xfunc.sgml

PostgreSQL 8.1.4的源码 适用于Linux下的开源数据库系统

     another function that returns the desired composite value.
    </para>     

    <para>
     We could call this function directly in either of two ways:

<screen>
SELECT new_emp();

         new_emp
--------------------------
 (None,1000.0,25,"(2,2)")

SELECT * FROM new_emp();

 name | salary | age | cubicle
------+--------+-----+---------
 None | 1000.0 |  25 | (2,2)
</screen>

     The second way is described more fully in <xref
     linkend="xfunc-sql-table-functions">.
    </para>     

    <para>
     When you use a function that returns a composite type,
     you might want only one field (attribute) from its result.
     You can do that with syntax like this:

<screen>
SELECT (new_emp()).name;

 name
------
 None
</screen>

     The extra parentheses are needed to keep the parser from getting
     confused.  If you try to do it without them, you get something like this:

<screen>
SELECT new_emp().name;
ERROR:  syntax error at or near "." at character 17
LINE 1: SELECT new_emp().name;
                        ^
</screen>
    </para>

    <para>
     Another option is to use
     functional notation for extracting an attribute.  The  simple  way 
     to explain this is that we can use the
     notations <literal>attribute(table)</>  and  <literal>table.attribute</>
     interchangeably.

<screen>
SELECT name(new_emp());

 name
------
 None
</screen>

<screen>
-- This is the same as:
-- SELECT emp.name AS youngster FROM emp WHERE emp.age &lt; 30;

SELECT name(emp) AS youngster FROM emp WHERE age(emp) &lt; 30;

 youngster
-----------
 Sam
 Andy
</screen>
    </para>

    <tip>
     <para>
      The equivalence between functional notation and attribute notation
      makes it possible to use functions on composite types to emulate
      <quote>computed fields</>.
      <indexterm>
       <primary>computed field</primary>
      </indexterm>
      <indexterm>
       <primary>field</primary>
       <secondary>computed</secondary>
      </indexterm>
      For example, using the previous definition
      for <literal>double_salary(emp)</>, we can write

<screen>
SELECT emp.name, emp.double_salary FROM emp;
</screen>

      An application using this wouldn't need to be directly aware that
      <literal>double_salary</> isn't a real column of the table.
      (You can also emulate computed fields with views.)
     </para>
    </tip>

    <para>
     Another way to use a function returning a composite type is to pass the
     result to another function that accepts the correct row type as input:

<screen>
CREATE FUNCTION getname(emp) RETURNS text AS $$
    SELECT $1.name;
$$ LANGUAGE SQL;

SELECT getname(new_emp());
 getname
---------
 None
(1 row)
</screen>
    </para>     

    <para>
     Still another way to use a function that returns a composite type is to
     call it as a table function, as described in <xref
     linkend="xfunc-sql-table-functions">.
    </para>
   </sect2>

   <sect2 id="xfunc-output-parameters">
    <title>Functions with Output Parameters</title>

   <indexterm>
    <primary>function</primary>
    <secondary>output parameter</secondary>
   </indexterm>

    <para>
     An alternative way of describing a function's results is to define it
     with <firstterm>output parameters</>, as in this example:

<screen>
CREATE FUNCTION add_em (IN x int, IN y int, OUT sum int)
AS 'SELECT $1 + $2'
LANGUAGE SQL;

SELECT add_em(3,7);
 add_em
--------
     10
(1 row)
</screen>

     This is not essentially different from the version of <literal>add_em</>
     shown in <xref linkend="xfunc-sql-base-functions">.  The real value of
     output parameters is that they provide a convenient way of defining
     functions that return several columns.  For example,

<screen>
CREATE FUNCTION sum_n_product (x int, y int, OUT sum int, OUT product int)
AS 'SELECT $1 + $2, $1 * $2'
LANGUAGE SQL;

 SELECT * FROM sum_n_product(11,42);
 sum | product
-----+---------
  53 |     462
(1 row)
</screen>

     What has essentially happened here is that we have created an anonymous
     composite type for the result of the function.  The above example has
     the same end result as

<screen>
CREATE TYPE sum_prod AS (sum int, product int);

CREATE FUNCTION sum_n_product (int, int) RETURNS sum_prod
AS 'SELECT $1 + $2, $1 * $2'
LANGUAGE SQL;
</screen>

     but not having to bother with the separate composite type definition
     is often handy.
    </para>

    <para>
     Notice that output parameters are not included in the calling argument
     list when invoking such a function from SQL.  This is because
     <productname>PostgreSQL</productname> considers only the input
     parameters to define the function's calling signature.  That means
     also that only the input parameters matter when referencing the function
     for purposes such as dropping it.  We could drop the above function
     with either of

<screen>
DROP FUNCTION sum_n_product (x int, y int, OUT sum int, OUT product int);
DROP FUNCTION sum_n_product (int, int);
</screen>
    </para>

    <para>
     Parameters can be marked as <literal>IN</> (the default),
     <literal>OUT</>, or <literal>INOUT</>.  An <literal>INOUT</>
     parameter serves as both an input parameter (part of the calling
     argument list) and an output parameter (part of the result record type).
    </para>
   </sect2>

   <sect2 id="xfunc-sql-table-functions">
    <title><acronym>SQL</acronym> Functions as Table Sources</title>

    <para>
     All SQL functions may be used in the <literal>FROM</> clause of a query,
     but it is particularly useful for functions returning composite types.
     If the function is defined to return a base type, the table function
     produces a one-column table.  If the function is defined to return
     a composite type, the table function produces a column for each attribute
     of the composite type.
    </para>

    <para>
     Here is an example:

<screen>
CREATE TABLE foo (fooid int, foosubid int, fooname text);
INSERT INTO foo VALUES (1, 1, 'Joe');
INSERT INTO foo VALUES (1, 2, 'Ed');
INSERT INTO foo VALUES (2, 1, 'Mary');

CREATE FUNCTION getfoo(int) RETURNS foo AS $$
    SELECT * FROM foo WHERE fooid = $1;
$$ LANGUAGE SQL;

SELECT *, upper(fooname) FROM getfoo(1) AS t1;

 fooid | foosubid | fooname | upper
-------+----------+---------+-------
     1 |        1 | Joe     | JOE
(1 row)
</screen>

     As the example shows, we can work with the columns of the function's
     result just the same as if they were columns of a regular table.
    </para>

    <para>
     Note that we only got one row out of the function.  This is because
     we did not use <literal>SETOF</>.  That is described in the next section.
    </para>
   </sect2>

   <sect2>
    <title><acronym>SQL</acronym> Functions Returning Sets</title>

    <para>
     When an SQL function is declared as returning <literal>SETOF
     <replaceable>sometype</></literal>, the function's final
     <command>SELECT</> query is executed to completion, and each row it
     outputs is returned as an element of the result set.
    </para>

    <para>
     This feature is normally used when calling the function in the <literal>FROM</>
     clause.  In this case each row returned by the function becomes
     a row of the table seen by the query.  For example, assume that
     table <literal>foo</> has the same contents as above, and we say:

<programlisting>
CREATE FUNCTION getfoo(int) RETURNS SETOF foo AS $$
    SELECT * FROM foo WHERE fooid = $1;
$$ LANGUAGE SQL;

SELECT * FROM getfoo(1) AS t1;
</programlisting>

     Then we would get:
<screen>
 fooid | foosubid | fooname
-------+----------+---------
     1 |        1 | Joe
     1 |        2 | Ed
(2 rows)
</screen>
    </para>

    <para>
     Currently, functions returning sets may also be called in the select list
     of a query.  For each row that the query
     generates by itself, the function returning set is invoked, and an output
     row is generated for each element of the function's result set. Note,
     however, that this capability is deprecated and may be removed in future
     releases. The following is an example function returning a set from the
     select list:

<screen>
CREATE FUNCTION listchildren(text) RETURNS SETOF text AS $$
    SELECT name FROM nodes WHERE parent = $1
$$ LANGUAGE SQL;

SELECT * FROM nodes;
   name    | parent
-----------+--------
 Top       |
 Child1    | Top
 Child2    | Top
 Child3    | Top
 SubChild1 | Child1
 SubChild2 | Child1
(6 rows)

SELECT listchildren('Top');
 listchildren
--------------
 Child1
 Child2
 Child3
(3 rows)

SELECT name, listchildren(name) FROM nodes;
  name  | listchildren
--------+--------------
 Top    | Child1
 Top    | Child2
 Top    | Child3
 Child1 | SubChild1
 Child1 | SubChild2
(5 rows)
</screen>

     In the last <command>SELECT</command>,
     notice that no output row appears for <literal>Child2</>, <literal>Child3</>, etc.
     This happens because <function>listchildren</function> returns an empty set
     for those arguments, so no result rows are generated.
    </para>
   </sect2>

   <sect2>
    <title>Polymorphic <acronym>SQL</acronym> Functions</title>

    <para>
     <acronym>SQL</acronym> functions may be declared to accept and
     return the polymorphic types <type>anyelement</type> and
     <type>anyarray</type>.  See <xref
     linkend="extend-types-polymorphic"> for a more detailed
     explanation of polymorphic functions. Here is a polymorphic
     function <function>make_array</function> that builds up an array
     from two arbitrary data type elements:
<screen>
CREATE FUNCTION make_array(anyelement, anyelement) RETURNS anyarray AS $$
    SELECT ARRAY[$1, $2];
$$ LANGUAGE SQL;

SELECT make_array(1, 2) AS intarray, make_array('a'::text, 'b') AS textarray;
 intarray | textarray
----------+-----------
 {1,2}    | {a,b}
(1 row)
</screen>
    </para>

    <para>
     Notice the use of the typecast <literal>'a'::text</literal>
     to specify that the argument is of type <type>text</type>. This is
     required if the argument is just a string literal, since otherwise
     it would be treated as type
     <type>unknown</type>, and array of <type>unknown</type> is not a valid
     type.
     Without the typecast, you will get errors like this:
<screen>
<computeroutput>
ERROR:  could not determine "anyarray"/"anyelement" type because input has type "unknown"
</computeroutput>
</screen>
    </para>

    <para>
     It is permitted to have polymorphic arguments with a fixed
     return type, but the converse is not. For example:
<screen>
CREATE FUNCTION is_greater(anyelement, anyelement) RETURNS boolean AS $$
    SELECT $1 &gt; $2;
$$ LANGUAGE SQL;

SELECT is_greater(1, 2);
 is_greater
------------