be performed for free in the
37 present implementation, but the reverse direction requires a function
38 that performs at least a syntax check. (Two types that are binary
39 coercible both ways are also referred to as binary compatible.)
40 You can define a cast as an I/O conversion cast by using the WITH INOUT
42 syntax. An I/O conversion cast is performed by invoking the output
43 function of the source data type, and passing the resulting string to
44 the input function of the target data type. In many common cases, this
45 feature avoids the need to write a separate cast function for
46 conversion. An I/O conversion cast acts the same as a regular
47 function-based cast; only the implementation is different.
48 By default, a cast can be invoked only by an explicit cast request,
50 that is an explicit CAST(x AS typename) or x::typename construct.
51 If the cast is marked AS ASSIGNMENT then it can be invoked implicitly
53 when assigning a value to a column of the target data type. For
54 example, supposing that foo.f1 is a column of type text, then:
55 INSERT INTO foo (f1) VALUES (42);
57 will be allowed if the cast from type integer to type text is marked AS
59 ASSIGNMENT, otherwise not. (We generally use the term assignment cast
60 to describe this kind of cast.)
61 If the cast is marked AS IMPLICIT then it can be invoked implicitly in
63 any context, whether assignment or internally in an expression. (We
64 generally use the term implicit cast to describe this kind of cast.)
65 For example, consider this query:
66 SELECT 2 + 4.0;
68 The parser initially marks the constants as being of type integer and
70 numeric respectively. There is no integer+numeric operator in the
71 system catalogs, but there is a numeric+numeric operator. The query
72 will therefore succeed if a cast from integer to numeric is available
73 and is marked AS IMPLICIT — which in fact it is. The parser will apply
74 the implicit cast and resolve the query as if it had been written
75 SELECT CAST ( 2 AS numeric ) + 4.0;
77 Now, the catalogs also provide a cast from numeric to integer. If that
79 cast were marked AS IMPLICIT — which it is not — then the parser would
80 be faced with choosing between the above interpretation and the
81 alternative of casting the numeric constant to integer and applying the
82 integer+integer operator. Lacking any knowledge of which choice to
83 prefer, it would give up and declare the query ambiguous. The fact that
84 only one of the two casts is implicit is the way in which we teach the
85 parser to prefer resolution of a mixed numeric-and-integer expression
86 as numeric; there is no built-in knowledge about that.
87 It is wise to be conservative about marking casts as implicit. An
89 overabundance of implicit casting paths can cause PostgreSQL to choose
90 surprising interpretations of commands, or to be unable to resolve
91 commands at all because there are multiple possible interpretations. A
92 good rule of thumb is to make a cast implicitly invokable only for
93 information-preserving transformations between types in the same
94 general type category. For example, the cast from int2 to int4 can
95 reasonably be implicit, but the cast from float8 to int4 should
96 probably be assignment-only. Cross-type-category casts, such as text to
97 int4, are best made explicit-only.
98 Note
100 Sometimes it is necessary for usability or standards-compliance
101 reasons to provide multiple implicit casts among a set of types,
102 resulting in ambiguity that cannot be avoided as above. The parser
103 has a fallback heuristic based on type categories and preferred
104 types that can help to provide desired behavior in such cases. See
105 CREATE TYPE (CREATE_TYPE(7)) for more information.
106 To be able to create a cast, you must own the source or the target data
108 type and have USAGE privilege on the other type. To create a
109 binary-coercible cast, you must be superuser. (This restriction is made
110 because an erroneous binary-coercible cast conversion can easily crash
111 the server.)
112
114 source_type
115 The name of the source data type of the cast.
116 target_type
118 The name of the target data type of the cast.
119 function_name(argument_type [, ...])
121 The function used to perform the cast. The function name can be
122 schema-qualified. If it is not, the function will be looked up in
123 the schema search path. The function's result data type must match
124 the target type of the cast. Its arguments are discussed below.
125 WITHOUT FUNCTION
127 Indicates that the source type is binary-coercible to the target
128 type, so no function is required to perform the cast.
129 WITH INOUT
131 Indicates that the cast is an I/O conversion cast, performed by
132 invoking the output function of the source data type, and passing
133 the resulting string to the input function of the target data type.
134 AS ASSIGNMENT
136 Indicates that the cast can be invoked implicitly in assignment
137 contexts.
138 AS IMPLICIT
140 Indicates that the cast can be invoked implicitly in any context.
141 Cast implementation functions can have one to three arguments. The
143 first argument type must be identical to or binary-coercible from the
144 cast's source type. The second argument, if present, must be type
145 integer; it receives the type modifier associated with the destination
146 type, or -1 if there is none. The third argument, if present, must be
147 type boolean; it receives true if the cast is an explicit cast, false
148 otherwise. (Bizarrely, the SQL standard demands different behaviors for
149 explicit and implicit casts in some cases. This argument is supplied
150 for functions that must implement such casts. It is not recommended
151 that you design your own data types so that this matters.)
152 The return type of a cast function must be identical to or
154 binary-coercible to the cast's target type.
155 Ordinarily a cast must have different source and target data types.
157 However, it is allowed to declare a cast with identical source and
158 target types if it has a cast implementation function with more than
159 one argument. This is used to represent type-specific length coercion
160 functions in the system catalogs. The named function is used to coerce
161 a value of the type to the type modifier value given by its second
162 argument.
163 When a cast has different source and target types and a function that
165 takes more than one argument, it supports converting from one type to
166 another and applying a length coercion in a single step. When no such
167 entry is available, coercion to a type that uses a type modifier
168 involves two cast steps, one to convert between data types and a second
169 to apply the modifier.
170 A cast to or from a domain type currently has no effect. Casting to or
172 from a domain uses the casts associated with its underlying type.
173
175 Use DROP CAST (DROP_CAST(7)) to remove user-defined casts.
176 Remember that if you want to be able to convert types both ways you
178 need to declare casts both ways explicitly.
179 It is normally not necessary to create casts between user-defined types
181 and the standard string types (text, varchar, and char(n), as well as
182 user-defined types that are defined to be in the string category).
183 PostgreSQL provides automatic I/O conversion casts for that. The
184 automatic casts to string types are treated as assignment casts, while
185 the automatic casts from string types are explicit-only. You can
186 override this behavior by declaring your own cast to replace an
187 automatic cast, but usually the only reason to do so is if you want the
188 conversion to be more easily invokable than the standard
189 assignment-only or explicit-only setting. Another possible reason is
190 that you want the conversion to behave differently from the type's I/O
191 function; but that is sufficiently surprising that you should think
192 twice about whether it's a good idea. (A small number of the built-in
193 types do indeed have different behaviors for conversions, mostly
194 because of requirements of the SQL standard.)
195 Prior to PostgreSQL 7.3, every function that had the same name as a
197 data type, returned that data type, and took one argument of a
198 different type was automatically a cast function. This convention has
199 been abandoned in face of the introduction of schemas and to be able to
200 represent binary-coercible casts in the system catalogs. The built-in
201 cast functions still follow this naming scheme, but they have to be
202 shown as casts in the system catalog pg_cast as well.
203 While not required, it is recommended that you continue to follow this
205 old convention of naming cast implementation functions after the target
206 data type. Many users are used to being able to cast data types using a
207 function-style notation, that is typename(x). This notation is in fact
208 nothing more nor less than a call of the cast implementation function;
209 it is not specially treated as a cast. If your conversion functions are
210 not named to support this convention then you will have surprised
211 users. Since PostgreSQL allows overloading of the same function name
212 with different argument types, there is no difficulty in having
213 multiple conversion functions from different types that all use the
214 target type's name.
215 Note
217 Actually the preceding paragraph is an oversimplification: there
218 are two cases in which a function-call construct will be treated as
219 a cast request without having matched it to an actual function. If
220 a function call name(x) does not exactly match any existing
221 function, but name is the name of a data type and pg_cast provides
222 a binary-coercible cast to this type from the type of x, then the
223 call will be construed as a binary-coercible cast. This exception
224 is made so that binary-coercible casts can be invoked using
225 functional syntax, even though they lack any function. Likewise, if
226 there is no pg_cast entry but the cast would be to or from a string
227 type, the call will be construed as an I/O conversion cast. This
228 exception allows I/O conversion casts to be invoked using
229 functional syntax.
230 Note
232 There is also an exception to the exception: I/O conversion casts
233 from composite types to string types cannot be invoked using
234 functional syntax, but must be written in explicit cast syntax
235 (either CAST or :: notation). This exception was added because
236 after the introduction of automatically-provided I/O conversion
237 casts, it was found too easy to accidentally invoke such a cast
238 when a function or column reference was intended.
239
241 To create an assignment cast from type bigint to type int4 using the
242 function int4(bigint):
243 CREATE CAST (bigint AS int4) WITH FUNCTION int4(bigint) AS ASSIGNMENT;
245 (This cast is already predefined in the system.)
247
249 The CREATE CAST command conforms to the SQL standard, except that SQL
250 does not make provisions for binary-coercible types or extra arguments
251 to implementation functions. AS IMPLICIT is a PostgreSQL extension,
252 too.
253
255 CREATE FUNCTION (CREATE_FUNCTION(7)), CREATE TYPE (CREATE_TYPE(7)),
256 DROP CAST (DROP_CAST(7))
257PostgreSQL 9.2.24 2017-11-06 CREATE CAST(7)