postgresql/doc/src/sgml/xtypes.sgml

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 <sect1 id="xtypes">
  <title>User-Defined Types</title>

  <indexterm zone="xtypes">
   <primary>data types</primary>
   <secondary>extending</secondary>
  </indexterm>

 <comment>
  This section needs to be updated for the version-1 function manager
  interface.
 </comment>

  <para>
   As described above, there are two kinds of data types in
   <productname>PostgreSQL</productname>: base types and composite
   types. This section describes how to define new base types.
  </para>

  <para>
   The examples in this section can be found in
   <filename>complex.sql</filename> and <filename>complex.c</filename>
   in the tutorial directory.
  </para>

 <para>
  <indexterm>
   <primary>input function</primary>
  </indexterm>
  <indexterm>
   <primary>output function</primary>
  </indexterm>
  A user-defined type must always have input and output functions.
  These functions determine how the type appears in strings (for input
  by the user and output to the user) and how the type is organized in
  memory.  The input function takes a null-terminated character string
  as its argument and returns the internal (in memory) representation of
  the type.  The output function takes the internal representation of
  the type as argument and returns a null-terminated character string.
 </para>

 <para>
  Suppose we want to define a type <type>complex</> that represents
  complex numbers. A natural way to to represent a complex number in
  memory would be the following C structure:

<programlisting>
typedef struct Complex {
    double      x;
    double      y;
} Complex;
</programlisting>

  As the external string representation of the type, we choose a
  string of the form <literal>(x,y)</literal>.
 </para>

 <para>
  The input and output functions are usually not hard to write,
  especially the output function.  But when defining the external
  string representation of the type, remember that you must eventually
  write a complete and robust parser for that representation as your
  input function.  For instance:

<programlisting>
Complex *
complex_in(char *str)
{
    double x, y;
    Complex *result;

    if (sscanf(str, " ( %lf , %lf )", &amp;x, &amp;y) != 2)
    {
        elog(ERROR, "complex_in: error in parsing %s", str);
        return NULL;
    }
    result = (Complex *) palloc(sizeof(Complex));
    result-&gt;x = x;
    result-&gt;y = y;
    return result;
}
</programlisting>

  The output function can simply be:

<programlisting>
char *
complex_out(Complex *complex)
{
    char *result;

    if (complex == NULL)
        return(NULL);
    result = (char *) palloc(60);
    sprintf(result, "(%g,%g)", complex-&gt;x, complex-&gt;y);
    return result;
}
</programlisting>
 </para>

 <para>
  You should try to make the input and output functions inverses of
  each other.  If you do not, you will have severe problems when you
  need to dump your data into a file and then read it back in.  This
  is a particularly common problem when floating-point numbers are
  involved.
 </para>

 <para>
  To define the <type>complex</type> type, we need to create the two
  user-defined functions <function>complex_in</function> and
  <function>complex_out</function> before creating the type:

<programlisting>
CREATE FUNCTION complex_in(cstring)
    RETURNS complex
    AS '<replaceable>filename</replaceable>'
    LANGUAGE C;

CREATE FUNCTION complex_out(complex)
    RETURNS cstring
    AS '<replaceable>filename</replaceable>'
    LANGUAGE C;
</programlisting>

  Notice that the declarations of the input and output functions must
  reference the not-yet-defined type.  This is allowed, but will draw
  warning messages that may be ignored.
 </para>

 <para>
  Finally, we can declare the data type:
<programlisting>
CREATE TYPE complex (
    internallength = 16,
    input = complex_in,
    output = complex_out
);
</programlisting>
 </para>

 <para>
  When you define a new base type,
  <productname>PostgreSQL</productname> automatically provides support
  for arrays of that
  type.<indexterm><primary>array</primary><secondary>of user-defined
  type</secondary></indexterm> For historical reasons, the array type
  has the same name as the base type with the underscore character
  (<literal>_</>) prepended.
 </para>

 <para>
  If the values of your data type might exceed a few hundred bytes in
  size (in internal form), you should mark them
  TOAST-able.<indexterm><primary>TOAST</primary><secondary>and
  user-defined types</secondary></indexterm> To do this, the internal
  representation must follow the standard layout for variable-length
  data: the first four bytes must be an <type>int32</type> containing
  the total length in bytes of the datum (including itself).  Also,
  when running the <command>CREATE TYPE</command> command, specify the
  internal length as <literal>variable</> and select the appropriate
  storage option.
 </para>

 <para>
  For further details see the description of the <command>CREATE
  TYPE</command> command in <xref linkend="reference">.
 </para>
</sect1>

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