1<!-- 2doc/src/sgml/ref/create_cast.sgml 3PostgreSQL documentation 4--> 5 6<refentry id="sql-createcast"> 7 <indexterm zone="sql-createcast"> 8 <primary>CREATE CAST</primary> 9 </indexterm> 10 11 <refmeta> 12 <refentrytitle>CREATE CAST</refentrytitle> 13 <manvolnum>7</manvolnum> 14 <refmiscinfo>SQL - Language Statements</refmiscinfo> 15 </refmeta> 16 17 <refnamediv> 18 <refname>CREATE CAST</refname> 19 <refpurpose>define a new cast</refpurpose> 20 </refnamediv> 21 22 <refsynopsisdiv> 23<synopsis> 24CREATE CAST (<replaceable>source_type</replaceable> AS <replaceable>target_type</replaceable>) 25 WITH FUNCTION <replaceable>function_name</replaceable> [ (<replaceable>argument_type</replaceable> [, ...]) ] 26 [ AS ASSIGNMENT | AS IMPLICIT ] 27 28CREATE CAST (<replaceable>source_type</replaceable> AS <replaceable>target_type</replaceable>) 29 WITHOUT FUNCTION 30 [ AS ASSIGNMENT | AS IMPLICIT ] 31 32CREATE CAST (<replaceable>source_type</replaceable> AS <replaceable>target_type</replaceable>) 33 WITH INOUT 34 [ AS ASSIGNMENT | AS IMPLICIT ] 35</synopsis> 36 </refsynopsisdiv> 37 38 <refsect1 id="sql-createcast-description"> 39 <title>Description</title> 40 41 <para> 42 <command>CREATE CAST</command> defines a new cast. A cast 43 specifies how to perform a conversion between 44 two data types. For example, 45<programlisting> 46SELECT CAST(42 AS float8); 47</programlisting> 48 converts the integer constant 42 to type <type>float8</type> by 49 invoking a previously specified function, in this case 50 <literal>float8(int4)</literal>. (If no suitable cast has been defined, the 51 conversion fails.) 52 </para> 53 54 <para> 55 Two types can be <firstterm>binary coercible</firstterm>, which 56 means that the conversion can be performed <quote>for free</quote> 57 without invoking any function. This requires that corresponding 58 values use the same internal representation. For instance, the 59 types <type>text</type> and <type>varchar</type> are binary 60 coercible both ways. Binary coercibility is not necessarily a 61 symmetric relationship. For example, the cast 62 from <type>xml</type> to <type>text</type> can be performed for 63 free in the present implementation, but the reverse direction 64 requires a function that performs at least a syntax check. (Two 65 types that are binary coercible both ways are also referred to as 66 binary compatible.) 67 </para> 68 69 <para> 70 You can define a cast as an <firstterm>I/O conversion cast</firstterm> by using 71 the <literal>WITH INOUT</literal> syntax. An I/O conversion cast is 72 performed by invoking the output function of the source data type, and 73 passing the resulting string to the input function of the target data type. 74 In many common cases, this feature avoids the need to write a separate 75 cast function for conversion. An I/O conversion cast acts the same as 76 a regular function-based cast; only the implementation is different. 77 </para> 78 79 <para> 80 By default, a cast can be invoked only by an explicit cast request, 81 that is an explicit <literal>CAST(<replaceable>x</replaceable> AS 82 <replaceable>typename</replaceable>)</literal> or 83 <replaceable>x</replaceable><literal>::</literal><replaceable>typename</replaceable> 84 construct. 85 </para> 86 87 <para> 88 If the cast is marked <literal>AS ASSIGNMENT</literal> then it can be invoked 89 implicitly when assigning a value to a column of the target data type. 90 For example, supposing that <literal>foo.f1</literal> is a column of 91 type <type>text</type>, then: 92<programlisting> 93INSERT INTO foo (f1) VALUES (42); 94</programlisting> 95 will be allowed if the cast from type <type>integer</type> to type 96 <type>text</type> is marked <literal>AS ASSIGNMENT</literal>, otherwise not. 97 (We generally use the term <firstterm>assignment 98 cast</firstterm> to describe this kind of cast.) 99 </para> 100 101 <para> 102 If the cast is marked <literal>AS IMPLICIT</literal> then it can be invoked 103 implicitly in any context, whether assignment or internally in an 104 expression. (We generally use the term <firstterm>implicit 105 cast</firstterm> to describe this kind of cast.) 106 For example, consider this query: 107<programlisting> 108SELECT 2 + 4.0; 109</programlisting> 110 The parser initially marks the constants as being of type <type>integer</type> 111 and <type>numeric</type> respectively. There is no <type>integer</type> 112 <literal>+</literal> <type>numeric</type> operator in the system catalogs, 113 but there is a <type>numeric</type> <literal>+</literal> <type>numeric</type> operator. 114 The query will therefore succeed if a cast from <type>integer</type> to 115 <type>numeric</type> is available and is marked <literal>AS IMPLICIT</literal> — 116 which in fact it is. The parser will apply the implicit cast and resolve 117 the query as if it had been written 118<programlisting> 119SELECT CAST ( 2 AS numeric ) + 4.0; 120</programlisting> 121 </para> 122 123 <para> 124 Now, the catalogs also provide a cast from <type>numeric</type> to 125 <type>integer</type>. If that cast were marked <literal>AS IMPLICIT</literal> — 126 which it is not — then the parser would be faced with choosing 127 between the above interpretation and the alternative of casting the 128 <type>numeric</type> constant to <type>integer</type> and applying the 129 <type>integer</type> <literal>+</literal> <type>integer</type> operator. Lacking any 130 knowledge of which choice to prefer, it would give up and declare the 131 query ambiguous. The fact that only one of the two casts is 132 implicit is the way in which we teach the parser to prefer resolution 133 of a mixed <type>numeric</type>-and-<type>integer</type> expression as 134 <type>numeric</type>; there is no built-in knowledge about that. 135 </para> 136 137 <para> 138 It is wise to be conservative about marking casts as implicit. An 139 overabundance of implicit casting paths can cause 140 <productname>PostgreSQL</productname> to choose surprising 141 interpretations of commands, or to be unable to resolve commands at 142 all because there are multiple possible interpretations. A good 143 rule of thumb is to make a cast implicitly invokable only for 144 information-preserving transformations between types in the same 145 general type category. For example, the cast from <type>int2</type> to 146 <type>int4</type> can reasonably be implicit, but the cast from 147 <type>float8</type> to <type>int4</type> should probably be 148 assignment-only. Cross-type-category casts, such as <type>text</type> 149 to <type>int4</type>, are best made explicit-only. 150 </para> 151 152 <note> 153 <para> 154 Sometimes it is necessary for usability or standards-compliance reasons 155 to provide multiple implicit casts among a set of types, resulting in 156 ambiguity that cannot be avoided as above. The parser has a fallback 157 heuristic based on <firstterm>type categories</firstterm> and <firstterm>preferred 158 types</firstterm> that can help to provide desired behavior in such cases. See 159 <xref linkend="sql-createtype"/> for 160 more information. 161 </para> 162 </note> 163 164 <para> 165 To be able to create a cast, you must own the source or the target data type 166 and have <literal>USAGE</literal> privilege on the other type. To create a 167 binary-coercible cast, you must be superuser. (This restriction is made 168 because an erroneous binary-coercible cast conversion can easily crash the 169 server.) 170 </para> 171 </refsect1> 172 173 <refsect1> 174 <title>Parameters</title> 175 176 <variablelist> 177 <varlistentry> 178 <term><replaceable>source_type</replaceable></term> 179 180 <listitem> 181 <para> 182 The name of the source data type of the cast. 183 </para> 184 </listitem> 185 </varlistentry> 186 187 <varlistentry> 188 <term><replaceable>target_type</replaceable></term> 189 190 <listitem> 191 <para> 192 The name of the target data type of the cast. 193 </para> 194 </listitem> 195 </varlistentry> 196 197 <varlistentry> 198 <term><literal><replaceable>function_name</replaceable>[(<replaceable>argument_type</replaceable> [, ...])]</literal></term> 199 200 <listitem> 201 <para> 202 The function used to perform the cast. The function name can 203 be schema-qualified. If it is not, the function will be looked 204 up in the schema search path. The function's result data type must 205 match the target type of the cast. Its arguments are discussed below. 206 If no argument list is specified, the function name must be unique in 207 its schema. 208 </para> 209 </listitem> 210 </varlistentry> 211 212 <varlistentry> 213 <term><literal>WITHOUT FUNCTION</literal></term> 214 215 <listitem> 216 <para> 217 Indicates that the source type is binary-coercible to the target type, 218 so no function is required to perform the cast. 219 </para> 220 </listitem> 221 </varlistentry> 222 223 <varlistentry> 224 <term><literal>WITH INOUT</literal></term> 225 226 <listitem> 227 <para> 228 Indicates that the cast is an I/O conversion cast, performed by 229 invoking the output function of the source data type, and passing the 230 resulting string to the input function of the target data type. 231 </para> 232 </listitem> 233 </varlistentry> 234 235 <varlistentry> 236 <term><literal>AS ASSIGNMENT</literal></term> 237 238 <listitem> 239 <para> 240 Indicates that the cast can be invoked implicitly in assignment 241 contexts. 242 </para> 243 </listitem> 244 </varlistentry> 245 246 <varlistentry> 247 <term><literal>AS IMPLICIT</literal></term> 248 249 <listitem> 250 <para> 251 Indicates that the cast can be invoked implicitly in any context. 252 </para> 253 </listitem> 254 </varlistentry> 255 </variablelist> 256 257 <para> 258 Cast implementation functions can have one to three arguments. 259 The first argument type must be identical to or binary-coercible from 260 the cast's source type. The second argument, 261 if present, must be type <type>integer</type>; it receives the type 262 modifier associated with the destination type, or <literal>-1</literal> 263 if there is none. The third argument, 264 if present, must be type <type>boolean</type>; it receives <literal>true</literal> 265 if the cast is an explicit cast, <literal>false</literal> otherwise. 266 (Bizarrely, the SQL standard demands different behaviors for explicit and 267 implicit casts in some cases. This argument is supplied for functions 268 that must implement such casts. It is not recommended that you design 269 your own data types so that this matters.) 270 </para> 271 272 <para> 273 The return type of a cast function must be identical to or 274 binary-coercible to the cast's target type. 275 </para> 276 277 <para> 278 Ordinarily a cast must have different source and target data types. 279 However, it is allowed to declare a cast with identical source and 280 target types if it has a cast implementation function with more than one 281 argument. This is used to represent type-specific length coercion 282 functions in the system catalogs. The named function is used to 283 coerce a value of the type to the type modifier value given by its 284 second argument. 285 </para> 286 287 <para> 288 When a cast has different source and 289 target types and a function that takes more than one argument, it 290 supports converting from one type to another and applying a length 291 coercion in a single step. When no such entry is available, coercion 292 to a type that uses a type modifier involves two cast steps, one to 293 convert between data types and a second to apply the modifier. 294 </para> 295 296 <para> 297 A cast to or from a domain type currently has no effect. Casting 298 to or from a domain uses the casts associated with its underlying type. 299 </para> 300 301 </refsect1> 302 303 <refsect1 id="sql-createcast-notes"> 304 <title>Notes</title> 305 306 <para> 307 Use <xref linkend="sql-dropcast"/> to remove user-defined casts. 308 </para> 309 310 <para> 311 Remember that if you want to be able to convert types both ways you 312 need to declare casts both ways explicitly. 313 </para> 314 315 <indexterm zone="sql-createcast"> 316 <primary>cast</primary> 317 <secondary>I/O conversion</secondary> 318 </indexterm> 319 320 <para> 321 It is normally not necessary to create casts between user-defined types 322 and the standard string types (<type>text</type>, <type>varchar</type>, and 323 <type>char(<replaceable>n</replaceable>)</type>, as well as user-defined types that 324 are defined to be in the string category). <productname>PostgreSQL</productname> 325 provides automatic I/O conversion casts for that. The automatic casts to 326 string types are treated as assignment casts, while the automatic casts 327 from string types are 328 explicit-only. You can override this behavior by declaring your own 329 cast to replace an automatic cast, but usually the only reason to 330 do so is if you want the conversion to be more easily invokable than the 331 standard assignment-only or explicit-only setting. Another possible 332 reason is that you want the conversion to behave differently from the 333 type's I/O function; but that is sufficiently surprising that you 334 should think twice about whether it's a good idea. (A small number of 335 the built-in types do indeed have different behaviors for conversions, 336 mostly because of requirements of the SQL standard.) 337 </para> 338 339 <para> 340 While not required, it is recommended that you continue to follow this old 341 convention of naming cast implementation functions after the target data 342 type. Many users are used to being able to cast data types using a 343 function-style notation, that is 344 <replaceable>typename</replaceable>(<replaceable>x</replaceable>). This notation is in fact 345 nothing more nor less than a call of the cast implementation function; it 346 is not specially treated as a cast. If your conversion functions are not 347 named to support this convention then you will have surprised users. 348 Since <productname>PostgreSQL</productname> allows overloading of the same function 349 name with different argument types, there is no difficulty in having 350 multiple conversion functions from different types that all use the 351 target type's name. 352 </para> 353 354 <note> 355 <para> 356 Actually the preceding paragraph is an oversimplification: there are 357 two cases in which a function-call construct will be treated as a cast 358 request without having matched it to an actual function. 359 If a function call <replaceable>name</replaceable>(<replaceable>x</replaceable>) does not 360 exactly match any existing function, but <replaceable>name</replaceable> is the name 361 of a data type and <structname>pg_cast</structname> provides a binary-coercible cast 362 to this type from the type of <replaceable>x</replaceable>, then the call will be 363 construed as a binary-coercible cast. This exception is made so that 364 binary-coercible casts can be invoked using functional syntax, even 365 though they lack any function. Likewise, if there is no 366 <structname>pg_cast</structname> entry but the cast would be to or from a string 367 type, the call will be construed as an I/O conversion cast. This 368 exception allows I/O conversion casts to be invoked using functional 369 syntax. 370 </para> 371 </note> 372 373 <note> 374 <para> 375 There is also an exception to the exception: I/O conversion casts from 376 composite types to string types cannot be invoked using functional 377 syntax, but must be written in explicit cast syntax (either 378 <literal>CAST</literal> or <literal>::</literal> notation). This exception was added 379 because after the introduction of automatically-provided I/O conversion 380 casts, it was found too easy to accidentally invoke such a cast when 381 a function or column reference was intended. 382 </para> 383 </note> 384 </refsect1> 385 386 387 <refsect1 id="sql-createcast-examples"> 388 <title>Examples</title> 389 390 <para> 391 To create an assignment cast from type <type>bigint</type> to type 392 <type>int4</type> using the function <literal>int4(bigint)</literal>: 393<programlisting> 394CREATE CAST (bigint AS int4) WITH FUNCTION int4(bigint) AS ASSIGNMENT; 395</programlisting> 396 (This cast is already predefined in the system.) 397 </para> 398 </refsect1> 399 400 <refsect1 id="sql-createcast-compat"> 401 <title>Compatibility</title> 402 403 <para> 404 The <command>CREATE CAST</command> command conforms to the 405 <acronym>SQL</acronym> standard, 406 except that SQL does not make provisions for binary-coercible 407 types or extra arguments to implementation functions. 408 <literal>AS IMPLICIT</literal> is a <productname>PostgreSQL</productname> 409 extension, too. 410 </para> 411 </refsect1> 412 413 414 <refsect1 id="sql-createcast-seealso"> 415 <title>See Also</title> 416 417 <para> 418 <xref linkend="sql-createfunction"/>, 419 <xref linkend="sql-createtype"/>, 420 <xref linkend="sql-dropcast"/> 421 </para> 422 </refsect1> 423 424</refentry> 425