create_type.sgml

PostgreSQL 8.1.4的源码 适用于Linux下的开源数据库系统

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$PostgreSQL: pgsql/doc/src/sgml/ref/create_type.sgml,v 1.59 2005/11/01 21:09:50 tgl Exp $
PostgreSQL documentation
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<refentry id="SQL-CREATETYPE">
 <refmeta>
  <refentrytitle id="sql-createtype-title">CREATE TYPE</refentrytitle>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

 <refnamediv>
  <refname>CREATE TYPE</refname>
  <refpurpose>define a new data type</refpurpose>
 </refnamediv>

 <indexterm zone="sql-createtype">
  <primary>CREATE TYPE</primary>
 </indexterm>

 <refsynopsisdiv>
<synopsis>
CREATE TYPE <replaceable class="parameter">name</replaceable> AS
    ( <replaceable class="PARAMETER">attribute_name</replaceable> <replaceable class="PARAMETER">data_type</replaceable> [, ... ] )

CREATE TYPE <replaceable class="parameter">name</replaceable> (
    INPUT = <replaceable class="parameter">input_function</replaceable>,
    OUTPUT = <replaceable class="parameter">output_function</replaceable>
    [ , RECEIVE = <replaceable class="parameter">receive_function</replaceable> ]
    [ , SEND = <replaceable class="parameter">send_function</replaceable> ]
    [ , ANALYZE = <replaceable class="parameter">analyze_function</replaceable> ]
    [ , INTERNALLENGTH = { <replaceable class="parameter">internallength</replaceable> | VARIABLE } ]
    [ , PASSEDBYVALUE ]
    [ , ALIGNMENT = <replaceable class="parameter">alignment</replaceable> ]
    [ , STORAGE = <replaceable class="parameter">storage</replaceable> ]
    [ , DEFAULT = <replaceable class="parameter">default</replaceable> ]
    [ , ELEMENT = <replaceable class="parameter">element</replaceable> ]
    [ , DELIMITER = <replaceable class="parameter">delimiter</replaceable> ]
)
</synopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   <command>CREATE TYPE</command> registers a new data type for use in
   the current database.  The user who defines a type becomes its
   owner.
  </para>

  <para>
   If a schema name is given then the type is created in the specified
   schema.  Otherwise it is created in the current schema.  The type
   name must be distinct from the name of any existing type or domain
   in the same schema.  (Because tables have associated data types,
   the type name must also be distinct from the name of any existing
   table in the same schema.)
  </para>

  <refsect2>
   <title>Composite Types</title>

  <para>
   The first form of <command>CREATE TYPE</command>
   creates a composite type.
   The composite type is specified by a list of attribute names and data types.
   This is essentially the same as the row type
   of a table, but using <command>CREATE TYPE</command> avoids the need to
   create an actual table when all that is wanted is to define a type.
   A stand-alone composite type is useful as the argument or return type of a
   function.
  </para>
  </refsect2>

  <refsect2>
   <title>Base Types</title>

  <para>
   The second form of <command>CREATE TYPE</command> creates a new base type
   (scalar type).  The parameters may appear in any order, not only that
   illustrated above, and most are optional.  You must register
   two or more functions (using <command>CREATE FUNCTION</command>) before
   defining the type.  The support functions 
   <replaceable class="parameter">input_function</replaceable> and
   <replaceable class="parameter">output_function</replaceable>
   are required, while the functions
   <replaceable class="parameter">receive_function</replaceable>,
   <replaceable class="parameter">send_function</replaceable> and
   <replaceable class="parameter">analyze_function</replaceable>
   are optional.  Generally these functions have to be coded in C
   or another low-level language.
  </para>

  <para>
   The <replaceable class="parameter">input_function</replaceable>
   converts the type's external textual representation to the internal
   representation  used by the
   operators and functions defined for the type.
   <replaceable class="parameter">output_function</replaceable>
   performs the reverse transformation.  The input function may be
   declared as taking one argument of type <type>cstring</type>,
   or as taking three arguments of types
   <type>cstring</type>, <type>oid</type>, <type>integer</type>.
   The first argument is the input text as a C string, the second
   argument is the type's own OID (except for array types, which instead
   receive their element type's OID),
   and the third is the <literal>typmod</> of the destination column, if known
   (-1 will be passed if not).
   The input function must return a value of the data type itself.
   The output function must be
   declared as taking one argument of the new data type.
   The output function must return type <type>cstring</type>.
  </para>

  <para>
   The optional <replaceable class="parameter">receive_function</replaceable>
   converts the type's external binary representation to the internal
   representation.  If this function is not supplied, the type cannot
   participate in binary input.  The binary representation should be
   chosen to be cheap to convert to internal form, while being reasonably
   portable.  (For example, the standard integer data types use network
   byte order as the external binary representation, while the internal
   representation is in the machine's native byte order.)  The receive
   function should perform adequate checking to ensure that the value is
   valid.
   The receive function may be declared as taking one argument of type
   <type>internal</type>, or as taking three arguments of types
   <type>internal</type>, <type>oid</type>, <type>integer</type>.
   The first argument is a pointer to a <type>StringInfo</type> buffer
   holding the received byte string; the optional arguments are the
   same as for the text input function.
   The receive function must return a value of the data type itself.
   Similarly, the optional
   <replaceable class="parameter">send_function</replaceable> converts
   from the internal representation to the external binary representation.
   If this function is not supplied, the type cannot participate in binary
   output.  The send function must be
   declared as taking one argument of the new data type.
   The send function must return type <type>bytea</type>.
  </para>

  <para>
   You should at this point be wondering how the input and output functions
   can be declared to have results or arguments of the new type, when they have
   to be created before the new type can be created.  The answer is that the
   input function must be created first, then the output function (and
   the binary I/O functions if wanted), and finally the data type.
   <productname>PostgreSQL</productname> will first see the name of the new
   data type as the return type of the input function.  It will create a
   <quote>shell</> type, which is simply a placeholder entry in
   the system catalog, and link the input function definition to the shell
   type.  Similarly the other functions will be linked to the (now already
   existing) shell type.  Finally, <command>CREATE TYPE</> replaces the
   shell entry with a complete type definition, and the new type can be used.
  </para>

  <para>
   The optional <replaceable class="parameter">analyze_function</replaceable>
   performs type-specific statistics collection for columns of the data type.
   By default, <command>ANALYZE</> will attempt to gather statistics using
   the type's <quote>equals</> and <quote>less-than</> operators, if there
   is a default b-tree operator class for the type.  For non-scalar types
   this behavior is likely to be unsuitable, so it can be overridden by
   specifying a custom analysis function.  The analysis function must be
   declared to take a single argument of type <type>internal</>, and return
   a <type>boolean</> result.  The detailed API for analysis functions appears
   in <filename>src/include/commands/vacuum.h</>.
  </para>

  <para>
   While the details of the new type's internal representation are only
   known to the I/O functions and other functions you create to work with
   the type, there are several properties of the internal representation
   that must be declared to <productname>PostgreSQL</productname>.
   Foremost of these is
   <replaceable class="parameter">internallength</replaceable>.
   Base data types can be fixed-length, in which case
   <replaceable class="parameter">internallength</replaceable> is a
   positive integer, or variable  length, indicated by setting
   <replaceable class="parameter">internallength</replaceable>
   to <literal>VARIABLE</literal>.  (Internally, this is represented
   by setting <literal>typlen</> to -1.)  The internal representation of all
   variable-length types must start with a 4-byte integer giving the total
   length of this value of the type.
  </para>

  <para>
   The optional flag <literal>PASSEDBYVALUE</literal> indicates that
   values of this data type are passed by value, rather than by
   reference.  You may not pass by value types whose internal
   representation is larger than the size of the <type>Datum</> type
   (4 bytes on most machines, 8 bytes on a few).
  </para>

  <para>
   The <replaceable class="parameter">alignment</replaceable> parameter
   specifies the storage alignment required for the data type.  The
   allowed values equate to alignment on 1, 2, 4, or 8 byte boundaries.
   Note that variable-length types must have an alignment of at least
   4, since they necessarily contain an <type>int4</> as their first component.
  </para>

  <para>
   The <replaceable class="parameter">storage</replaceable> parameter
   allows selection of storage strategies for variable-length data
   types.  (Only <literal>plain</literal> is allowed for fixed-length
   types.)  <literal>plain</literal> specifies that data of the type
   will always be stored in-line and not compressed.
   <literal>extended</literal> specifies that the system will first
   try to compress a long data value, and will move the value out of
   the main table row if it's still too long.
   <literal>external</literal> allows the value to be moved out of the
   main table, but the system will not try to compress it.
   <literal>main</literal> allows compression, but discourages moving
   the value out of the main table.  (Data items with this storage
   strategy may still be moved out of the main table if there is no
   other way to make a row fit, but they will be kept in the main
   table preferentially over <literal>extended</literal> and
   <literal>external</literal> items.)
  </para>

  <para>
   A default value may be specified, in case a user wants columns of the
   data type to default to something other than the null value.
   Specify the default with the <literal>DEFAULT</literal> key word.
   (Such a default may be overridden by an explicit <literal>DEFAULT</literal>
   clause attached to a particular column.)
  </para>

  <para>
   To indicate that a type is an array, specify the type of the array
   elements using the <literal>ELEMENT</> key word.  For example, to
   define an array of 4-byte integers (<type>int4</type>), specify
   <literal>ELEMENT = int4</literal>. More details about array types
   appear below.
  </para>

  <para>
   To indicate the delimiter to be used between values in the external
   representation of arrays of this type, <replaceable
   class="parameter">delimiter</replaceable> can be
   set to a specific character.  The default delimiter is the comma
   (<literal>,</literal>).  Note that the delimiter is associated
   with the array element type, not the array type itself.
  </para>

  </refsect2>

  <refsect2>
   <title>Array Types</title>

   <para>
    Whenever a user-defined base data type is created, 
    <productname>PostgreSQL</productname> automatically creates an
    associated array type, whose name consists of the base type's
    name prepended with an underscore.  The parser understands this
    naming convention, and translates requests for columns of type
    <literal>foo[]</> into requests for type <literal>_foo</>.
    The implicitly-created array type is variable length and uses the
    built-in input and output functions <literal>array_in</> and
    <literal>array_out</>.
   </para>

   <para>
    You might reasonably ask why there is an <option>ELEMENT</>
    option, if the system makes the correct array type automatically.
    The only case where it's useful to use <option>ELEMENT</> is when you are
    making a fixed-length type that happens to be internally an array of a number of
    identical things, and you want to allow these things to be accessed
    directly by subscripting, in addition to whatever operations you plan
    to provide for the type as a whole.  For example, type <type>name</>
    allows its constituent <type>char</> elements to be accessed this way.
    A 2-D <type>point</> type could allow its two component numbers to be
    accessed like <literal>point[0]</> and <literal>point[1]</>.
    Note that
    this facility only works for fixed-length types whose internal form
    is exactly a sequence of identical fixed-length fields.  A subscriptable
    variable-length type must have the generalized internal representation
    used by <literal>array_in</> and <literal>array_out</>.
    For historical reasons (i.e., this is clearly wrong but it's far too
    late to change it), subscripting of fixed-length array types starts from
    zero, rather than from one as for variable-length arrays.
   </para>
  </refsect2>