1CREATE CAST(7)                   SQL Commands                   CREATE CAST(7)
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NAME

6       CREATE CAST - define a new cast
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SYNOPSIS

10       CREATE CAST (sourcetype AS targettype)
11           WITH FUNCTION funcname (argtypes)
12           [ AS ASSIGNMENT | AS IMPLICIT ]
13
14       CREATE CAST (sourcetype AS targettype)
15           WITHOUT FUNCTION
16           [ AS ASSIGNMENT | AS IMPLICIT ]
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18       CREATE CAST (sourcetype AS targettype)
19           WITH INOUT
20           [ AS ASSIGNMENT | AS IMPLICIT ]
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DESCRIPTION

24       CREATE  CAST defines a new cast. A cast specifies how to perform a con‐
25       version between two data types. For example:
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27       SELECT CAST(42 AS float8);
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29       converts the integer constant 42 to type float8 by  invoking  a  previ‐
30       ously  specified  function,  in this case float8(int4). (If no suitable
31       cast has been defined, the conversion fails.)
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33       Two types can be binary coercible, which means that the conversion  can
34       be  performed ``for free'' without invoking any function. This requires
35       that corresponding values use the  same  internal  representation.  For
36       instance,  the  types  text and varchar are binary coercible both ways.
37       Binary coercibility is not necessarily a  symmetric  relationship.  For
38       example,  the  cast  from  xml to text can be performed for free in the
39       present implementation, but the reverse direction requires  a  function
40       that  performs  at  least  a  syntax  check. (Two types that are binary
41       coercible both ways are also referred to as binary compatible.)
42
43       You can define a cast as an I/O conversion cast using  the  WITH  INOUT
44       syntax.  An  I/O  conversion  cast  is performed by invoking the output
45       function of the source data type, and passing the result to  the  input
46       function of the target data type.
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48       By  default,  a  cast  can be invoked only by an explicit cast request,
49       that is an explicit CAST(x AS typename) or x::typename construct.
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51       If the cast is marked AS ASSIGNMENT then it can be  invoked  implicitly
52       when  assigning a value to a column of the target data type.  For exam‐
53       ple, supposing that foo.f1 is a column of type text, then:
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55       INSERT INTO foo (f1) VALUES (42);
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57       will be allowed if the cast from type integer to type text is marked AS
58       ASSIGNMENT,  otherwise not.  (We generally use the term assignment cast
59       to describe this kind of cast.)
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61       If the cast is marked AS IMPLICIT then it can be invoked implicitly  in
62       any  context,  whether  assignment  or internally in an expression. (We
63       generally use the term implicit cast to describe this  kind  of  cast.)
64       For example, consider this query:
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66       SELECT 2 + 4.0;
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68       The  parser  initially marks the constants as being of type integer and
69       numeric respectively. There is no integer +  numeric  operator  in  the
70       system  catalogs, but there is a numeric + numeric operator.  The query
71       will therefore succeed if a cast from integer to numeric  is  available
72       and  is marked AS IMPLICIT — which in fact it is. The parser will apply
73       the implicit cast and resolve the query as if it had been written
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75       SELECT CAST ( 2 AS numeric ) + 4.0;
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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  alter‐
81       native  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 over‐
89       abundance of implicit casting paths can cause PostgreSQL to choose sur‐
90       prising interpretations of commands, or to be unable  to  resolve  com‐
91       mands  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 gen‐
94       eral type category. For example, the cast from int2 to int4 can reason‐
95       ably  be  implicit, but the cast from float8 to int4 should probably be
96       assignment-only. Cross-type-category casts, such as text to  int4,  are
97       best made explicit-only.
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99              Note:  Sometimes it is necessary for usability or standards-com‐
100              pliance reasons to provide multiple implicit casts among  a  set
101              of  types,  resulting  in  ambiguity  that  cannot be avoided as
102              above. The parser has a fallback heuristic based on  type  cate‐
103              gories  and  preferred  types  that  can help to provide desired
104              behavior in such cases. See  CREATE  TYPE  [create_type(7)]  for
105              more information.
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107
108       To be able to create a cast, you must own the source or the target data
109       type. To create a binary-coercible cast, you must be superuser.   (This
110       restriction  is made because an erroneous binary-coercible cast conver‐
111       sion can easily crash the server.)
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PARAMETERS

114       sourcetype
115              The name of the source data type of the cast.
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117       targettype
118              The name of the target data type of the cast.
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120       funcname(argtypes)
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
123              in the schema search path. The function's result data type  must
124              match  the  target type of the cast. Its arguments are discussed
125              below.
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127       WITHOUT FUNCTION
128              Indicates that the source type is binary-coercible to the target
129              type, so no function is required to perform the cast.
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131       WITH INOUT
132              Indicates  that the cast is an I/O conversion cast, performed by
133              invoking the output function of the source data type, and  pass‐
134              ing the result to the input function of the target data type.
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136       AS ASSIGNMENT
137              Indicates  that the cast can be invoked implicitly in assignment
138              contexts.
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140       AS IMPLICIT
141              Indicates that the cast can be invoked implicitly  in  any  con‐
142              text.
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 inte‐
147       ger; 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
151       for explicit and implicit casts in some cases. This  argument  is  sup‐
152       plied  for  functions  that must implement such casts. It is not recom‐
153       mended 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  binary-
156       coercible to the cast's target type.
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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 tar‐
160       get  types  if it has a cast implementation function with more than one
161       argument. This is used to represent type-specific length coercion func‐
162       tions  in  the  system catalogs. The named function is used to coerce a
163       value of the type to the type modifier value given by its second  argu‐
164       ment.
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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 represents 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  steps, one to convert between data types and a second to
171       apply the modifier.
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NOTES

174       Use DROP CAST [drop_cast(7)] to remove user-defined casts.
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176       Remember that if you want to be able to convert  types  both  ways  you
177       need to declare casts both ways explicitly.
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180       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  auto‐
184       matic  casts to string types are treated as assignment casts, while the
185       automatic casts from string types are explicit-only. You  can  override
186       this  behavior by declaring your own cast to replace an automatic cast,
187       but usually the only reason to do so is if you want the  conversion  to
188       be more easily invokable than the standard assignment-only or explicit-
189       only setting. Another possible reason is that you want  the  conversion
190       to  behave differently from the type's I/O function; but that is suffi‐
191       ciently surprising that you should think twice  about  whether  it's  a
192       good idea. (A small number of the built-in types do indeed have differ‐
193       ent behaviors for conversions, mostly because of  requirements  of  the
194       SQL standard.)
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196       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  differ‐
198       ent  type  was automatically a cast function.  This convention has been
199       abandoned in face of the introduction of schemas and to be able to rep‐
200       resent binary-coercible casts in the system catalogs. The built-in cast
201       functions still follow this naming scheme, but they have to be shown as
202       casts in the system catalog pg_cast as well.
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204       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 multi‐
213       ple conversion functions from different types that all use  the  target
214       type's name.
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216              Note: Actually the preceding paragraph is an oversimplification:
217              there are two cases in which a function-call construct  will  be
218              treated as a cast request without having matched it to an actual
219              function.  If a function call name(x) does not exactly match any
220              existing  function,  but  name  is  the  name of a data type and
221              pg_cast provides a binary-coercible cast to this type  from  the
222              type of x, then the call will be construed as a binary-coercible
223              cast. This exception is made so that binary-coercible casts  can
224              be  invoked  using  functional syntax, even though they lack any
225              function. Likewise, if there is no pg_cast entry  but  the  cast
226              would be to or from a string type, the call will be construed as
227              an I/O conversion cast. This  exception  allows  I/O  conversion
228              casts to be invoked using functional syntax.
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EXAMPLES

232       To  create  an  assignment cast from type bigint to type int4 using the
233       function int4(bigint):
234
235       CREATE CAST (bigint AS int4) WITH FUNCTION int4(bigint) AS ASSIGNMENT;
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237       (This cast is already predefined in the system.)
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COMPATIBILITY

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

246       CREATE  FUNCTION  [create_function(7)],  CREATE  TYPE [create_type(7)],
247       DROP CAST [drop_cast(7)]
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251SQL - Language Statements         2011-09-22                    CREATE CAST(7)
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