1CREATE CAST(7)           PostgreSQL 10.7 Documentation          CREATE CAST(7)
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NAME

6       CREATE_CAST - define a new cast
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SYNOPSIS

9       CREATE CAST (source_type AS target_type)
10           WITH FUNCTION function_name [ (argument_type [, ...]) ]
11           [ AS ASSIGNMENT | AS IMPLICIT ]
12
13       CREATE CAST (source_type AS target_type)
14           WITHOUT FUNCTION
15           [ AS ASSIGNMENT | AS IMPLICIT ]
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17       CREATE CAST (source_type AS target_type)
18           WITH INOUT
19           [ AS ASSIGNMENT | AS IMPLICIT ]
20

DESCRIPTION

22       CREATE CAST defines a new cast. A cast specifies how to perform a
23       conversion between two data types. For example,
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25           SELECT CAST(42 AS float8);
26
27       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
31       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
41       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
49       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
52       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:
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56           INSERT INTO foo (f1) VALUES (42);
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58       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
62       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:
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67           SELECT 2 + 4.0;
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69       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
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76           SELECT CAST ( 2 AS numeric ) + 4.0;
77
78       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
88       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.
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99           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
107       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.)
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PARAMETERS

114       source_type
115           The name of the source data type of the cast.
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117       target_type
118           The name of the target data type of the cast.
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120       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. If
125           no argument list is specified, the function name must be unique in
126           its schema.
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128       WITHOUT FUNCTION
129           Indicates that the source type is binary-coercible to the target
130           type, so no function is required to perform the cast.
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132       WITH INOUT
133           Indicates that the cast is an I/O conversion cast, performed by
134           invoking the output function of the source data type, and passing
135           the resulting string to the input function of the target data type.
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137       AS ASSIGNMENT
138           Indicates that the cast can be invoked implicitly in assignment
139           contexts.
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141       AS IMPLICIT
142           Indicates that the cast can be invoked implicitly in any context.
143
144       Cast implementation functions can have one to three arguments. The
145       first argument type must be identical to or binary-coercible from the
146       cast's source type. The second argument, if present, must be type
147       integer; it receives the type modifier associated with the destination
148       type, or -1 if there is none. The third argument, if present, must be
149       type boolean; it receives true if the cast is an explicit cast, false
150       otherwise. (Bizarrely, the SQL standard demands different behaviors for
151       explicit and implicit casts in some cases. This argument is supplied
152       for functions that must implement such casts. It is not recommended
153       that you design your own data types so that this matters.)
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155       The return type of a cast function must be identical to or
156       binary-coercible to the cast's target type.
157
158       Ordinarily a cast must have different source and target data types.
159       However, it is allowed to declare a cast with identical source and
160       target types if it has a cast implementation function with more than
161       one argument. This is used to represent type-specific length coercion
162       functions in the system catalogs. The named function is used to coerce
163       a value of the type to the type modifier value given by its second
164       argument.
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166       When a cast has different source and target types and a function that
167       takes more than one argument, it supports converting from one type to
168       another and applying a length coercion in a single step. When no such
169       entry is available, coercion to a type that uses a type modifier
170       involves two cast steps, one to convert between data types and a second
171       to apply the modifier.
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173       A cast to or from a domain type currently has no effect. Casting to or
174       from a domain uses the casts associated with its underlying type.
175

NOTES

177       Use DROP CAST (DROP_CAST(7)) to remove user-defined casts.
178
179       Remember that if you want to be able to convert types both ways you
180       need to declare casts both ways explicitly.
181
182       It is normally not necessary to create casts between user-defined types
183       and the standard string types (text, varchar, and char(n), as well as
184       user-defined types that are defined to be in the string category).
185       PostgreSQL provides automatic I/O conversion casts for that. The
186       automatic casts to string types are treated as assignment casts, while
187       the automatic casts from string types are explicit-only. You can
188       override this behavior by declaring your own cast to replace an
189       automatic cast, but usually the only reason to do so is if you want the
190       conversion to be more easily invokable than the standard
191       assignment-only or explicit-only setting. Another possible reason is
192       that you want the conversion to behave differently from the type's I/O
193       function; but that is sufficiently surprising that you should think
194       twice about whether it's a good idea. (A small number of the built-in
195       types do indeed have different behaviors for conversions, mostly
196       because of requirements of the SQL standard.)
197
198       While not required, it is recommended that you continue to follow this
199       old convention of naming cast implementation functions after the target
200       data type. Many users are used to being able to cast data types using a
201       function-style notation, that is typename(x). This notation is in fact
202       nothing more nor less than a call of the cast implementation function;
203       it is not specially treated as a cast. If your conversion functions are
204       not named to support this convention then you will have surprised
205       users. Since PostgreSQL allows overloading of the same function name
206       with different argument types, there is no difficulty in having
207       multiple conversion functions from different types that all use the
208       target type's name.
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210           Note
211           Actually the preceding paragraph is an oversimplification: there
212           are two cases in which a function-call construct will be treated as
213           a cast request without having matched it to an actual function. If
214           a function call name(x) does not exactly match any existing
215           function, but name is the name of a data type and pg_cast provides
216           a binary-coercible cast to this type from the type of x, then the
217           call will be construed as a binary-coercible cast. This exception
218           is made so that binary-coercible casts can be invoked using
219           functional syntax, even though they lack any function. Likewise, if
220           there is no pg_cast entry but the cast would be to or from a string
221           type, the call will be construed as an I/O conversion cast. This
222           exception allows I/O conversion casts to be invoked using
223           functional syntax.
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225           Note
226           There is also an exception to the exception: I/O conversion casts
227           from composite types to string types cannot be invoked using
228           functional syntax, but must be written in explicit cast syntax
229           (either CAST or :: notation). This exception was added because
230           after the introduction of automatically-provided I/O conversion
231           casts, it was found too easy to accidentally invoke such a cast
232           when a function or column reference was intended.
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EXAMPLES

235       To create an assignment cast from type bigint to type int4 using the
236       function int4(bigint):
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238           CREATE CAST (bigint AS int4) WITH FUNCTION int4(bigint) AS ASSIGNMENT;
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240       (This cast is already predefined in the system.)
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COMPATIBILITY

243       The CREATE CAST command conforms to the SQL standard, except that SQL
244       does not make provisions for binary-coercible types or extra arguments
245       to implementation functions.  AS IMPLICIT is a PostgreSQL extension,
246       too.
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SEE ALSO

249       CREATE FUNCTION (CREATE_FUNCTION(7)), CREATE TYPE (CREATE_TYPE(7)),
250       DROP CAST (DROP_CAST(7))
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254PostgreSQL 10.7                      2019                       CREATE CAST(7)
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