postgresql/doc/src/sgml/xtypes.sgml

<!-- doc/src/sgml/xtypes.sgml -->

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

  <indexterm zone="xtypes">
   <primary>data type</primary>
   <secondary>user-defined</secondary>
  </indexterm>

  <para>
   As described in <xref linkend="extend-type-system"/>,
   <productname>PostgreSQL</productname> can be extended to support new
   data types.  This section describes how to define new base types,
   which are data types defined below the level of the <acronym>SQL</acronym>
   language.  Creating a new base type requires implementing functions
   to operate on the type in a low-level language, usually C.
  </para>

  <para>
   The examples in this section can be found in
   <filename>complex.sql</filename> and <filename>complex.c</filename>
   in the <filename>src/tutorial</filename> directory of the source distribution.
   See the <filename>README</filename> file in that directory for instructions
   about running the examples.
  </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.  If we want to do anything more with the type than merely
  store it, we must provide additional functions to implement whatever
  operations we'd like to have for the type.
 </para>

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

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

  We will need to make this a pass-by-reference type, since it's too
  large to fit into a single <type>Datum</type> value.
 </para>

 <para>
  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><![CDATA[
PG_FUNCTION_INFO_V1(complex_in);

Datum
complex_in(PG_FUNCTION_ARGS)
{
    char       *str = PG_GETARG_CSTRING(0);
    double      x,
                y;
    Complex    *result;

    if (sscanf(str, " ( %lf , %lf )", &x, &y) != 2)
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                 errmsg("invalid input syntax for type %s: \"%s\"",
                        "complex", str)));

    result = (Complex *) palloc(sizeof(Complex));
    result->x = x;
    result->y = y;
    PG_RETURN_POINTER(result);
}
]]>
</programlisting>

  The output function can simply be:

<programlisting><![CDATA[
PG_FUNCTION_INFO_V1(complex_out);

Datum
complex_out(PG_FUNCTION_ARGS)
{
    Complex    *complex = (Complex *) PG_GETARG_POINTER(0);
    char       *result;

    result = psprintf("(%g,%g)", complex->x, complex->y);
    PG_RETURN_CSTRING(result);
}
]]>
</programlisting>
 </para>

 <para>
  You should be careful 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>
  Optionally, a user-defined type can provide binary input and output
  routines.  Binary I/O is normally faster but less portable than textual
  I/O.  As with textual I/O, it is up to you to define exactly what the
  external binary representation is.  Most of the built-in data types
  try to provide a machine-independent binary representation.  For
  <type>complex</type>, we will piggy-back on the binary I/O converters
  for type <type>float8</type>:

<programlisting><![CDATA[
PG_FUNCTION_INFO_V1(complex_recv);

Datum
complex_recv(PG_FUNCTION_ARGS)
{
    StringInfo  buf = (StringInfo) PG_GETARG_POINTER(0);
    Complex    *result;

    result = (Complex *) palloc(sizeof(Complex));
    result->x = pq_getmsgfloat8(buf);
    result->y = pq_getmsgfloat8(buf);
    PG_RETURN_POINTER(result);
}

PG_FUNCTION_INFO_V1(complex_send);

Datum
complex_send(PG_FUNCTION_ARGS)
{
    Complex    *complex = (Complex *) PG_GETARG_POINTER(0);
    StringInfoData buf;

    pq_begintypsend(&buf);
    pq_sendfloat8(&buf, complex->x);
    pq_sendfloat8(&buf, complex->y);
    PG_RETURN_BYTEA_P(pq_endtypsend(&buf));
}
]]>
</programlisting>
 </para>

 <para>
  Once we have written the I/O functions and compiled them into a shared
  library, we can define the <type>complex</type> type in SQL.
  First we declare it as a shell type:

<programlisting>
CREATE TYPE complex;
</programlisting>

  This serves as a placeholder that allows us to reference the type while
  defining its I/O functions.  Now we can define the I/O functions:

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

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

CREATE FUNCTION complex_recv(internal)
   RETURNS complex
   AS '<replaceable>filename</replaceable>'
   LANGUAGE C IMMUTABLE STRICT;

CREATE FUNCTION complex_send(complex)
   RETURNS bytea
   AS '<replaceable>filename</replaceable>'
   LANGUAGE C IMMUTABLE STRICT;
</programlisting>
 </para>

 <para>
  Finally, we can provide the full definition of the data type:
<programlisting>
CREATE TYPE complex (
   internallength = 16,
   input = complex_in,
   output = complex_out,
   receive = complex_recv,
   send = complex_send,
   alignment = double
);
</programlisting>
 </para>

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

 <para>
  Once the data type exists, we can declare additional functions to
  provide useful operations on the data type.  Operators can then be
  defined atop the functions, and if needed, operator classes can be
  created to support indexing of the data type.  These additional
  layers are discussed in following sections.
 </para>

 <para>
  If the internal representation of the data type is variable-length, the
  internal representation must follow the standard layout for variable-length
  data: the first four bytes must be a <type>char[4]</type> field which is
  never accessed directly (customarily named <structfield>vl_len_</structfield>). You
  must use the <function>SET_VARSIZE()</function> macro to store the total
  size of the datum (including the length field itself) in this field
  and <function>VARSIZE()</function> to retrieve it.  (These macros exist
  because the length field may be encoded depending on platform.)
 </para>

 <para>
  For further details see the description of the
  <xref linkend="sql-createtype"/> command.
 </para>

 <sect2 id="xtypes-toast">
  <title>TOAST Considerations</title>
   <indexterm>
    <primary>TOAST</primary>
    <secondary>and user-defined types</secondary>
   </indexterm>

 <para>
  If the values of your data type vary in size (in internal form), it's
  usually desirable to make the data type <acronym>TOAST</acronym>-able (see <xref
  linkend="storage-toast"/>). You should do this even if the values are always
  too small to be compressed or stored externally, because
  <acronym>TOAST</acronym> can save space on small data too, by reducing header
  overhead.
 </para>

 <para>
  To support <acronym>TOAST</acronym> storage, the C functions operating on the data
  type must always be careful to unpack any toasted values they are handed
  by using <function>PG_DETOAST_DATUM</function>.  (This detail is customarily hidden
  by defining type-specific <function>GETARG_DATATYPE_P</function> macros.)
  Then, when running the <command>CREATE TYPE</command> command, specify the
  internal length as <literal>variable</literal> and select some appropriate storage
  option other than <literal>plain</literal>.
 </para>

 <para>
  If data alignment is unimportant (either just for a specific function or
  because the data type specifies byte alignment anyway) then it's possible
  to avoid some of the overhead of <function>PG_DETOAST_DATUM</function>. You can use
  <function>PG_DETOAST_DATUM_PACKED</function> instead (customarily hidden by
  defining a <function>GETARG_DATATYPE_PP</function> macro) and using the macros
  <function>VARSIZE_ANY_EXHDR</function> and <function>VARDATA_ANY</function> to access
  a potentially-packed datum.
  Again, the data returned by these macros is not aligned even if the data
  type definition specifies an alignment. If the alignment is important you
  must go through the regular <function>PG_DETOAST_DATUM</function> interface.
 </para>

 <note>
  <para>
   Older code frequently declares <structfield>vl_len_</structfield> as an
   <type>int32</type> field instead of <type>char[4]</type>.  This is OK as long as
   the struct definition has other fields that have at least <type>int32</type>
   alignment.  But it is dangerous to use such a struct definition when
   working with a potentially unaligned datum; the compiler may take it as
   license to assume the datum actually is aligned, leading to core dumps on
   architectures that are strict about alignment.
  </para>
 </note>

 <para>
  Another feature that's enabled by <acronym>TOAST</acronym> support is the
  possibility of having an <firstterm>expanded</firstterm> in-memory data
  representation that is more convenient to work with than the format that
  is stored on disk.  The regular or <quote>flat</quote> varlena storage format
  is ultimately just a blob of bytes; it cannot for example contain
  pointers, since it may get copied to other locations in memory.
  For complex data types, the flat format may be quite expensive to work
  with, so <productname>PostgreSQL</productname> provides a way to <quote>expand</quote>
  the flat format into a representation that is more suited to computation,
  and then pass that format in-memory between functions of the data type.
 </para>

 <para>
  To use expanded storage, a data type must define an expanded format that
  follows the rules given in <filename>src/include/utils/expandeddatum.h</filename>,
  and provide functions to <quote>expand</quote> a flat varlena value into
  expanded format and <quote>flatten</quote> the expanded format back to the
  regular varlena representation.  Then ensure that all C functions for
  the data type can accept either representation, possibly by converting
  one into the other immediately upon receipt.  This does not require fixing
  all existing functions for the data type at once, because the standard
  <function>PG_DETOAST_DATUM</function> macro is defined to convert expanded inputs
  into regular flat format.  Therefore, existing functions that work with
  the flat varlena format will continue to work, though slightly
  inefficiently, with expanded inputs; they need not be converted until and
  unless better performance is important.
 </para>

 <para>
  C functions that know how to work with an expanded representation
  typically fall into two categories: those that can only handle expanded
  format, and those that can handle either expanded or flat varlena inputs.
  The former are easier to write but may be less efficient overall, because
  converting a flat input to expanded form for use by a single function may
  cost more than is saved by operating on the expanded format.
  When only expanded format need be handled, conversion of flat inputs to
  expanded form can be hidden inside an argument-fetching macro, so that
  the function appears no more complex than one working with traditional
  varlena input.
  To handle both types of input, write an argument-fetching function that
  will detoast external, short-header, and compressed varlena inputs, but
  not expanded inputs.  Such a function can be defined as returning a
  pointer to a union of the flat varlena format and the expanded format.
  Callers can use the <function>VARATT_IS_EXPANDED_HEADER()</function> macro to
  determine which format they received.
 </para>

 <para>
  The <acronym>TOAST</acronym> infrastructure not only allows regular varlena
  values to be distinguished from expanded values, but also
  distinguishes <quote>read-write</quote> and <quote>read-only</quote> pointers to
  expanded values.  C functions that only need to examine an expanded
  value, or will only change it in safe and non-semantically-visible ways,
  need not care which type of pointer they receive.  C functions that
  produce a modified version of an input value are allowed to modify an
  expanded input value in-place if they receive a read-write pointer, but
  must not modify the input if they receive a read-only pointer; in that
  case they have to copy the value first, producing a new value to modify.
  A C function that has constructed a new expanded value should always
  return a read-write pointer to it.  Also, a C function that is modifying
  a read-write expanded value in-place should take care to leave the value
  in a sane state if it fails partway through.
 </para>

 <para>
  For examples of working with expanded values, see the standard array
  infrastructure, particularly
  <filename>src/backend/utils/adt/array_expanded.c</filename>.
 </para>

 </sect2>

</sect1>