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 <chapter id="xfunc">
  <title id="xfunc-title">Extending <acronym>SQL</acronym>: Functions</title>

  <indexterm zone="xfunc"><primary>function</></>

  <sect1 id="xfunc-intro">
   <title>Introduction</title>

  <comment>
   Historically, functions were perhaps considered a tool for creating
   types.  Today, few people build their own types but many write
   their own functions.  This introduction ought to be changed to
   reflect this.
  </comment>

  <para>
   As  it  turns  out,  part of defining a new type is the
   definition of functions  that  describe  its  behavior.
   Consequently,  while  it  is  possible  to define a new
   function without defining a new type,  the  reverse  is
   not  true.   We therefore describe how to add new functions 
   to <productname>Postgres</productname> before  describing  
   how  to  add  new types.
  </para>

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

   <itemizedlist>
    <listitem>
     <para>
      query language functions 
      (functions written in <acronym>SQL</acronym>)
     </para>
    </listitem>
    <listitem>
     <para>
      procedural language 
      functions (functions written in, for example, <application>PL/Tcl</> or <application>PL/pgSQL</>)
     </para>
    </listitem>
    <listitem>
     <para>
      internal functions
     </para>
    </listitem>
    <listitem>
     <para>
      C language functions
     </para>
    </listitem>
   </itemizedlist>
  </para>

  <para>
   Every kind
   of  function  can take a base type, a composite type or
   some combination as arguments (parameters).   In  addition, 
   every kind of function can return a base type or
   a composite type.  It's easiest to define <acronym>SQL</acronym> 
   functions, so we'll start with those.  Examples in this section 
   can also be found in <filename>funcs.sql</filename> 
   and <filename>funcs.c</filename> in the tutorial directory.
  </para>
  </sect1>

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

   <indexterm zone="xfunc-sql"><primary>function</><secondary>SQL</></>

   <para>
    SQL functions execute an arbitrary list of SQL statements, returning
    the result of the last query in the list, which must be a
    <literal>SELECT</>.
    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 multi-row
    result is not well-defined unless you use <literal>ORDER BY</>.)
    If the last query happens
    to return no rows at all, NULL 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</literal> <replaceable>sometype</>.  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 should be a list of one or more SQL
    statements separated by semicolons.  Note that because the syntax
    of the <command>CREATE FUNCTION</command> command requires the body of the
    function to be enclosed in single quotes, single quote marks
    (<literal>'</>) used
    in the body of the function must be escaped, by writing two single
    quotes (<literal>''</>) or a backslash (<literal>\'</>) where each
    quote is desired.
   </para>

   <para>
    Arguments to the SQL function may be referenced in the function
    body using the syntax <literal>$<replaceable>n</></>: $1 refers to
    the first argument, $2 to the second, and so on.  If an argument
    is of a composite type, then the <quote>dot notation</quote>,
    e.g., <literal>$1.emp</literal>, may be used to access attributes
    of the argument.
   </para>

   <sect2>
    <title>Examples</title>

    <para>
     To illustrate a simple SQL function, consider the following,
     which might be used to debit a bank account:

<programlisting>
CREATE FUNCTION tp1 (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 tp1(17, 100.0);
</programlisting>
    </para>

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

<programlisting>
CREATE FUNCTION tp1 (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>

    <para>
     Any collection of commands in the  <acronym>SQL</acronym>
     language can be packaged together and defined as a function.
     The commands can include data modification (i.e.,
     <command>INSERT</command>, <command>UPDATE</command>, and
     <command>DELETE</command>) as well
     as <command>SELECT</command> queries.  However, the final command 
     must be a <command>SELECT</command> that returns whatever is
     specified as the function's return type.

<programlisting>
CREATE FUNCTION clean_EMP () RETURNS integer AS '
    DELETE FROM EMP 
        WHERE EMP.salary &lt;= 0;
    SELECT 1 AS ignore_this;
' LANGUAGE SQL;

SELECT clean_EMP();
</programlisting>

<screen>
 x
---
 1
</screen>
    </para>
   </sect2>

   <sect2>
    <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>:
     
<programlisting>
CREATE FUNCTION one() RETURNS integer AS '
    SELECT 1 as RESULT;
' LANGUAGE SQL;

SELECT one();
</programlisting>

<screen>
 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 labelled <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</>:

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

SELECT add_em(1, 2) AS answer;
</programlisting>

<screen>
 answer
--------
      3
</screen>
    </para>
   </sect2>

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

    <para>
     When  specifying  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 attributes 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 your
     salary would be if it were doubled:

<programlisting>
CREATE FUNCTION double_salary(EMP) RETURNS integer AS '
    SELECT $1.salary * 2 AS salary;
' LANGUAGE SQL;

SELECT name, double_salary(EMP) AS dream
    FROM EMP
    WHERE EMP.cubicle ~= point '(2,1)';
</programlisting>

<screen>
 name | dream
------+-------
 Sam  |  2400
</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 SELECT command uses a table name to denote
     the entire current row of that table as a composite value.
    </para>

    <para>
     It is also possible to build a function that returns a composite type.
     (However, as we'll see below, there are some
     unfortunate restrictions on how the function may be used.)
     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 AS salary,
        25 AS age,
        point ''(2,2)'' AS cubicle;
' LANGUAGE SQL;
</programlisting>
    </para>

    <para>
     In this case we have specified each of  the  attributes
     with  a  constant value, but any computation or expression 
     could have been substituted for these constants.
     Note two important things about defining the function:

     <itemizedlist>
      <listitem>
       <para>
	The  target  list  order must be exactly the same as
	that in which the columns appear in the table associated
	with the composite type.
       </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>
     In the present release of <productname>PostgreSQL</productname>
     there are some unpleasant restrictions on how functions returning
     composite types can be used.  Briefly, when calling a function that
     returns a row, we cannot retrieve the entire row.  We must either
     project a single attribute out of the row or pass the entire row into
     another function.  (Trying to display the entire row value will yield
     a meaningless number.)  For example,

<programlisting>
SELECT name(new_emp());
</programlisting>

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

    <para>
     This example makes use of the
     function notation for projecting attributes.  The  simple  way 
     to explain this is that we can usually use the
     notations <literal>attribute(table)</>  and  <literal>table.attribute</>
     interchangeably:

<programlisting>
--
-- 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;
</programlisting>

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

    <para>
	The reason why, in general, we must use the function
        syntax  for projecting attributes of function return
        values is that the parser  just  doesn't  understand
        the  dot syntax for projection when combined
        with function calls.

<screen>
SELECT new_emp().name AS nobody;
ERROR:  parser: parse error at or near "."
</screen>
    </para>

    <para>
     Another way to use a function returning a row result is to declare a
     second function accepting a rowtype parameter, and pass the function
     result to it:

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

<screen>
SELECT getname(new_emp());
 getname
---------
 None
(1 row)
</screen>
    </para>     
   </sect2>

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

    <para>
     As previously mentioned, an SQL function may be declared as
     returning <literal>SETOF</literal> <replaceable>sometype</>.
     In this case the function's final SELECT query is executed to
     completion, and each row it outputs is returned as an element
     of the set.
    </para>

    <para>
     Functions returning sets may only be called in the target list
     of a SELECT query.  For each row that the SELECT generates by itself,
     the function returning set is invoked, and an output row is generated
     for each element of the function's result set.  An example:

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

<screen>
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 SELECT,
     notice that no output row appears for Child2, Child3, etc.
     This happens because listchildren() returns an empty set
     for those inputs, so no output rows are generated.
    </para>
   </sect2>
  </sect1>

  <sect1 id="xfunc-pl">
   <title>Procedural Language Functions</title>

   <para>
    Procedural languages aren't built into the <productname>PostgreSQL</productname> server; they are offered
    by loadable modules. Please refer to the documentation of the
    procedural language in question for details about the syntax and how the function body
    is interpreted for each language.
   </para>

   <para>
    There are currently four procedural languages available in the
    standard <productname>PostgreSQL</productname> distribution:
    <application>PL/pgSQL</application>, <application>PL/Tcl</application>,
    <application>PL/Perl</application>, and <application>PL/Python</application>.  Other languages can be
    defined by users.  Refer to <xref linkend="xplang"> for more
    information.  The basics of developing a new procedural language are covered in <xref linkend="xfunc-plhandler">.
   </para>
  </sect1>

  <sect1 id="xfunc-internal">
   <title>Internal Functions</title>

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

   <para>
    Internal functions are functions written in C that have been statically
    linked into the <productname>PostgreSQL</productname> server.
    The <quote>body</quote> of the function definition
    specifies the C-language name of the function, which need not be the
    same as the name being declared for SQL use.
    (For reasons of backwards compatibility, an empty body
    is accepted as meaning that the C-language function name is the
    same as the SQL name.)
   </para>

   <para>
    Normally, all internal functions present in the
    backend are declared during the initialization of the database cluster (<command>initdb</command>),
    but a user could use <command>CREATE FUNCTION</command>
    to create additional alias names for an internal function.
    Internal functions are declared in <command>CREATE FUNCTION</command>
    with language name <literal>internal</literal>.  For instance, to
    create an alias for the <function>sqrt</function> function:
<programlisting>
CREATE FUNCTION square_root(double precision) RETURNS double precision
    AS 'dsqrt'
    LANGUAGE INTERNAL
    WITH (isStrict);
</programlisting>
    (Most internal functions expect to be declared <quote>strict</quote>.)
   </para>

   <note>
    <para>
     Not all <quote>predefined</quote> functions are
     <quote>internal</quote> in the above sense.  Some predefined
     functions are written in SQL.
    </para>
   </note>
  </sect1>

  <sect1 id="xfunc-c">
   <title>C Language Functions</title>

   <para>
    User-defined functions can be written in C (or a language that can
    be made compatible with C, such as C++).  Such functions are
    compiled into dynamically loadable objects (also called shared
    libraries) and are loaded by the server on demand.  The dynamic
    loading feature is what distinguishes <quote>C language</> functions
    from <quote>internal</> functions --- the actual coding conventions
    are essentially the same for both.  (Hence, the standard internal
    function library is a rich source of coding examples for user-defined
    C functions.)
   </para>

   <para>
    Two different calling conventions are currently used for C functions.
    The newer <quote>version 1</quote> calling convention is indicated by writing
    a <literal>PG_FUNCTION_INFO_V1()</literal> macro call for the function,
    as illustrated below.  Lack of such a macro indicates an old-style
    ("version 0") function.  The language name specified in <command>CREATE FUNCTION</command>
    is <literal>C</literal> in either case.  Old-style functions are now deprecated
    because of portability problems and lack of functionality, but they
    are still supported for compatibility reasons.
   </para>

  <sect2 id="xfunc-c-dynload">
   <title>Dynamic Loading</title>

   <para>
    The first time a user-defined function in a particular
    loadable object file is called in a backend session,
    the dynamic loader loads that object file into memory so that the
    function can be called.  The <command>CREATE FUNCTION</command>
    for a user-defined C function must therefore specify two pieces of
    information for the function: the name of the loadable
    object file, and the C name (link symbol) of the specific function to call
    within that object file.  If the C name is not explicitly specified then
    it is assumed to be the same as the SQL function name.
   </para>

   <para>
    The following algorithm is used to locate the shared object file
    based on the name given in the <command>CREATE FUNCTION</command>
    command:

    <orderedlist>
     <listitem>
      <para>
       If the name is an absolute path, the given file is loaded.
      </para>
     </listitem>

     <listitem>
      <para>
       If the name starts with the string <literal>$libdir</literal>,
       that part is replaced by the PostgreSQL package library directory
       name, which is determined at build time.<indexterm><primary>$libdir</></>
      </para>
     </listitem>

     <listitem>
      <para>
       If the name does not contain a directory part, the file is
       searched for in the path specified by the configuration variable
       <varname>dynamic_library_path</varname>.<indexterm><primary>dynamic_library_path</></>
      </para>
     </listitem>

     <listitem>
      <para>
       Otherwise (the file was not found in the path, or it contains a
       non-absolute directory part), the dynamic loader will try to
       take the name as given, which will most likely fail.  (It is
       unreliable to depend on the current working directory.)
      </para>
     </listitem>
    </orderedlist>

    If this sequence does not work, the platform-specific shared
    library file name extension (often <filename>.so</filename>) is
    appended to the given name and this sequence is tried again.  If
    that fails as well, the load will fail.
   </para>

   <note>
    <para>
     The user id the <application>PostgreSQL</application> server runs
     as must be able to traverse the path to the file you intend to
     load.  Making the file or a higher-level directory not readable
     and/or not executable by the <quote>postgres</quote> user is a
     common mistake.
    </para>
   </note>

   <para>
    In any case, the file name that is given in the
    <command>CREATE FUNCTION</command> command is recorded literally
    in the system catalogs, so if the file needs to be loaded again
    the same procedure is applied.
   </para>

   <note>
    <para>
     <application>PostgreSQL</application> will not compile a C function
     automatically.  The object file must be compiled before it is referenced
     in a <command>CREATE
     FUNCTION</> command.  See <xref linkend="dfunc"> for additional
     information.
    </para>
   </note>

   <note>
    <para>
     After it is used for the first time, a dynamically loaded object
     file is retained in memory.  Future calls in the same session to the
     function(s) in that file will only incur the small overhead of a symbol
     table lookup.  If you need to force a reload of an object file, for
     example after recompiling it, use the <command>LOAD</> command or
     begin a fresh session.
    </para>
   </note>

   <para>
    It is recommended to locate shared libraries either relative to
    <literal>$libdir</literal> or through the dynamic library path.
    This simplifies version upgrades if the new installation is at a
    different location.  The actual directory that
    <literal>$libdir</literal> stands for can be found out with the
    command <literal>pg_config --pkglibdir</literal>.
   </para>

   <note>
    <para>
     Before <application>PostgreSQL</application> release 7.2, only exact
     absolute paths to object files could be specified in <command>CREATE
     FUNCTION</>.  This approach is now deprecated since it makes the
     function definition unnecessarily unportable.  It's best to specify
     just the shared library name with no path nor extension, and let
     the search mechanism provide that information instead.
    </para>
   </note>

  </sect2>

   <sect2>
    <title>Base Types in C-Language Functions</title>

    <para>
     <xref linkend="xfunc-c-type-table"> gives the C type required for
     parameters in the C functions that will be loaded into Postgres.
     The <quote>Defined In</quote> column gives the header file that
     needs to be included to get the type definition.  (The actual
     definition may be in a different file that is included by the
     listed file.  It is recommended that users stick to the defined
     interface.)  Note that you should always include
     <filename>postgres.h</filename> first in any source file, because
     it declares a number of things that you will need anyway.
    </para>

     <table tocentry="1" id="xfunc-c-type-table">
      <title>Equivalent C Types
       for Built-In <productname>PostgreSQL</productname> Types</title>
      <titleabbrev>Equivalent C Types</titleabbrev>
      <tgroup cols="3">
       <thead>
	<row>
	 <entry>
	  SQL Type
	 </entry>
	 <entry>
	  C Type
	 </entry>
	 <entry>
	  Defined In
	 </entry>
	</row>
       </thead>
       <tbody>
	<row>
	 <entry><type>abstime</type></entry>
	 <entry><type>AbsoluteTime</type></entry>
	 <entry><filename>utils/nabstime.h</filename></entry>
	</row>
	<row>
	 <entry><type>boolean</type></entry>
	 <entry><type>bool</type></entry>
	 <entry><filename>postgres.h</filename> (maybe compiler built-in)</entry>
	</row>
	<row>
	 <entry><type>box</type></entry>
	 <entry><type>BOX*</type></entry>
	 <entry><filename>utils/geo-decls.h</filename></entry>
	</row>
	<row>
	 <entry><type>bytea</type></entry>
	 <entry><type>bytea*</type></entry>
	 <entry><filename>postgres.h</filename></entry>
	</row>
	<row>
	 <entry><type>"char"</type></entry>
	 <entry><type>char</type></entry>
	 <entry>(compiler built-in)</entry>
	</row>
	<row>
	 <entry><type>character</type></entry>
	 <entry><type>BpChar*</type></entry>
	 <entry><filename>postgres.h</filename></entry>
	</row>
	<row>
	 <entry><type>cid</type></entry>
	 <entry><type>CommandId</type></entry>
	 <entry><filename>postgres.h</filename></entry>
	</row>
	<row>
	 <entry><type>date</type></entry>
	 <entry><type>DateADT</type></entry>
	 <entry><filename>utils/date.h</filename></entry>
	</row>
	<row>
	 <entry><type>smallint</type> (<type>int2</type>)</entry>
	 <entry><type>int2</type> or <type>int16</type></entry>
	 <entry><filename>postgres.h</filename></entry>
	</row>
	<row>
	 <entry><type>int2vector</type></entry>
	 <entry><type>int2vector*</type></entry>
	 <entry><filename>postgres.h</filename></entry>
	</row>
	<row>
	 <entry><type>integer</type> (<type>int4</type>)</entry>
	 <entry><type>int4</type> or <type>int32</type></entry>
	 <entry><filename>postgres.h</filename></entry>
	</row>
	<row>
	 <entry><type>real</type> (<type>float4</type>)</entry>
	 <entry><type>float4*</type></entry>
	<entry><filename>postgres.h</filename></entry>
	</row>
	<row>
	 <entry><type>double precision</type> (<type>float8</type>)</entry>
	 <entry><type>float8*</type></entry>
	 <entry><filename>postgres.h</filename></entry>
	</row>
	<row>
	 <entry><type>interval</type></entry>
	 <entry><type>Interval*</type></entry>
	 <entry><filename>utils/timestamp.h</filename></entry>
	</row>
	<row>
	 <entry><type>lseg</type></entry>
	 <entry><type>LSEG*</type></entry>
	 <entry><filename>utils/geo-decls.h</filename></entry>
	</row>
	<row>
	 <entry><type>name</type></entry>
	 <entry><type>Name</type></entry>
	 <entry><filename>postgres.h</filename></entry>
	</row>
	<row>
	 <entry><type>oid</type></entry>
	 <entry><type>Oid</type></entry>
	 <entry><filename>postgres.h</filename></entry>
	</row>
	<row>
	 <entry><type>oidvector</type></entry>
	 <entry><type>oidvector*</type></entry>
	 <entry><filename>postgres.h</filename></entry>
	</row>
	<row>
	 <entry><type>path</type></entry>
	 <entry><type>PATH*</type></entry>
	 <entry><filename>utils/geo-decls.h</filename></entry>
	</row>
	<row>
	 <entry><type>point</type></entry>
	 <entry><type>POINT*</type></entry>
	 <entry><filename>utils/geo-decls.h</filename></entry>
	</row>
	<row>
	 <entry><type>regproc</type></entry>
	 <entry><type>regproc</type></entry>
	 <entry><filename>postgres.h</filename></entry>
	</row>
	<row>
	 <entry><type>reltime</type></entry>
	 <entry><type>RelativeTime</type></entry>
	 <entry><filename>utils/nabstime.h</filename></entry>
	</row>
	<row>
	 <entry><type>text</type></entry>
	 <entry><type>text*</type></entry>
	 <entry><filename>postgres.h</filename></entry>
	</row>
	<row>
	 <entry><type>tid</type></entry>
	 <entry><type>ItemPointer</type></entry>
	 <entry><filename>storage/itemptr.h</filename></entry>
	</row>
	<row>
	 <entry><type>time</type></entry>
	 <entry><type>TimeADT</type></entry>
	 <entry><filename>utils/date.h</filename></entry>
	</row>
	<row>
	 <entry><type>time with time zone</type></entry>
	 <entry><type>TimeTzADT</type></entry>
	 <entry><filename>utils/date.h</filename></entry>
	</row>
	<row>
	 <entry><type>timestamp</type></entry>
	 <entry><type>Timestamp*</type></entry>
	 <entry><filename>utils/timestamp.h</filename></entry>
	</row>
	<row>
	 <entry><type>tinterval</type></entry>
	 <entry><type>TimeInterval</type></entry>
	 <entry><filename>utils/nabstime.h</filename></entry>
	</row>
	<row>
	 <entry><type>varchar</type></entry>
	 <entry><type>VarChar*</type></entry>
	 <entry><filename>postgres.h</filename></entry>
	</row>
	<row>
	 <entry><type>xid</type></entry>
	 <entry><type>TransactionId</type></entry>
	 <entry><filename>postgres.h</filename></entry>
	</row>
       </tbody>
      </tgroup>
     </table>

    <para>
     Internally, <productname>Postgres</productname> regards a
     base type as a <quote>blob  of memory</quote>.   The  user-defined  
     functions that you define over a type in turn define the 
     way  that  <productname>Postgres</productname> can operate  
     on  it.  That is, <productname>Postgres</productname> will 
     only store and retrieve the data from disk and use  your  
     user-defined functions to input, process, and output the data.
     Base types can have one of three internal formats:

     <itemizedlist>
      <listitem>
       <para>
	pass by value, fixed-length
       </para>
      </listitem>
      <listitem>
       <para>
	pass by reference, fixed-length
       </para>
      </listitem>
      <listitem>
       <para>
	pass by reference, variable-length
       </para>
      </listitem>
     </itemizedlist>
    </para>

    <para>
     By-value  types  can  only be 1, 2 or 4 bytes in length
     (also 8 bytes, if <literal>sizeof(Datum)</literal> is 8 on your machine).
     You should be careful 
     to define your types such that  they  will  be  the  same  
     size (in bytes) on all architectures.  For example, the 
     <literal>long</literal> type is dangerous because  it  
     is 4 bytes on some machines and 8 bytes on others, whereas 
     <type>int</type>  type  is  4  bytes  on  most  
     Unix machines.  A reasonable implementation of  
     the  <type>int4</type>  type  on  Unix
     machines might be:
     
<programlisting>
/* 4-byte integer, passed by value */
typedef int int4;
</programlisting>

     <productname>PostgreSQL</productname> automatically figures
     things out so that the integer types really have the size they
     advertise.
    </para>

    <para>
     On  the  other hand, fixed-length types of any size may
     be passed by-reference.  For example, here is a  sample
     implementation of a <productname>PostgreSQL</productname> type:
     
<programlisting>
/* 16-byte structure, passed by reference */
typedef struct
{
    double  x, y;
} Point;
</programlisting>
    </para>

    <para>
     Only  pointers  to  such types can be used when passing
     them in and out of <productname>Postgres</productname> functions.
     To return a value of such a type, allocate the right amount of
     memory with <literal>palloc()</literal>, fill in the allocated memory,
     and return a pointer to it.  (Alternatively, you can return an input
     value of the same type by returning its pointer.  <emphasis>Never</>
     modify the contents of a pass-by-reference input value, however.)
    </para>

    <para>
     Finally, all variable-length types must also be  passed
     by  reference.   All  variable-length  types must begin
     with a length field of exactly 4 bytes, and all data to
     be  stored within that type must be located in the memory 
     immediately  following  that  length  field.   The
     length  field  is  the  total  length  of the structure
     (i.e.,  it  includes  the  size  of  the  length  field
     itself).  We can define the text type as follows:

<programlisting>
typedef struct {
    int4 length;
    char data[1];
} text;
</programlisting>
    </para>

    <para>
     Obviously,  the  data  field shown here is not long enough to hold
     all possible strings; it's impossible to declare such
     a  structure  in  <acronym>C</acronym>.  When manipulating 
     variable-length types, we must  be  careful  to  allocate  
     the  correct amount  of memory and initialize the length field.  
     For example, if we wanted to  store  40  bytes  in  a  text
     structure, we might use a code fragment like this:

<programlisting>
#include "postgres.h"
...
char buffer[40]; /* our source data */
...
text *destination = (text *) palloc(VARHDRSZ + 40);
destination-&gt;length = VARHDRSZ + 40;
memmove(destination-&gt;data, buffer, 40);
...
</programlisting>
    </para>

    <para>
     Now that we've gone over all of the possible structures
     for base types, we can show some examples of real functions.
    </para>
   </sect2>

   <sect2>
    <title>Version-0 Calling Conventions for C-Language Functions</title>

    <para>
     We present the <quote>old style</quote> calling convention first --- although
     this approach is now deprecated, it's easier to get a handle on
     initially.  In the version-0 method, the arguments and result
     of the C function are just declared in normal C style, but being
     careful to use the C representation of each SQL data type as shown
     above.
    </para>

    <para>
     Here are some examples:

<programlisting>
#include "postgres.h"
#include &lt;string.h&gt;

/* By Value */
         
int
add_one(int arg)
{
    return arg + 1;
}

/* By Reference, Fixed Length */

float8 *
add_one_float8(float8 *arg)
{
    float8    *result = (float8 *) palloc(sizeof(float8));

    *result = *arg + 1.0;
       
    return result;
}

Point *
makepoint(Point *pointx, Point *pointy)
{
    Point     *new_point = (Point *) palloc(sizeof(Point));

    new_point->x = pointx->x;
    new_point->y = pointy->y;
       
    return new_point;
}

/* By Reference, Variable Length */

text *
copytext(text *t)
{
    /*
     * VARSIZE is the total size of the struct in bytes.
     */
    text *new_t = (text *) palloc(VARSIZE(t));
    VARATT_SIZEP(new_t) = VARSIZE(t);
    /*
     * VARDATA is a pointer to the data region of the struct.
     */
    memcpy((void *) VARDATA(new_t), /* destination */
           (void *) VARDATA(t),     /* source */
           VARSIZE(t)-VARHDRSZ);    /* how many bytes */
    return new_t;
}

text *
concat_text(text *arg1, text *arg2)
{
    int32 new_text_size = VARSIZE(arg1) + VARSIZE(arg2) - VARHDRSZ;
    text *new_text = (text *) palloc(new_text_size);

    VARATT_SIZEP(new_text) = new_text_size;
    memcpy(VARDATA(new_text), VARDATA(arg1), VARSIZE(arg1)-VARHDRSZ);
    memcpy(VARDATA(new_text) + (VARSIZE(arg1)-VARHDRSZ),
           VARDATA(arg2), VARSIZE(arg2)-VARHDRSZ);
    return new_text;
}
</programlisting>
    </para>

    <para>
     Supposing that the above code has been prepared in file
     <filename>funcs.c</filename> and compiled into a shared object,
     we could define the functions to <productname>Postgres</productname>
     with commands like this:
     
<programlisting>
CREATE FUNCTION add_one(int4) RETURNS int4
     AS '<replaceable>PGROOT</replaceable>/tutorial/funcs' LANGUAGE C
     WITH (isStrict);

-- note overloading of SQL function name add_one()
CREATE FUNCTION add_one(float8) RETURNS float8
     AS '<replaceable>PGROOT</replaceable>/tutorial/funcs',
        'add_one_float8'
     LANGUAGE C WITH (isStrict);

CREATE FUNCTION makepoint(point, point) RETURNS point
     AS '<replaceable>PGROOT</replaceable>/tutorial/funcs' LANGUAGE C
     WITH (isStrict);
                         
CREATE FUNCTION copytext(text) RETURNS text
     AS '<replaceable>PGROOT</replaceable>/tutorial/funcs' LANGUAGE C
     WITH (isStrict);

CREATE FUNCTION concat_text(text, text) RETURNS text
     AS '<replaceable>PGROOT</replaceable>/tutorial/funcs' LANGUAGE C
     WITH (isStrict);
</programlisting>
    </para>

    <para>
     Here <replaceable>PGROOT</replaceable> stands for the full path to
     the <productname>Postgres</productname> source tree. (Better style would
     be to use just <literal>'funcs'</> in the <literal>AS</> clause,
     after having added <replaceable>PGROOT</replaceable><literal>/tutorial</>
     to the search path.  In any case, we may omit the system-specific
     extension for a shared library, commonly <literal>.so</literal> or
     <literal>.sl</literal>.)
    </para>

    <para>
     Notice that we have specified the functions as <quote>strict</quote>,
     meaning that
     the system should automatically assume a NULL result if any input
     value is NULL.  By doing this, we avoid having to check for NULL inputs
     in the function code.  Without this, we'd have to check for NULLs
     explicitly, for example by checking for a null pointer for each
     pass-by-reference argument.  (For pass-by-value arguments, we don't
     even have a way to check!)
    </para>

    <para>
     Although this calling convention is simple to use,
     it is not very portable; on some architectures there are problems
     with passing smaller-than-int data types this way.  Also, there is
     no simple way to return a NULL result, nor to cope with NULL arguments
     in any way other than making the function strict.  The version-1
     convention, presented next, overcomes these objections.
    </para>
   </sect2>

   <sect2>
    <title>Version-1 Calling Conventions for C-Language Functions</title>

    <para>
     The version-1 calling convention relies on macros to suppress most
     of the complexity of passing arguments and results.  The C declaration
     of a version-1 function is always
<programlisting>
Datum funcname(PG_FUNCTION_ARGS)
</programlisting>
     In addition, the macro call
<programlisting>
PG_FUNCTION_INFO_V1(funcname);
</programlisting>
     must appear in the same source file (conventionally it's written
     just before the function itself).  This macro call is not needed
     for <literal>internal</>-language functions, since Postgres currently
     assumes all internal functions are version-1.  However, it is
     <emphasis>required</emphasis> for dynamically-loaded functions.
    </para>

    <para>
     In a version-1 function, each actual argument is fetched using a
     <function>PG_GETARG_<replaceable>xxx</replaceable>()</function>
     macro that corresponds to the argument's datatype, and the result
     is returned using a
     <function>PG_RETURN_<replaceable>xxx</replaceable>()</function>
     macro for the return type.
    </para>

    <para>
     Here we show the same functions as above, coded in version-1 style:

<programlisting>
#include "postgres.h"
#include &lt;string.h&gt;
#include "fmgr.h"

/* By Value */

PG_FUNCTION_INFO_V1(add_one);
         
Datum
add_one(PG_FUNCTION_ARGS)
{
    int32   arg = PG_GETARG_INT32(0);

    PG_RETURN_INT32(arg + 1);
}

/* By Reference, Fixed Length */

PG_FUNCTION_INFO_V1(add_one_float8);

Datum
add_one_float8(PG_FUNCTION_ARGS)
{
    /* The macros for FLOAT8 hide its pass-by-reference nature */
    float8   arg = PG_GETARG_FLOAT8(0);

    PG_RETURN_FLOAT8(arg + 1.0);
}

PG_FUNCTION_INFO_V1(makepoint);

Datum
makepoint(PG_FUNCTION_ARGS)
{
    /* Here, the pass-by-reference nature of Point is not hidden */
    Point     *pointx = PG_GETARG_POINT_P(0);
    Point     *pointy = PG_GETARG_POINT_P(1);
    Point     *new_point = (Point *) palloc(sizeof(Point));

    new_point->x = pointx->x;
    new_point->y = pointy->y;
       
    PG_RETURN_POINT_P(new_point);
}

/* By Reference, Variable Length */

PG_FUNCTION_INFO_V1(copytext);

Datum
copytext(PG_FUNCTION_ARGS)
{
    text     *t = PG_GETARG_TEXT_P(0);
    /*
     * VARSIZE is the total size of the struct in bytes.
     */
    text     *new_t = (text *) palloc(VARSIZE(t));
    VARATT_SIZEP(new_t) = VARSIZE(t);
    /*
     * VARDATA is a pointer to the data region of the struct.
     */
    memcpy((void *) VARDATA(new_t), /* destination */
           (void *) VARDATA(t),     /* source */
           VARSIZE(t)-VARHDRSZ);    /* how many bytes */
    PG_RETURN_TEXT_P(new_t);
}

PG_FUNCTION_INFO_V1(concat_text);

Datum
concat_text(PG_FUNCTION_ARGS)
{
    text  *arg1 = PG_GETARG_TEXT_P(0);
    text  *arg2 = PG_GETARG_TEXT_P(1);
    int32 new_text_size = VARSIZE(arg1) + VARSIZE(arg2) - VARHDRSZ;
    text *new_text = (text *) palloc(new_text_size);

    VARATT_SIZEP(new_text) = new_text_size;
    memcpy(VARDATA(new_text), VARDATA(arg1), VARSIZE(arg1)-VARHDRSZ);
    memcpy(VARDATA(new_text) + (VARSIZE(arg1)-VARHDRSZ),
           VARDATA(arg2), VARSIZE(arg2)-VARHDRSZ);
    PG_RETURN_TEXT_P(new_text);
}
</programlisting>
    </para>

    <para>
     The <command>CREATE FUNCTION</command> commands are the same as
     for the version-0 equivalents.
    </para>

    <para>
     At first glance, the version-1 coding conventions may appear to
     be just pointless obscurantism.  However, they do offer a number
     of improvements, because the macros can hide unnecessary detail.
     An example is that in coding add_one_float8, we no longer need to
     be aware that float8 is a pass-by-reference type.  Another
     example is that the GETARG macros for variable-length types hide
     the need to deal with fetching <quote>toasted</quote> (compressed or
     out-of-line) values.  The old-style <function>copytext</function>
     and <function>concat_text</function> functions shown above are
     actually wrong in the presence of toasted values, because they
     don't call <function>pg_detoast_datum()</function> on their
     inputs.  (The handler for old-style dynamically-loaded functions
     currently takes care of this detail, but it does so less
     efficiently than is possible for a version-1 function.)
    </para>

    <para>
     One big improvement in version-1 functions is better handling of NULL
     inputs and results.  The macro <function>PG_ARGISNULL(n)</function>
     allows a function to test whether each input is NULL (of course, doing
     this is only necessary in functions not declared <quote>strict</>).
     As with the
     <function>PG_GETARG_<replaceable>xxx</replaceable>()</function> macros,
     the input arguments are counted beginning at zero.  Note that one
     should refrain from executing
     <function>PG_GETARG_<replaceable>xxx</replaceable>()</function> until
     one has verified that the argument isn't NULL.
     To return a NULL result, execute <function>PG_RETURN_NULL()</function>;
     this works in both strict and non-strict functions.
    </para>

    <para>
     The version-1 function call conventions make it possible to
     return <quote>set</quote> results and implement trigger functions and
     procedural-language call handlers.  Version-1 code is also more
     portable than version-0, because it does not break ANSI C restrictions
     on function call protocol.  For more details see
     <filename>src/backend/utils/fmgr/README</filename> in the source
     distribution.
    </para>
   </sect2>

   <sect2>
    <title>Composite Types in C-Language Functions</title>

    <para>
     Composite types do not  have  a  fixed  layout  like  C
     structures.   Instances of a composite type may contain
     null fields.  In addition,  composite  types  that  are
     part  of  an  inheritance  hierarchy may have different
     fields than other members of the same inheritance hierarchy.    
     Therefore,  <productname>Postgres</productname>  provides  
     a  procedural interface for accessing fields of composite types  
     from C.  As <productname>Postgres</productname> processes 
     a set of rows, each row will be passed into your 
     function as an  opaque  structure of type <literal>TUPLE</literal>.
     Suppose we want to write a function to answer the query

<programlisting>
SELECT name, c_overpaid(emp, 1500) AS overpaid
FROM emp
WHERE name = 'Bill' OR name = 'Sam';
</programlisting>

     In the query above, we can define c_overpaid as:
     
<programlisting>
#include "postgres.h"
#include "executor/executor.h"  /* for GetAttributeByName() */

bool
c_overpaid(TupleTableSlot *t, /* the current row of EMP */
           int32 limit)
{
    bool isnull;
    int32 salary;

    salary = DatumGetInt32(GetAttributeByName(t, "salary", &amp;isnull));
    if (isnull)
        return (false);
    return salary &gt; limit;
}

/* In version-1 coding, the above would look like this: */

PG_FUNCTION_INFO_V1(c_overpaid);

Datum
c_overpaid(PG_FUNCTION_ARGS)
{
    TupleTableSlot  *t = (TupleTableSlot *) PG_GETARG_POINTER(0);
    int32            limit = PG_GETARG_INT32(1);
    bool isnull;
    int32 salary;

    salary = DatumGetInt32(GetAttributeByName(t, "salary", &amp;isnull));
    if (isnull)
        PG_RETURN_BOOL(false);
    /* Alternatively, we might prefer to do PG_RETURN_NULL() for null salary */

    PG_RETURN_BOOL(salary &gt; limit);
}
</programlisting>
    </para>

    <para>
     <function>GetAttributeByName</function> is the 
     <productname>Postgres</productname> system function that
     returns attributes out of the current row.  It has
     three arguments: the argument of type <type>TupleTableSlot*</type> passed into
     the  function, the name of the desired attribute, and a
     return parameter that tells whether  the  attribute
     is  null.   <function>GetAttributeByName</function> returns a Datum
     value that you can convert to the proper datatype by using the
     appropriate <function>DatumGet<replaceable>XXX</replaceable>()</function> macro.
    </para>

    <para>
     The  following  query  lets  <productname>Postgres</productname>  
     know  about  the <function>c_overpaid</function> function:

<programlisting>
CREATE FUNCTION c_overpaid(emp, int4) 
RETURNS bool
AS '<replaceable>PGROOT</replaceable>/tutorial/funcs' 
LANGUAGE C;
</programlisting>
    </para>

    <para>
     While there are ways to construct new rows or modify  
     existing rows from within a C function, these
     are far too complex to discuss in this manual.
     Consult the backend source code for examples.
    </para>
   </sect2>

   <sect2>
    <title>Writing Code</title>

    <para>
     We now turn to the more difficult task of writing  
     programming  language  functions.  Be warned: this section
     of the manual will not make you a programmer.  You must
     have  a  good  understanding of <acronym>C</acronym>
     (including the use of pointers and the malloc memory manager)  
     before  trying to write <acronym>C</acronym> functions for 
     use with <productname>Postgres</productname>. While  it may 
     be possible to load functions written in languages other 
     than <acronym>C</acronym> into  <productname>Postgres</productname>,  
     this  is  often difficult  (when  it  is possible at all) 
     because other languages, such as <acronym>FORTRAN</acronym> 
     and <acronym>Pascal</acronym> often do not follow the same 
     <firstterm>calling convention</firstterm>
     as <acronym>C</acronym>.  That is, other
     languages  do  not  pass  argument  and  return  values
     between functions in the same way.  For this reason, we
     will assume that your  programming  language  functions
     are written in <acronym>C</acronym>.
    </para>

    <para>
     The  basic  rules  for building <acronym>C</acronym> functions 
     are as follows:

     <itemizedlist>
      <listitem>
       <para>
	Use <literal>pg_config --includedir-server</literal><indexterm><primary>pg_config</></> to find
	out where the PostgreSQL server header files are installed on
	your system (or the system that your users will be running
	on).  This option is new with PostgreSQL 7.2.  For PostgreSQL
	7.1 you should use the option <option>--includedir</option>.
	(<command>pg_config</command> will exit with a non-zero status
	if it encounters an unknown option.)  For releases prior to
	7.1 you will have to guess, but since that was before the
	current calling conventions were introduced, it is unlikely
	that you want to support those releases.
       </para>
      </listitem>

      <listitem>
       <para>
	When allocating memory, use the
	<productname>Postgres</productname> routines
	<function>palloc</function> and <function>pfree</function>
	instead of the corresponding <acronym>C</acronym> library
	routines <function>malloc</function> and
	<function>free</function>.  The memory allocated by
	<function>palloc</function> will be freed automatically at the
	end of each transaction, preventing memory leaks.
       </para>
      </listitem>

      <listitem>
       <para>
	Always zero the bytes of your structures using
	<function>memset</function> or <function>bzero</function>.
	Several routines (such as the hash access method, hash join
	and the sort algorithm) compute functions of the raw bits
	contained in your structure.  Even if you initialize all
	fields of your structure, there may be several bytes of
	alignment padding (holes in the structure) that may contain
	garbage values.
       </para>
      </listitem>

      <listitem>
       <para>
        Most of the internal <productname>Postgres</productname> types
	are declared in <filename>postgres.h</filename>, while the function
	manager interfaces (<symbol>PG_FUNCTION_ARGS</symbol>, etc.)
	are in <filename>fmgr.h</filename>, so you will need to
	include at least these two files.  For portability reasons it's best
	to include <filename>postgres.h</filename> <emphasis>first</>,
	before any other system or user header files.
	Including <filename>postgres.h</filename> will also include
	<filename>elog.h</filename> and <filename>palloc.h</filename>
	for you.
       </para>
      </listitem>

      <listitem>
       <para>
        Symbol names defined within object files must not conflict
        with each other or with symbols defined in the
        <productname>PostgreSQL</productname> server executable.  You
        will have to rename your functions or variables if you get
        error messages to this effect.
       </para>
      </listitem>

      <listitem>
       <para>
	Compiling and linking your object code  so  that
	it  can  be  dynamically  loaded  into  
	<productname>Postgres</productname>
	always requires special flags.
	See <xref linkend="dfunc">
	for  a  detailed explanation of how to do it for
	your particular operating system.
       </para>
      </listitem>
     </itemizedlist>
    </para>
   </sect2>

&dfunc;

  </sect1>

  <sect1 id="xfunc-overload">
   <title>Function Overloading</title>

   <indexterm zone="xfunc-overload"><primary>overloading</></>

   <para>
    More than one function may be defined with the same SQL name, so long
    as the arguments they take are different.  In other words,
    function names can be <firstterm>overloaded</firstterm>.  When a
    query is executed, the server will determine which function to
    call from the data types and the number of the provided arguments.
    Overloading can also be used to simulate functions with a variable
    number of arguments, up to a finite maximum number.
   </para>

   <para>
    A function may also have the same name as an attribute.  In the case
    that there is an ambiguity between a function on a complex type and
    an attribute of the complex type, the attribute will always be used.
   </para>

   <para>
    When creating a family of overloaded functions, one should be
    careful not to create ambiguities.  For instance, given the
    functions
<programlisting>
CREATE FUNCTION test(int, real) RETURNS ...
CREATE FUNCTION test(smallint, double precision) RETURNS ...
</programlisting>
    it is not immediately clear which function would be called with
    some trivial input like <literal>test(1, 1.5)</literal>.  The
    currently implemented resolution rules are described in the
    <citetitle>User's Guide</citetitle>, but it is unwise to design a
    system that subtly relies on this behavior.
   </para>

   <para>
    When overloading C language functions, there is an additional
    constraint: The C name of each function in the family of
    overloaded functions must be different from the C names of all
    other functions, either internal or dynamically loaded.  If this
    rule is violated, the behavior is not portable.  You might get a
    run-time linker error, or one of the functions will get called
    (usually the internal one).  The alternative form of the
    <literal>AS</> clause for the SQL <command>CREATE
    FUNCTION</command> command decouples the SQL function name from
    the function name in the C source code.  E.g.,
<programlisting>
CREATE FUNCTION test(int) RETURNS int
    AS '<replaceable>filename</>', 'test_1arg'
    LANGUAGE C;
CREATE FUNCTION test(int, int) RETURNS int
    AS '<replaceable>filename</>', 'test_2arg'
    LANGUAGE C;
</programlisting>
    The names of the C functions here reflect one of many possible conventions.
   </para>

   <para>
    Prior to <productname>PostgreSQL</productname> 7.0, this
    alternative syntax did not exist.  There is a trick to get around
    the problem, by defining a set of C functions with different names
    and then define a set of identically-named SQL function wrappers
    that take the appropriate argument types and call the matching C
    function.
   </para>
  </sect1>


  <sect1 id="xfunc-plhandler">
   <title>Procedural Language Handlers</title>

   <para>
    All calls to functions that are written in a language other than
    the current <quote>version 1</quote> interface for compiled
    languages (this includes functions in user-defined procedural languages,
    functions written in SQL, and functions using the version 0 compiled
    language interface), go through a <firstterm>call handler</firstterm>
    function for the specific language.  It is the responsibility of
    the call handler to execute the function in a meaningful way, such
    as by interpreting the supplied source text.  This section
    describes how a language call handler can be written.  This is not
    a common task, in fact, it has only been done a handful of times
    in the history of <productname>PostgreSQL</productname>, but the
    topic naturally belongs in this chapter, and the material might
    give some insight into the extensible nature of the
    <productname>PostgreSQL</productname> system.
   </para>

   <para>
    The call handler for a procedural language is a
    <quote>normal</quote> function, which must be written in a
    compiled language such as C and registered with
    <productname>PostgreSQL</productname> as taking no arguments and
    returning the <type>opaque</type> type, a placeholder for
    unspecified or undefined types.  This prevents the call handler
    from being called directly as a function from queries.  (However,
    arguments may be supplied in the actual call to the handler when a
    function in the language offered by the handler is to be
    executed.)
   </para>

   <note>
    <para>
     In <productname>PostgreSQL</productname> 7.1 and later, call
     handlers must adhere to the <quote>version 1</quote> function
     manager interface, not the old-style interface.
    </para>
   </note>

   <para>
    The call handler is called in the same way as any other function:
    It receives a pointer to a
    <structname>FunctionCallInfoData</structname> struct containing
    argument values and information about the called function, and it
    is expected to return a <type>Datum</type> result (and possibly
    set the <structfield>isnull</structfield> field of the
    <structname>FunctionCallInfoData</structname> struct, if it wishes
    to return an SQL NULL result).  The difference between a call
    handler and an ordinary callee function is that the
    <structfield>flinfo-&gt;fn_oid</structfield> field of the
    <structname>FunctionCallInfoData</structname> struct will contain
    the OID of the actual function to be called, not of the call
    handler itself.  The call handler must use this field to determine
    which function to execute.  Also, the passed argument list has
    been set up according to the declaration of the target function,
    not of the call handler.
   </para>

   <para>
    It's up to the call handler to fetch the
    <classname>pg_proc</classname> entry and to analyze the argument
    and return types of the called procedure. The AS clause from the
    <command>CREATE FUNCTION</command> of the procedure will be found
    in the <literal>prosrc</literal> attribute of the
    <classname>pg_proc</classname> table entry. This may be the source
    text in the procedural language itself (like for PL/Tcl), a
    path name to a file, or anything else that tells the call handler
    what to do in detail.
   </para>

   <para>
    Often, the same function is called many times per SQL statement.
    A call handler can avoid repeated lookups of information about the
    called function by using the
    <structfield>flinfo-&gt;fn_extra</structfield> field.  This will
    initially be NULL, but can be set by the call handler to point at
    information about the PL function.  On subsequent calls, if
    <structfield>flinfo-&gt;fn_extra</structfield> is already non-NULL
    then it can be used and the information lookup step skipped.  The
    call handler must be careful that
    <structfield>flinfo-&gt;fn_extra</structfield> is made to point at
    memory that will live at least until the end of the current query,
    since an <structname>FmgrInfo</structname> data structure could be
    kept that long.  One way to do this is to allocate the extra data
    in the memory context specified by
    <structfield>flinfo-&gt;fn_mcxt</structfield>; such data will
    normally have the same lifespan as the
    <structname>FmgrInfo</structname> itself.  But the handler could
    also choose to use a longer-lived context so that it can cache
    function definition information across queries.
   </para>

   <para>
    When a PL function is invoked as a trigger, no explicit arguments
    are passed, but the
    <structname>FunctionCallInfoData</structname>'s
    <structfield>context</structfield> field points at a
    <structname>TriggerData</structname> node, rather than being NULL
    as it is in a plain function call.  A language handler should
    provide mechanisms for PL functions to get at the trigger
    information.
   </para>

   <para>
    This is a template for a PL handler written in C:
<programlisting>
#include "postgres.h"
#include "executor/spi.h"
#include "commands/trigger.h"
#include "utils/elog.h"
#include "fmgr.h"
#include "access/heapam.h"
#include "utils/syscache.h"
#include "catalog/pg_proc.h"
#include "catalog/pg_type.h"

PG_FUNCTION_INFO_V1(plsample_call_handler);

Datum
plsample_call_handler(PG_FUNCTION_ARGS)
{
    Datum          retval;

    if (CALLED_AS_TRIGGER(fcinfo))
    {
        /*
         * Called as a trigger procedure
         */
        TriggerData    *trigdata = (TriggerData *) fcinfo->context;

        retval = ...
    }
    else {
        /*
         * Called as a function
         */

        retval = ...
    }

    return retval;
}
</programlisting>
   </para>

   <para>
    Only a few thousand lines of code have to be added instead of the
    dots to complete the call handler.  See <xref linkend="xfunc-c">
    for information on how to compile it into a loadable module.
   </para>

   <para>
    The following commands then register the sample procedural
    language:
<programlisting>
CREATE FUNCTION plsample_call_handler () RETURNS opaque
    AS '/usr/local/pgsql/lib/plsample'
    LANGUAGE C;
CREATE LANGUAGE plsample
    HANDLER plsample_call_handler;
</programlisting>
   </para>
  </sect1>
 </chapter>

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