xfunc.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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 <sect1 id="xfunc">
  <title>User-Defined Functions</title>

  <indexterm zone="xfunc">
   <primary>function</primary>
   <secondary>user-defined</secondary>
  </indexterm>

  <para>
   <productname>PostgreSQL</productname> provides four kinds of
   functions:

   <itemizedlist>
    <listitem>
     <para>
      query language functions (functions written in
      <acronym>SQL</acronym>) (<xref linkend="xfunc-sql">)
     </para>
    </listitem>
    <listitem>
     <para>
      procedural language functions (functions written in, for
      example, <application>PL/pgSQL</> or <application>PL/Tcl</>)
      (<xref linkend="xfunc-pl">)
     </para>
    </listitem>
    <listitem>
     <para>
      internal functions (<xref linkend="xfunc-internal">)
     </para>
    </listitem>
    <listitem>
     <para>
      C-language functions (<xref linkend="xfunc-c">)
     </para>
    </listitem>
   </itemizedlist>
  </para>

  <para>
   Every kind
   of  function  can take base types, composite types, or
   combinations of these as arguments (parameters). In addition,
   every kind of function can return a base type or
   a composite type.  Functions may also be defined to return
   sets of base or composite values.
  </para>

  <para>
   Many kinds of functions can take or return certain pseudo-types
   (such as polymorphic types), but the available facilities vary.
   Consult the description of each kind of function for more details.
  </para>

  <para>
   It's easiest to define <acronym>SQL</acronym>
   functions, so we'll start by discussing those.
   Most of the concepts presented for <acronym>SQL</acronym> functions
   will carry over to the other types of functions.
  </para>

  <para>
   Throughout this chapter, it can be useful to look at the reference
   page of the <xref linkend="sql-createfunction"
   endterm="sql-createfunction-title"> command to
   understand the examples better.  Some examples from this chapter
   can be found in <filename>funcs.sql</filename> and
   <filename>funcs.c</filename> in the <filename>src/tutorial</>
   directory in the <productname>PostgreSQL</productname> source
   distribution.
  </para>
  </sect1>

  <sect1 id="xfunc-sql">
   <title>Query Language (<acronym>SQL</acronym>) Functions</title>

   <indexterm zone="xfunc-sql">
    <primary>function</primary>
    <secondary>user-defined</secondary>
    <tertiary>in SQL</tertiary>
   </indexterm>

   <para>
    SQL functions execute an arbitrary list of SQL statements, returning
    the result of the last query in the list.
    In the simple (non-set)
    case, the first row of the last query's result will be returned.
    (Bear in mind that <quote>the first row</quote> of a multirow
    result is not well-defined unless you use <literal>ORDER BY</>.)
    If the last query happens
    to return no rows at all, the null value will be returned.
   </para>

   <para>
    <indexterm><primary>SETOF</><seealso>function</></> Alternatively,
    an SQL function may be declared to return a set, by specifying the
    function's return type as <literal>SETOF
    <replaceable>sometype</></literal>.  In this case all rows of the
    last query's result are returned.  Further details appear below.
   </para>

   <para>
    The body of an SQL function must be a list of SQL
    statements separated by semicolons.  A semicolon after the last
    statement is optional.  Unless the function is declared to return
    <type>void</>, the last statement must be a <command>SELECT</>.
   </para>

    <para>
     Any collection of commands in the  <acronym>SQL</acronym>
     language can be packaged together and defined as a function.
     Besides <command>SELECT</command> queries, the commands can include data
     modification queries (<command>INSERT</command>,
     <command>UPDATE</command>, and <command>DELETE</command>), as well as
     other SQL commands. (The only exception is that you can't put
     <command>BEGIN</>, <command>COMMIT</>, <command>ROLLBACK</>, or
     <command>SAVEPOINT</> commands into a <acronym>SQL</acronym> function.)
     However, the final command 
     must be a <command>SELECT</command> that returns whatever is
     specified as the function's return type.  Alternatively, if you
     want to define a SQL function that performs actions but has no
     useful value to return, you can define it as returning <type>void</>.
     In that case, the function body must not end with a <command>SELECT</command>.
     For example, this function removes rows with negative salaries from
     the <literal>emp</> table:

<screen>
CREATE FUNCTION clean_emp() RETURNS void AS '
    DELETE FROM emp
        WHERE salary &lt; 0;
' LANGUAGE SQL;

SELECT clean_emp();

 clean_emp
-----------

(1 row)
</screen>
    </para>

   <para>
    The syntax of the <command>CREATE FUNCTION</command> command requires
    the function body to be written as a string constant.  It is usually
    most convenient to use dollar quoting (see <xref
    linkend="sql-syntax-dollar-quoting">) for the string constant.
    If you choose to use regular single-quoted string constant syntax,
    you must escape single quote marks (<literal>'</>) and backslashes
    (<literal>\</>) used in the body of the function, typically by
    doubling them (see <xref linkend="sql-syntax-strings">).
   </para>

   <para>
    Arguments to the SQL function are referenced in the function
    body using the syntax <literal>$<replaceable>n</></>: <literal>$1</>
    refers to the first argument, <literal>$2</> to the second, and so on.
    If an argument is of a composite type, then the dot notation,
    e.g., <literal>$1.name</literal>, may be used to access attributes
    of the argument.  The arguments can only be used as data values,
    not as identifiers.  Thus for example this is reasonable:
<programlisting>
INSERT INTO mytable VALUES ($1);
</programlisting>
but this will not work:
<programlisting>
INSERT INTO $1 VALUES (42);
</programlisting>
   </para>

   <sect2 id="xfunc-sql-base-functions">
    <title><acronym>SQL</acronym> Functions on Base Types</title>

    <para>
     The simplest possible <acronym>SQL</acronym> function has no arguments and
     simply returns a base type, such as <type>integer</type>:
     
<screen>
CREATE FUNCTION one() RETURNS integer AS $$
    SELECT 1 AS result;
$$ LANGUAGE SQL;

-- Alternative syntax for string literal:
CREATE FUNCTION one() RETURNS integer AS '
    SELECT 1 AS result;
' LANGUAGE SQL;

SELECT one();

 one
-----
   1
</screen>
    </para>

    <para>
     Notice that we defined a column alias within the function body for the result of the function
     (with  the  name <literal>result</>),  but this column alias is not visible
     outside the function.  Hence,  the  result  is labeled <literal>one</>
     instead of <literal>result</>.
    </para>

    <para>
     It is almost as easy to define <acronym>SQL</acronym> functions  
     that take base types as arguments.  In the example below, notice
     how we refer to the arguments within the function as <literal>$1</>
     and <literal>$2</>.

<screen>
CREATE FUNCTION add_em(integer, integer) RETURNS integer AS $$
    SELECT $1 + $2;
$$ LANGUAGE SQL;

SELECT add_em(1, 2) AS answer;

 answer
--------
      3
</screen>
    </para>

    <para>
     Here is a more useful function, which might be used to debit a
     bank account:

<programlisting>
CREATE FUNCTION tf1 (integer, numeric) RETURNS integer AS $$
    UPDATE bank 
        SET balance = balance - $2
        WHERE accountno = $1;
    SELECT 1;
$$ LANGUAGE SQL;
</programlisting>

     A user could execute this function to debit account 17 by $100.00 as
     follows:

<programlisting>
SELECT tf1(17, 100.0);
</programlisting>
    </para>

    <para>
     In practice one would probably like a more useful result from the
     function than a constant 1, so a more likely definition
     is

<programlisting>
CREATE FUNCTION tf1 (integer, numeric) RETURNS numeric AS $$
    UPDATE bank 
        SET balance = balance - $2
        WHERE accountno = $1;
    SELECT balance FROM bank WHERE accountno = $1;
$$ LANGUAGE SQL;
</programlisting>

     which adjusts the balance and returns the new balance.
    </para>
   </sect2>

   <sect2>
    <title><acronym>SQL</acronym> Functions on Composite Types</title>

    <para>
     When writing  functions with arguments of composite
     types, we must  not  only  specify  which
     argument  we  want (as we did above with <literal>$1</> and <literal>$2</literal>) but
     also the desired attribute (field) of  that  argument.   For  example,
     suppose that 
     <type>emp</type> is a table containing employee data, and therefore
     also the name of the composite type of each row of the table.  Here
     is a function <function>double_salary</function> that computes what someone's
     salary would be if it were doubled:

<screen>
CREATE TABLE emp (
    name        text,
    salary      numeric,
    age         integer,
    cubicle     point
);

CREATE FUNCTION double_salary(emp) RETURNS numeric AS $$
    SELECT $1.salary * 2 AS salary;
$$ LANGUAGE SQL;

SELECT name, double_salary(emp.*) AS dream
    FROM emp
    WHERE emp.cubicle ~= point '(2,1)';

 name | dream
------+-------
 Bill |  8400
</screen>
    </para>

    <para>
     Notice the use of the syntax <literal>$1.salary</literal>
     to select one field of the argument row value.  Also notice
     how the calling <command>SELECT</> command uses <literal>*</>
     to select
     the entire current row of a table as a composite value.  The table
     row can alternatively be referenced using just the table name,
     like this:
<screen>
SELECT name, double_salary(emp) AS dream
    FROM emp
    WHERE emp.cubicle ~= point '(2,1)';
</screen>
     but this usage is deprecated since it's easy to get confused.
    </para>

    <para>
     Sometimes it is handy to construct a composite argument value
     on-the-fly.  This can be done with the <literal>ROW</> construct.
     For example, we could adjust the data being passed to the function:
<screen>
SELECT name, double_salary(ROW(name, salary*1.1, age, cubicle)) AS dream
    FROM emp;
</screen>
    </para>

    <para>
     It is also possible to build a function that returns a composite type.
     This is an example of a function 
     that returns a single <type>emp</type> row:

<programlisting>
CREATE FUNCTION new_emp() RETURNS emp AS $$
    SELECT text 'None' AS name,
        1000.0 AS salary,
        25 AS age,
        point '(2,2)' AS cubicle;
$$ LANGUAGE SQL;
</programlisting>

     In this example we have specified each of  the  attributes
     with  a  constant value, but any computation
     could have been substituted for these constants.
    </para>

    <para>
     Note two important things about defining the function:

     <itemizedlist>
      <listitem>
       <para>
        The select list order in the query must be exactly the same as
        that in which the columns appear in the table associated
        with the composite type.  (Naming the columns, as we did above,
        is irrelevant to the system.)
       </para>
      </listitem>
      <listitem>
       <para>
        You must typecast the expressions to match the
        definition of the composite type, or you will get errors like this:
<screen>
<computeroutput>
ERROR:  function declared to return emp returns varchar instead of text at column 1
</computeroutput>
</screen>
       </para>
      </listitem>
     </itemizedlist>
    </para>     

    <para>
     A different way to define the same function is:

<programlisting>
CREATE FUNCTION new_emp() RETURNS emp AS $$
    SELECT ROW('None', 1000.0, 25, '(2,2)')::emp;
$$ LANGUAGE SQL;
</programlisting>

     Here we wrote a <command>SELECT</> that returns just a single
     column of the correct composite type.  This isn't really better
     in this situation, but it is a handy alternative in some cases
     &mdash; for example, if we need to compute the result by calling