doc/src/sgml/ref/create_cast.sgml

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   2 doc/src/sgml/ref/create_cast.sgml
   3 PostgreSQL documentation
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   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>
  24 CREATE 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
  28 CREATE CAST (<replaceable>source_type</replaceable> AS <replaceable>target_type</replaceable>)
  29     WITHOUT FUNCTION
  30     [ AS ASSIGNMENT | AS IMPLICIT ]
  31
  32 CREATE 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>
  46 SELECT 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>
  93 INSERT 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>
 108 SELECT 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> &mdash;
 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>
 119 SELECT 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> &mdash;
 126    which it is not &mdash; 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 <link linkend="sql-dropcast"><command>DROP CAST</command></link> 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>
 394 CREATE 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>