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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/Tcl</> or <application>PL/pgSQL</>)
      (<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.
  </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"> 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>.<indexterm><primary>SETOF</></>
    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>
    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.
   </para>

   <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>:
     
<screen>
CREATE FUNCTION one() RETURNS integer AS $$
    SELECT 1 AS result;
$$ LANGUAGE SQL;

-- Alternative syntax:
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>

    <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 (i.e.,
     <command>INSERT</command>, <command>UPDATE</command>, and
     <command>DELETE</command>).  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:

<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>
   </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
------+-------
 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 <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 $$
    SELECT text 'None' AS name,
        1000 AS salary,
        25 AS age,
        point '(2,2)' AS cubicle;
$$ 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 function that returns a row (composite type) can be used as a table
     function, as described below.  It can also be called in the context
     of an SQL expression, but only when you
     extract a single attribute out of the row or pass the entire row into
     another function that accepts the same composite type.
    </para>

    <para>
     This is an example of extracting an attribute out of a row type:

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

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

     We need the extra parentheses to keep the parser from getting confused:

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

    <para>
     The other way to use a function returning a row result is to declare a
     second function accepting a row type argument and pass the
     result of the first function to it:

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

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

   <sect2>
    <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
(2 rows)
</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</>.  This 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 deterministic
     return type, but the converse is not. For example:
<screen>
CREATE FUNCTION is_greater(anyelement, anyelement) RETURNS boolean AS $$
    SELECT $1 > $2;
$$ LANGUAGE SQL;

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

CREATE FUNCTION invalid_func() RETURNS anyelement AS $$
    SELECT 1;
$$ LANGUAGE SQL;
ERROR:  cannot determine result data type
DETAIL:  A function returning "anyarray" or "anyelement" must have at least one argument of either type.
</screen>
    </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>.
    Refer to <xref linkend="xplang"> for more information.
    Other languages can be defined by users.
    The basics of developing a new procedural language are covered in <xref
    linkend="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
    server 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
    STRICT;
</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>

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

   <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
    (<quote>version 0</quote>) 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>

   <indexterm zone="xfunc-c-dynload">
    <primary>dynamic loading</primary>
   </indexterm>

   <para>
    The first time a user-defined function in a particular
    loadable object file is called in a 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>$libdir</literal>,
       that part is replaced by the <productname>PostgreSQL</> package
	library directory
       name, which is determined at build time.<indexterm><primary>$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
       <xref linkend="guc-dynamic-library-path">.<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>

   <para>
    The user ID the <productname>PostgreSQL</productname> 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 <systemitem>postgres</systemitem>
    user is a common mistake.
   </para>

   <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>
     <productname>PostgreSQL</productname> 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>

   <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>

   <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>

   <para>
    Before <productname>PostgreSQL</productname> 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>
  </sect2>

   <sect2 id="xfunc-c-basetype">
    <title>Base Types in C-Language Functions</title>

    <indexterm zone="xfunc-c-basetype">
     <primary>data type</primary>
     <secondary>internal organisation</secondary>
    </indexterm>

    <para>
     To know how to write C-language functions, you need to know how
     <productname>PostgreSQL</productname> internally represents base
     data types and how they can be passed to and from functions.
     Internally, <productname>PostgreSQL</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>PostgreSQL</productname> can operate on it.  That
     is, <productname>PostgreSQL</productname> will only store and
     retrieve the data from disk and use your user-defined functions
     to input, process, and output the data.
    </para>

    <para>
     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>
    </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>

     Only  pointers  to  such types can be used when passing
     them in and out of <productname>PostgreSQL</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.  (You can also return an input value
     that has the same type as the return value directly by returning
     the pointer to the input value.  <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 contains the total length of the structure,
     that is,  it  includes  the  size  of  the  length  field
     itself.
    </para>

    <para>
     As an example, we can define the type <type>text</type> as
     follows:

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

     Obviously,  the  data  field declared here is not long enough to hold
     all possible strings.  Since it's impossible to declare a variable-size
     structure in <acronym>C</acronym>, we rely on the knowledge that the
     <acronym>C</acronym> compiler won't range-check array subscripts.  We
     just allocate the necessary amount of space and then access the array as
     if it were declared the right length.  (This is a common trick, which
     you can read about in many textbooks about C.)
    </para>

    <para>
     When manipulating 
     variable-length types, we must  be  careful  to  allocate  
     the  correct amount  of memory and set the length field correctly.
     For example, if we wanted to  store  40  bytes  in  a <structname>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;
memcpy(destination-&gt;data, buffer, 40);
...
</programlisting>

     <literal>VARHDRSZ</> is the same as <literal>sizeof(int4)</>, but
     it's considered good style to use the macro <literal>VARHDRSZ</>
     to refer to the size of the overhead for a variable-length type.
    </para>

    <para>
     <xref linkend="xfunc-c-type-table"> specifies which C type
     corresponds to which SQL type when writing a C-language function
     that uses a built-in type of <productname>PostgreSQL</>.
     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 SQL Types</title>
      <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>
     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>Calling Conventions Version 0 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>PostgreSQL</productname>
     with commands like this:
     
<programlisting>
CREATE FUNCTION add_one(integer) RETURNS integer
     AS '<replaceable>DIRECTORY</replaceable>/funcs', 'add_one'
     LANGUAGE C STRICT;

-- note overloading of SQL function name "add_one"
CREATE FUNCTION add_one(double precision) RETURNS double precision
     AS '<replaceable>DIRECTORY</replaceable>/funcs', 'add_one_float8'
     LANGUAGE C STRICT;

CREATE FUNCTION makepoint(point, point) RETURNS point
     AS '<replaceable>DIRECTORY</replaceable>/funcs', 'makepoint'
     LANGUAGE C STRICT;
                         
CREATE FUNCTION copytext(text) RETURNS text
     AS '<replaceable>DIRECTORY</replaceable>/funcs', 'copytext'
     LANGUAGE C STRICT;

CREATE FUNCTION concat_text(text, text) RETURNS text
     AS '<replaceable>DIRECTORY</replaceable>/funcs', 'concat_text',
     LANGUAGE C STRICT;
</programlisting>
    </para>

    <para>
     Here, <replaceable>DIRECTORY</replaceable> stands for the
     directory of the shared library file (for instance the
     <productname>PostgreSQL</productname> tutorial directory, which
     contains the code for the examples used in this section).
     (Better style would be to use just <literal>'funcs'</> in the
     <literal>AS</> clause, after having added
     <replaceable>DIRECTORY</replaceable> 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 null values
     explicitly, 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 data types that are smaller than <type>int</type> 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>Calling Conventions Version 1 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
     <productname>PostgreSQL</> assumes that all internal functions
     use the version-1 convention.  It is, however, required 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 data type, and the
     result is returned using a
     <function>PG_RETURN_<replaceable>xxx</replaceable>()</function>
     macro for the return type.
     <function>PG_GETARG_<replaceable>xxx</replaceable>()</function>
     takes as its argument the number of the function argument to
     fetch, where the count starts at 0.
     <function>PG_RETURN_<replaceable>xxx</replaceable>()</function>
     takes as its argument the actual value to return.
    </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);
}

/* b 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.  They do, however, offer a number
     of improvements, because the macros can hide unnecessary detail.
     An example is that in coding <function>add_one_float8</>, we no longer need to
     be aware that <type>float8</type> is a pass-by-reference type.  Another
     example is that the <literal>GETARG</> macros for variable-length types allow
     for more efficient fetching of <quote>toasted</quote> (compressed or
     out-of-line) values.
    </para>

    <para>
     One big improvement in version-1 functions is better handling of null
     inputs and results.  The macro <function>PG_ARGISNULL(<replaceable>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 nonstrict functions.
    </para>

    <para>
     Other options provided in the new-style interface are two
     variants of the
     <function>PG_GETARG_<replaceable>xxx</replaceable>()</function>
     macros. The first of these,
     <function>PG_GETARG_<replaceable>xxx</replaceable>_COPY()</function>,
     guarantees to return a copy of the specified argument that is
     safe for writing into. (The normal macros will sometimes return a
     pointer to a value that is physically stored in a table, which
     must not be written to. Using the
     <function>PG_GETARG_<replaceable>xxx</replaceable>_COPY()</function>
     macros guarantees a writable result.)
    The second variant consists of the
    <function>PG_GETARG_<replaceable>xxx</replaceable>_SLICE()</function>
    macros which take three arguments. The first is the number of the
    function argument (as above). The second and third are the offset and
    length of the segment to be returned. Offsets are counted from
    zero, and a negative length requests that the remainder of the
    value be returned. These macros provide more efficient access to
    parts of large values in the case where they have storage type
    <quote>external</quote>. (The storage type of a column can be specified using
    <literal>ALTER TABLE <replaceable>tablename</replaceable> ALTER
    COLUMN <replaceable>colname</replaceable> SET STORAGE
    <replaceable>storagetype</replaceable></literal>. <replaceable>storagetype</replaceable> is one of
    <literal>plain</>, <literal>external</>, <literal>extended</literal>,
     or <literal>main</>.)
    </para>

    <para>
     Finally, the version-1 function call conventions make it possible
     to return set results (<xref linkend="xfunc-c-return-set">) and
     implement trigger functions (<xref linkend="triggers">) and
     procedural-language call handlers (<xref
     linkend="plhandler">).  Version-1 code is also more
     portable than version-0, because it does not break restrictions
     on function call protocol in the C standard.  For more details
     see <filename>src/backend/utils/fmgr/README</filename> in the
     source distribution.
    </para>
   </sect2>

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

    <para>
     Before we turn to the more advanced topics, we should discuss
     some coding rules for <productname>PostgreSQL</productname>
     C-language functions.  While it may be possible to load functions
     written in languages other than C into
     <productname>PostgreSQL</productname>, this is usually difficult
     (when it is possible at all) because other languages, such as
     C++, FORTRAN, or Pascal often do not follow the same calling
     convention as C.  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 C-language functions are
     actually written in C.
    </para>

    <para>
     The basic rules for writing and building C functions are as follows:

     <itemizedlist>
      <listitem>
       <para>
        Use <literal>pg_config
        --includedir-server</literal><indexterm><primary>pg_config</><secondary>with user-defined C functions</></>
        to find out where the <productname>PostgreSQL</> server header
        files are installed on your system (or the system that your
        users will be running on).  This option is new with
        <productname>PostgreSQL</> 7.2.  For
        <productname>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>PostgreSQL</productname> functions
        <function>palloc</function><indexterm><primary>palloc</></> and <function>pfree</function><indexterm><primary>pfree</></>
        instead of the corresponding C library functions
        <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>.  Without this, it's difficult to
	support hash indexes or hash joins, as you must pick out only
	the significant bits of your data structure to compute a hash.
        Even if you initialize all fields of your structure, there may be
        alignment padding (holes in the structure) that may contain
        garbage values.
       </para>
      </listitem>

      <listitem>
       <para>
        Most of the internal <productname>PostgreSQL</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 code so that it can be dynamically
        loaded into <productname>PostgreSQL</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;

   <sect2 id="xfunc-c-pgxs">
    <title>Extension build infrastructure</title>

   <indexterm zone="xfunc-c-pgxs">
    <primary>pgxs</primary>
   </indexterm>

   <para>
    If you are thinking about distributing your
    <productname>PostgreSQL</> extension modules, setting up a
    portable build system for them can be fairly difficult.  Therefore
    the <productname>PostgreSQL</> installation provides a build
    infrastructure for extensions, called <acronym>PGXS</acronym>, so
    that simple extension modules can be built simply against an
    already installed server.  Note that this infrastructure is not
    intended to be a universal build system framework that can be used
    to build all software interfacing to <productname>PostgreSQL</>;
    it simply automates common build rules for simple server extension
    modules.  For more complicated packages, you need to write your
    own build system.
   </para>

   <para>
    To use the infrastructure for your extension, you must write a
    simple makefile.  In that makefile, you need to set some variables
    and finally include the global <acronym>PGXS</acronym> makefile.
    Here is an example that builds an extension module named
    <literal>isbn_issn</literal> consisting of a shared library, an
    SQL script, and a documentation text file:
<programlisting>
MODULES = isbn_issn
DATA_built = isbn_issn.sql
DOCS = README.isbn_issn

PGXS := $(shell pg_config --pgxs)
include $(PGXS)
</programlisting>
    The last two lines should always be the same.  Earlier in the
    file, you assign variables or add custom
    <application>make</application> rules.
   </para>

   <para>
    The following variables can be set:

    <variablelist>
     <varlistentry>
      <term><varname>MODULES</varname></term>
      <listitem>
       <para>
        list of shared objects to be build from source file with same
        stem (do not include suffix in this list)
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>DATA</varname></term>
      <listitem>
       <para>
        random files to install into <literal><replaceable>prefix</replaceable>/share/contrib</literal>
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>DATA_built</varname></term>
      <listitem>
       <para>
        random files to install into
        <literal><replaceable>prefix</replaceable>/share/contrib</literal>,
        which need to be built first
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>DOCS</varname></term>
      <listitem>
       <para>
        random files to install under
        <literal><replaceable>prefix</replaceable>/doc/contrib</literal>
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>SCRIPTS</varname></term>
      <listitem>
       <para>
        script files (not binaries) to install into
        <literal><replaceable>prefix</replaceable>/bin</literal>
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>SCRIPTS_built</varname></term>
      <listitem>
       <para>
        script files (not binaries) to install into
        <literal><replaceable>prefix</replaceable>/bin</literal>,
        which need to be built first
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>REGRESS</varname></term>
      <listitem>
       <para>
        list of regression test cases (without suffix)
       </para>
      </listitem>
     </varlistentry>
    </variablelist>

    or at most one of these two:

    <variablelist>
     <varlistentry>
      <term><varname>PROGRAM</varname></term>
      <listitem>
       <para>
        a binary program to build (list objects files in <varname>OBJS</varname>)
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>MODULE_big</varname></term>
      <listitem>
       <para>
        a shared object to build (list object files in <varname>OBJS</varname>)
       </para>
      </listitem>
     </varlistentry>
    </variablelist>

    The following can also be set:

    <variablelist>

     <varlistentry>
      <term><varname>EXTRA_CLEAN</varname></term>
      <listitem>
       <para>
        extra files to remove in <literal>make clean</literal>
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>PG_CPPFLAGS</varname></term>
      <listitem>
       <para>
        will be added to <varname>CPPFLAGS</varname>
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>PG_LIBS</varname></term>
      <listitem>
       <para>
        will be added to <varname>PROGRAM</varname> link line
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>SHLIB_LINK</varname></term>
      <listitem>
       <para>
        will be added to <varname>MODULE_big</varname> link line
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>

   <para>
    Put this makefile as <literal>Makefile</literal> in the directory
    which holds your extension. Then you can do
    <literal>make</literal> to compile, and later <literal>make
    install</literal> to install your module.  The extension is
    compiled and installed for the
    <productname>PostgreSQL</productname> installation that
    corresponds to the first <command>pg_config</command> command
    found in your path.
   </para>
  </sect2>


   <sect2>
    <title>Composite-Type Arguments 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>PostgreSQL</productname> provides a function
     interface for accessing fields of composite types from C.
    </para>

    <para>
     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>

     Using call conventions version 0, we can define
     <function>c_overpaid</> as:
     
<programlisting>
#include "postgres.h"
#include "executor/executor.h"  /* for GetAttributeByName() */

bool
c_overpaid(HeapTupleHeader 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;
}
</programlisting>

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

<programlisting>
#include "postgres.h"
#include "executor/executor.h"  /* for GetAttributeByName() */

PG_FUNCTION_INFO_V1(c_overpaid);

Datum
c_overpaid(PG_FUNCTION_ARGS)
{
    HeapTupleHeader  t = PG_GETARG_HEAPTUPLEHEADER(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>PostgreSQL</productname> system function that
     returns attributes out of the specified row.  It has
     three arguments: the argument of type <type>HeapTupleHeader</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 <type>Datum</type>
     value that you can convert to the proper data type by using the
     appropriate <function>DatumGet<replaceable>XXX</replaceable>()</function> macro.
    </para>

    <para>
     There is also <function>GetAttributeByNum</function>, which selects
     the target attribute by column number instead of name.
    </para>

    <para>
     The following command declares the function
     <function>c_overpaid</function> in SQL:

<programlisting>
CREATE FUNCTION c_overpaid(emp, integer) RETURNS boolean
    AS '<replaceable>DIRECTORY</replaceable>/funcs', 'c_overpaid'
    LANGUAGE C STRICT;
</programlisting>

     Notice we have used <literal>STRICT</> so that we did not have to
     check whether the input arguments were NULL.
    </para>
   </sect2>

   <sect2>
    <title>Returning Rows (Composite Types) from C-Language Functions</title>

    <para>
     To return a row or composite-type value from a C-language
     function, you can use a special API that provides macros and
     functions to hide most of the complexity of building composite
     data types.  To use this API, the source file must include:
<programlisting>
#include "funcapi.h"
</programlisting>
    </para>

    <para>
     There are two ways you can build a composite data value (henceforth
     a <quote>tuple</>): you can build it from an array of Datum values,
     or from an array of C strings that can be passed to the input
     conversion functions of the tuple's column datatypes.  In either
     case, you first need to obtain or construct a <structname>TupleDesc</>
     descriptor for the tuple structure.  When working with Datums, you
     pass the <structname>TupleDesc</> to <function>BlessTupleDesc</>,
     and then call <function>heap_formtuple</> for each row.  When working
     with C strings, you pass the <structname>TupleDesc</> to
     <function>TupleDescGetAttInMetadata</>, and then call
     <function>BuildTupleFromCStrings</> for each row.  In the case of a
     function returning a set of tuples, the setup steps can all be done
     once during the first call of the function.
    </para>

    <para>
     Several helper functions are available for setting up the initial
     <structname>TupleDesc</>.  If you want to use a named composite type,
     you can fetch the information from the system catalogs.  Use
<programlisting>
TupleDesc RelationNameGetTupleDesc(const char *relname)
</programlisting>
     to get a <structname>TupleDesc</> for a named relation, or
<programlisting>
TupleDesc TypeGetTupleDesc(Oid typeoid, List *colaliases)
</programlisting>
     to get a <structname>TupleDesc</> based on a type OID. This can
     be used to get a <structname>TupleDesc</> for a base or
     composite type.  When writing a function that returns
     <structname>record</>, the expected <structname>TupleDesc</> 
     must be passed in by the caller.
    </para>

    <para>
     Once you have a <structname>TupleDesc</>, call
<programlisting>
TupleDesc BlessTupleDesc(TupleDesc tupdesc)
</programlisting>
     if you plan to work with Datums, or
<programlisting>
AttInMetadata *TupleDescGetAttInMetadata(TupleDesc tupdesc)
</programlisting>
     if you plan to work with C strings.  If you are writing a function
     returning set, you can save the results of these functions in the
     <structname>FuncCallContext</> structure --- use the
     <structfield>tuple_desc</> or <structfield>attinmeta</> field
     respectively.
    </para>

    <para>
     When working with Datums, use
<programlisting>
HeapTuple heap_formtuple(TupleDesc tupdesc, Datum *values, char *nulls)
</programlisting>
     to build a <structname>HeapTuple</> given user data in Datum form.
    </para>

    <para>
     When working with C strings, use
<programlisting>
HeapTuple BuildTupleFromCStrings(AttInMetadata *attinmeta, char **values)
</programlisting>
     to build a <structname>HeapTuple</> given user data
     in C string form.  <literal>values</literal> is an array of C strings,
     one for each attribute of the return row. Each C string should be in
     the form expected by the input function of the attribute data
     type. In order to return a null value for one of the attributes,
     the corresponding pointer in the <parameter>values</> array
     should be set to <symbol>NULL</>.  This function will need to
     be called again for each row you return.
    </para>

    <para>
     Once you have built a tuple to return from your function, it
     must be converted into a <type>Datum</>. Use
<programlisting>
HeapTupleGetDatum(HeapTuple tuple)
</programlisting>
     to convert a <structname>HeapTuple</> into a valid Datum.  This
     <type>Datum</> can be returned directly if you intend to return
     just a single row, or it can be used as the current return value
     in a set-returning function.
    </para>

    <para>
     An example appears in the next section.
    </para>

   </sect2>

   <sect2 id="xfunc-c-return-set">
    <title>Returning Sets from C-Language Functions</title>

    <para>
     There is also a special API that provides support for returning
     sets (multiple rows) from a C-language function.  A set-returning
     function must follow the version-1 calling conventions.  Also,
     source files must include <filename>funcapi.h</filename>, as
     above.
    </para>

    <para>
     A set-returning function (<acronym>SRF</>) is called
     once for each item it returns.  The <acronym>SRF</> must
     therefore save enough state to remember what it was doing and
     return the next item on each call.
     The structure <structname>FuncCallContext</> is provided to help
     control this process.  Within a function, <literal>fcinfo-&gt;flinfo-&gt;fn_extra</>
     is used to hold a pointer to <structname>FuncCallContext</>
     across calls.
<programlisting>
typedef struct
{
    /*
     * Number of times we've been called before
     * 
     * call_cntr is initialized to 0 for you by SRF_FIRSTCALL_INIT(), and
     * incremented for you every time SRF_RETURN_NEXT() is called.
     */
    uint32 call_cntr;

    /*
     * OPTIONAL maximum number of calls
     *
     * max_calls is here for convenience only and setting it is optional.
     * If not set, you must provide alternative means to know when the
     * function is done.
     */
    uint32 max_calls;

    /*
     * OPTIONAL pointer to result slot
     * 
     * This is obsolete and only present for backwards compatibility, viz,
     * user-defined SRFs that use the deprecated TupleDescGetSlot().
     */
    TupleTableSlot *slot;

    /*
     * OPTIONAL pointer to miscellaneous user-provided context information
     * 
     * user_fctx is for use as a pointer to your own data to retain
     * arbitrary context information between calls of your function.
     */
    void *user_fctx;

    /*
     * OPTIONAL pointer to struct containing attribute type input metadata
     * 
     * attinmeta is for use when returning tuples (i.e., composite data types)
     * and is not used when returning base data types. It is only needed
     * if you intend to use BuildTupleFromCStrings() to create the return
     * tuple.
     */
    AttInMetadata *attinmeta;

    /*
     * memory context used for structures that must live for multiple calls
     *
     * multi_call_memory_ctx is set by SRF_FIRSTCALL_INIT() for you, and used
     * by SRF_RETURN_DONE() for cleanup. It is the most appropriate memory
     * context for any memory that is to be reused across multiple calls
     * of the SRF.
     */
    MemoryContext multi_call_memory_ctx;

    /*
     * OPTIONAL pointer to struct containing tuple description
     *
     * tuple_desc is for use when returning tuples (i.e. composite data types)
     * and is only needed if you are going to build the tuples with
     * heap_formtuple() rather than with BuildTupleFromCStrings().  Note that
     * the TupleDesc pointer stored here should usually have been run through
     * BlessTupleDesc() first.
     */
    TupleDesc tuple_desc;

} FuncCallContext;
</programlisting>
    </para>

    <para>
     An <acronym>SRF</> uses several functions and macros that
     automatically manipulate the <structname>FuncCallContext</>
     structure (and expect to find it via <literal>fn_extra</>).  Use
<programlisting>
SRF_IS_FIRSTCALL()
</programlisting>
     to determine if your function is being called for the first or a
     subsequent time. On the first call (only) use
<programlisting>
SRF_FIRSTCALL_INIT()
</programlisting>
     to initialize the <structname>FuncCallContext</>. On every function call,
     including the first, use
<programlisting>
SRF_PERCALL_SETUP()
</programlisting>
     to properly set up for using the <structname>FuncCallContext</>
     and clearing any previously returned data left over from the
     previous pass.
    </para>

    <para>
     If your function has data to return, use
<programlisting>
SRF_RETURN_NEXT(funcctx, result)
</programlisting>
     to return it to the caller.  (<literal>result</> must be of type
     <type>Datum</>, either a single value or a tuple prepared as
     described above.)  Finally, when your function is finished
     returning data, use
<programlisting>
SRF_RETURN_DONE(funcctx)
</programlisting>
     to clean up and end the <acronym>SRF</>.
    </para>

    <para>
     The memory context that is current when the <acronym>SRF</> is called is
     a transient context that will be cleared between calls.  This means
     that you do not need to call <function>pfree</> on everything
     you allocated using <function>palloc</>; it will go away anyway.  However, if you want to allocate
     any data structures to live across calls, you need to put them somewhere
     else.  The memory context referenced by
     <structfield>multi_call_memory_ctx</> is a suitable location for any
     data that needs to survive until the <acronym>SRF</> is finished running.  In most
     cases, this means that you should switch into
     <structfield>multi_call_memory_ctx</> while doing the first-call setup.
    </para>

    <para>
     A complete pseudo-code example looks like the following:
<programlisting>
Datum
my_set_returning_function(PG_FUNCTION_ARGS)
{
    FuncCallContext  *funcctx;
    Datum             result;
    MemoryContext     oldcontext;
    <replaceable>further declarations as needed</replaceable>

    if (SRF_IS_FIRSTCALL())
    {
        funcctx = SRF_FIRSTCALL_INIT();
        oldcontext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);
        /* One-time setup code appears here: */
        <replaceable>user code</replaceable>
        <replaceable>if returning composite</replaceable>
            <replaceable>build TupleDesc, and perhaps AttInMetadata</replaceable>
        <replaceable>endif returning composite</replaceable>
        <replaceable>user code</replaceable>
        MemoryContextSwitchTo(oldcontext);
    }

    /* Each-time setup code appears here: */
    <replaceable>user code</replaceable>
    funcctx = SRF_PERCALL_SETUP();
    <replaceable>user code</replaceable>

    /* this is just one way we might test whether we are done: */
    if (funcctx-&gt;call_cntr &lt; funcctx-&gt;max_calls)
    {
        /* Here we want to return another item: */
        <replaceable>user code</replaceable>
        <replaceable>obtain result Datum</replaceable>
        SRF_RETURN_NEXT(funcctx, result);
    }
    else
    {
        /* Here we are done returning items and just need to clean up: */
        <replaceable>user code</replaceable>
        SRF_RETURN_DONE(funcctx);
    }
}
</programlisting>
    </para>

    <para>
     A complete example of a simple <acronym>SRF</> returning a composite type looks like:
<programlisting>
PG_FUNCTION_INFO_V1(testpassbyval);

Datum
testpassbyval(PG_FUNCTION_ARGS)
{
    FuncCallContext     *funcctx;
    int                  call_cntr;
    int                  max_calls;
    TupleDesc            tupdesc;
    AttInMetadata       *attinmeta;

     /* stuff done only on the first call of the function */
     if (SRF_IS_FIRSTCALL())
     {
        MemoryContext	oldcontext;

        /* create a function context for cross-call persistence */
        funcctx = SRF_FIRSTCALL_INIT();

        /* switch to memory context appropriate for multiple function calls */
        oldcontext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);

        /* total number of tuples to be returned */
        funcctx-&gt;max_calls = PG_GETARG_UINT32(0);

        /* Build a tuple description for a __testpassbyval tuple */
        tupdesc = RelationNameGetTupleDesc("__testpassbyval");

        /*
         * generate attribute metadata needed later to produce tuples from raw
         * C strings
         */
        attinmeta = TupleDescGetAttInMetadata(tupdesc);
        funcctx-&gt;attinmeta = attinmeta;

        MemoryContextSwitchTo(oldcontext);
    }

    /* stuff done on every call of the function */
    funcctx = SRF_PERCALL_SETUP();

    call_cntr = funcctx-&gt;call_cntr;
    max_calls = funcctx-&gt;max_calls;
    attinmeta = funcctx-&gt;attinmeta;
 
    if (call_cntr &lt; max_calls)    /* do when there is more left to send */
    {
        char       **values;
        HeapTuple    tuple;
        Datum        result;

        /*
         * Prepare a values array for building the returned tuple.
         * This should be an array of C strings which will
         * be processed later by the type input functions.
         */
        values = (char **) palloc(3 * sizeof(char *));
        values[0] = (char *) palloc(16 * sizeof(char));
        values[1] = (char *) palloc(16 * sizeof(char));
        values[2] = (char *) palloc(16 * sizeof(char));

        snprintf(values[0], 16, "%d", 1 * PG_GETARG_INT32(1));
        snprintf(values[1], 16, "%d", 2 * PG_GETARG_INT32(1));
        snprintf(values[2], 16, "%d", 3 * PG_GETARG_INT32(1));

        /* build a tuple */
        tuple = BuildTupleFromCStrings(attinmeta, values);

        /* make the tuple into a datum */
        result = HeapTupleGetDatum(tuple);

        /* clean up (this is not really necessary) */
        pfree(values[0]);
        pfree(values[1]);
        pfree(values[2]);
        pfree(values);

        SRF_RETURN_NEXT(funcctx, result);
    }
    else    /* do when there is no more left */
    {
        SRF_RETURN_DONE(funcctx);
    }
}
</programlisting>

     The SQL code to declare this function is:
<programlisting>
CREATE TYPE __testpassbyval AS (f1 integer, f2 integer, f3 integer);

CREATE OR REPLACE FUNCTION testpassbyval(integer, integer) RETURNS SETOF __testpassbyval
    AS '<replaceable>filename</>', 'testpassbyval'
    LANGUAGE C IMMUTABLE STRICT;
</programlisting>
    </para>

    <para>
     The directory <filename>contrib/tablefunc</> in the source
     distribution contains more examples of set-returning functions.
    </para>
   </sect2>

   <sect2>
    <title>Polymorphic Arguments and Return Types</title>

    <para>
     C-language 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. When function arguments or return types
     are defined as polymorphic types, the function author cannot know
     in advance what data type it will be called with, or
     need to return. There are two routines provided in <filename>fmgr.h</>
     to allow a version-1 C function to discover the actual data types
     of its arguments and the type it is expected to return. The routines are
     called <literal>get_fn_expr_rettype(FmgrInfo *flinfo)</> and
     <literal>get_fn_expr_argtype(FmgrInfo *flinfo, int argnum)</>.
     They return the result or argument type OID, or <symbol>InvalidOid</symbol> if the
     information is not available.
     The structure <literal>flinfo</> is normally accessed as
     <literal>fcinfo-&gt;flinfo</>. The parameter <literal>argnum</>
     is zero based.
    </para>

    <para>
     For example, suppose we want to write a function to accept a single
     element of any type, and return a one-dimensional array of that type:

<programlisting>
PG_FUNCTION_INFO_V1(make_array);
Datum
make_array(PG_FUNCTION_ARGS)
{
    ArrayType  *result;
    Oid         element_type = get_fn_expr_argtype(fcinfo-&gt;flinfo, 0);
    Datum       element;
    int16       typlen;
    bool        typbyval;
    char        typalign;
    int         ndims;
    int         dims[MAXDIM];
    int         lbs[MAXDIM];

    if (!OidIsValid(element_type))
        elog(ERROR, "could not determine data type of input");

    /* get the provided element */
    element = PG_GETARG_DATUM(0);

    /* we have one dimension */
    ndims = 1;
    /* and one element */
    dims[0] = 1;
    /* and lower bound is 1 */
    lbs[0] = 1;

    /* get required info about the element type */
    get_typlenbyvalalign(element_type, &amp;typlen, &amp;typbyval, &amp;typalign);

    /* now build the array */
    result = construct_md_array(&amp;element, ndims, dims, lbs,
                                element_type, typlen, typbyval, typalign);

    PG_RETURN_ARRAYTYPE_P(result);
}
</programlisting>
    </para>

    <para>
     The following command declares the function
     <function>make_array</function> in SQL:

<programlisting>
CREATE FUNCTION make_array(anyelement) RETURNS anyarray
    AS '<replaceable>DIRECTORY</replaceable>/funcs', 'make_array'
    LANGUAGE C STRICT;
</programlisting>

     Note the use of <literal>STRICT</literal>; this is essential
     since the code is not bothering to test for a null input.
    </para>
   </sect2>
  </sect1>

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

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

   <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>
    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
    <xref linkend="typeconv">, but it is unwise to design a system that subtly
    relies on this behavior.
   </para>

   <para>
    A function that takes a single argument of a composite type should
    generally not have the same name as any attribute (field) of that type.
    Recall that <literal>attribute(table)</literal> is considered equivalent
    to <literal>table.attribute</literal>.  In the case that there is an
    ambiguity between a function on a composite type and an attribute of
    the composite type, the attribute will always be used.  It is possible
    to override that choice by schema-qualifying the function name
    (that is, <literal>schema.func(table)</literal>) but it's better to
    avoid the problem by not choosing conflicting names.
   </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.  For instance,
<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>
  </sect1>

  <sect1 id="xfunc-volatility">
   <title>Function Volatility Categories</title>

   <indexterm zone="xfunc-volatility">
    <primary>volatility</primary>
    <secondary>functions</secondary>
   </indexterm>

   <para>
    Every function has a <firstterm>volatility</> classification, with
    the possibilities being <literal>VOLATILE</>, <literal>STABLE</>, or
    <literal>IMMUTABLE</>.  <literal>VOLATILE</> is the default if the
    <command>CREATE FUNCTION</command> command does not specify a category.
    The volatility category is a promise to the optimizer about the behavior
    of the function:

   <itemizedlist>
    <listitem>
     <para>
      A <literal>VOLATILE</> function can do anything, including modifying
      the database.  It can return different results on successive calls with
      the same arguments.  The optimizer makes no assumptions about the
      behavior of such functions.  A query using a volatile function will
      re-evaluate the function at every row where its value is needed.
     </para>
    </listitem>
    <listitem>
     <para>
      A <literal>STABLE</> function cannot modify the database and is
      guaranteed to return the same results given the same arguments
      for all calls within a single surrounding query.  This category
      allows the optimizer to optimize away multiple calls of the function
      within a single query.  In particular, it is safe to use an expression
      containing such a function in an indexscan condition.  (Since an
      indexscan will evaluate the comparison value only once, not once at
      each row, it is not valid to use a <literal>VOLATILE</> function in
      an indexscan condition.)
     </para>
    </listitem>
    <listitem>
     <para>
      An <literal>IMMUTABLE</> function cannot modify the database and is
      guaranteed to return the same results given the same arguments forever.
      This category allows the optimizer to pre-evaluate the function when
      a query calls it with constant arguments.  For example, a query like
      <literal>SELECT ... WHERE x = 2 + 2</> can be simplified on sight to
      <literal>SELECT ... WHERE x = 4</>, because the function underlying
      the integer addition operator is marked <literal>IMMUTABLE</>.
     </para>
    </listitem>
   </itemizedlist>
   </para>

   <para>
    For best optimization results, you should label your functions with the
    strictest volatility category that is valid for them.
   </para>

   <para>
    Any function with side-effects <emphasis>must</> be labeled
    <literal>VOLATILE</>, so that calls to it cannot be optimized away.
    Even a function with no side-effects needs to be labeled
    <literal>VOLATILE</> if its value can change within a single query;
    some examples are <literal>random()</>, <literal>currval()</>,
    <literal>timeofday()</>.
   </para>

   <para>
    There is relatively little difference between <literal>STABLE</> and
    <literal>IMMUTABLE</> categories when considering simple interactive
    queries that are planned and immediately executed: it doesn't matter
    a lot whether a function is executed once during planning or once during
    query execution startup.  But there is a big difference if the plan is
    saved and reused later.  Labeling a function <literal>IMMUTABLE</> when
    it really isn't may allow it to be prematurely folded to a constant during
    planning, resulting in a stale value being re-used during subsequent uses
    of the plan.  This is a hazard when using prepared statements or when
    using function languages that cache plans (such as
    <application>PL/pgSQL</>).
   </para>

   <para>
    Because of the snapshotting behavior of MVCC (see <xref linkend="mvcc">)
    a function containing only <command>SELECT</> commands can safely be
    marked <literal>STABLE</>, even if it selects from tables that might be
    undergoing modifications by concurrent queries.
    <productname>PostgreSQL</productname> will execute a <literal>STABLE</>
    function using the snapshot established for the calling query, and so it
    will see a fixed view of the database throughout that query.
    Also note
    that the <function>current_timestamp</> family of functions qualify
    as stable, since their values do not change within a transaction.
   </para>

   <para>
    The same snapshotting behavior is used for <command>SELECT</> commands
    within <literal>IMMUTABLE</> functions.  It is generally unwise to select
    from database tables within an <literal>IMMUTABLE</> function at all,
    since the immutability will be broken if the table contents ever change.
    However, <productname>PostgreSQL</productname> does not enforce that you
    do not do that.
   </para>

   <para>
    A common error is to label a function <literal>IMMUTABLE</> when its
    results depend on a configuration parameter.  For example, a function
    that manipulates timestamps might well have results that depend on the
    <xref linkend="guc-timezone"> setting.  For safety, such functions should
    be labeled <literal>STABLE</> instead.
   </para>

   <note>
    <para>
     Before <productname>PostgreSQL</productname> release 8.0, the requirement
     that <literal>STABLE</> and <literal>IMMUTABLE</> functions cannot modify
     the database was not enforced by the system.  Release 8.0 enforces it
     by requiring SQL functions and procedural language functions of these
     categories to contain no SQL commands other than <command>SELECT</>.
    </para>
   </note>
  </sect1>

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