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

  <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>Postgres</productname>  <acronym>SQL</acronym>  
   provides  three types 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, PLTCL or PLSQL)
     </para>
    </listitem>
    <listitem>
     <para>
      programming  
      language  functions  (functions  written in a compiled 
      programming language such as <acronym>C</acronym>)
     </para>
    </listitem>
   </itemizedlist>

   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>.
  </para>

  <sect1>
   <title>Query Language (<acronym>SQL</acronym>) Functions</title>

   <para>
    SQL functions execute an arbitrary list of SQL queries, returning
    the results of the last query in the list.  SQL functions in general
    return sets.  If their returntype is not specified as a
    <literal>setof</literal>,
    then an arbitrary element of the last query's result will be returned.
   </para>

   <para>
    The body of a SQL function following AS
    should be a list of queries separated by semicolons and
    bracketed within single-quote marks.  Note that quote marks used in
    the queries must be escaped, by preceding them with a backslash.
   </para>

   <para>
    Arguments to the SQL function may be referenced in the queries using
    a $n syntax: $1 refers to the first argument, $2 to the second, and so
    on.  If an argument is complex, then a <firstterm>dot</firstterm>
    notation (e.g. "$1.emp") may be
    used to access attributes of the argument or
    to invoke functions.
   </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 (int4, float8) 
    RETURNS int4
    AS 'UPDATE bank 
        SET balance = bank.balance - $2
        WHERE bank.acctountno = $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>
     The following more interesting example takes a single argument of type
     EMP, and retrieves multiple results:

     <programlisting>
CREATE FUNCTION hobbies (EMP) RETURNS SET OF hobbies
    AS 'SELECT hobbies.* FROM hobbies
        WHERE $1.name = hobbies.person'
    LANGUAGE 'sql';
     </programlisting>
    </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 <literal>int4</literal>:
     
     <programlisting>
CREATE FUNCTION one() 
    RETURNS int4
    AS 'SELECT 1 as RESULT;' 
    LANGUAGE 'sql';

SELECT one() AS answer;

+-------+
|answer |
+-------+
|1      |
+-------+
     </programlisting>
    </para>
    <para>
     Notice that we defined a column name for  the  function's result
     (with  the  name  RESULT),  but this column name is not visible
     outside the function.  Hence,  the  result  is labelled answer
     instead of one.
    </para>
    <para>
     It's 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 $1
     and $2:

     <programlisting>
CREATE FUNCTION add_em(int4, int4) 
    RETURNS int4
    AS 'SELECT $1 + $2;' 
    LANGUAGE 'sql';

SELECT add_em(1, 2) AS answer;

+-------+
|answer |
+-------+
|3      |
+-------+
     </programlisting>
    </para>
   </sect2>

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

    <para>
     When  specifying  functions with arguments of composite
     types (such as EMP), we must  not  only  specify  which
     argument  we  want (as we did above with $1 and $2) but
     also the attributes of  that  argument.   For  example,
     take the function double_salary that computes what your
     salary would be if it were doubled:

     <programlisting>
CREATE FUNCTION double_salary(EMP) 
    RETURNS int4
    AS 'SELECT $1.salary * 2 AS salary;' 
    LANGUAGE 'sql';

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


+-----+-------+
|name | dream |
+-----+-------+
|Sam  | 2400  |
+-----+-------+
     </programlisting>
    </para>
    <para>
     Notice the use of the syntax $1.salary.
     Before launching into the  subject  of  functions  that
     return  composite  types,  we  must first introduce the
     function notation for projecting attributes.  The  simple  way 
     to explain this is that we can usually use the
     notations attribute(class)  and  class.attribute  interchangably:

     <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;

+----------+
|youngster |
+----------+
|Sam       |
+----------+
     </programlisting>
    </para>
    <para>
     As  we shall see, however, this is not always the case.
     This function notation is important when we want to use
     a  function that returns a single instance.  We do this
     by assembling the entire instance within the  function,
     attribute  by attribute.  This is an example of a function 
     that returns a single EMP instance:

     <programlisting>
CREATE FUNCTION new_emp() 
    RETURNS EMP
    AS 'SELECT \'None\'::text AS name,
        1000 AS salary,
        25 AS age,
        \'(2,2)\'::point 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.
     Defining a function like this can be tricky.   Some  of
     the more important caveats are as follows:

     <itemizedlist>
      <listitem>
       <para>
	The  target  list  order must be exactly the same as
	that in which the attributes appear  in  the  CREATE
	TABLE statement that defined the composite type.
       </para>
      </listitem>
      <listitem>
       <para>
	You must typecast the expressions (using ::) to match the
	composite type's definition, or you will get errors like this:
	<programlisting>
	 <computeroutput>
ERROR:  function declared to return emp returns varchar instead of text at column 1
	 </computeroutput>
	</programlisting>
       </para>
      </listitem>
      <listitem>
       <para>
	When calling a function that returns an instance, we
        cannot retrieve the entire instance.  We must either
        project an attribute out of the instance or pass the
        entire instance into another function.

	<programlisting>
SELECT name(new_emp()) AS nobody;

+-------+
|nobody |
+-------+
|None   |
+-------+
	</programlisting>
       </para>
      </listitem>
      <listitem>
       <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  other (dot) syntax for projection when combined
        with function calls.

	<programlisting>
SELECT new_emp().name AS nobody;
NOTICE:parser: syntax error at or near "."
	</programlisting>
       </para>
      </listitem>
     </itemizedlist>
    </para>     
    <para>
     Any collection of commands in the  <acronym>SQL</acronym>  query  
     language can be packaged together and defined as a function.
     The commands can include updates (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 returntype.

     <programlisting>
CREATE FUNCTION clean_EMP () 
    RETURNS int4
    AS 'DELETE FROM EMP 
        WHERE EMP.salary &lt;= 0;
        SELECT 1 AS ignore_this;'
    LANGUAGE 'sql';

SELECT clean_EMP();

+--+
|x |
+--+
|1 |
+--+
     </programlisting>
    </para>
   </sect2>
  </sect1>

  <sect1>
   <title>Procedural Language Functions</title>

   <para>
    Procedural languages aren't built into Postgres. They are offered
    by loadable modules. Please refer to the documentation for the
    PL in question for details about the syntax and how the AS
    clause is interpreted by the PL handler.
   </para>

   <para>
    There are two procedural languages available with the standard
    <productname>Postgres</productname> distribution (PLTCL and PLSQL), and other
    languages can be defined.
    Refer to <xref linkend="xplang-title" endterm="xplang-title"> for
    more information.
   </para>
  </sect1>

  <sect1>
   <title>Internal Functions</title>

   <para>
    Internal functions are functions written in C which have been statically
    linked into the <productname>Postgres</productname> backend
    process. The AS
    clause gives 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 AS
    string is accepted as meaning that the C-language function name is the
    same as the SQL name.)  Normally, all internal functions present in the
    backend are declared as SQL functions during database initialization,
    but a user could use <command>CREATE FUNCTION</command>
    to create additional alias names for an internal function.
   </para>

   <para>
    Internal functions are declared in <command>CREATE FUNCTION</command>
    with language name <literal>internal</literal> or
    <literal>newinternal</literal>, depending on whether they follow the
    old (pre-7.1) or new (7.1 and later) function call conventions.
    The details of the call conventions are the same as for
    <literal>C</literal> and <literal>newC</literal> functions respectively;
    see the next section for details.
   </para>
  </sect1>

  <sect1>
   <title>Compiled (C) Language Functions</title>

   <para>
    Functions written in C can be compiled into dynamically loadable
    objects (also called shared libraries), and used to implement user-defined
    SQL functions.  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 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.

    <note>
     <para>
      After it is used for the first time, a dynamically loaded user
      function is retained in memory, and future calls to the function
      in the same session will only incur the small overhead of a symbol table
      lookup.
     </para>
    </note>
   </para>

   <para>
    The string which specifies the object file (the first string in the AS
    clause) should be the <emphasis>full path</emphasis> of the object
    code file for the function, bracketed by quotation marks.  If a
    link symbol is given in the AS clause, the link symbol should also be
    bracketed by single quotation marks, and should be exactly the
    same as the name of the function in the C source code. On Unix systems
    the command <command>nm</command> will print all of the link
    symbols in a dynamically loadable object.

    <note>
     <para>
      <productname>Postgres</productname> will not compile a function
      automatically; it must be compiled before it is used in a CREATE
      FUNCTION command.  See below for additional information.
     </para>
    </note>
   </para>

   <para>
    Two different calling conventions are currently used for C functions.
    The "old style" (pre-<productname>Postgres</productname>-7.1) method
    is selected by writing language name '<literal>C</literal>' in the
    <command>CREATE FUNCTION</command> command, while the "new style"
    (7.1 and later) method is selecting by writing language name
    '<literal>newC</literal>'.  Old-style functions are now deprecated
    because of portability problems and lack of functionality, but they
    are still supported for compatibility reasons.
   </para>

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

    <para>
     The following table gives the C type required for parameters in the C
     functions that will be loaded into Postgres.  The "Defined In"
     column gives the actual header file (in the
     <filename>.../src/backend/</filename>
     directory) that the equivalent C type is defined.  However, if you
     include <filename>utils/builtins.h</filename>,
     these files will automatically be
     included.

     <table tocentry="1">
      <title>Equivalent C Types
       for Built-In <productname>Postgres</productname> Types</title>
      <titleabbrev>Equivalent C Types</titleabbrev>
      <tgroup cols="3">
       <thead>
	<row>
	 <entry>
	  Built-In Type
	 </entry>
	 <entry>
	  C Type
	 </entry>
	 <entry>
	  Defined In
	 </entry>
	</row>
       </thead>
       <tbody>
	<row>
	 <entry>abstime</entry>
	 <entry>AbsoluteTime</entry>
	 <entry>utils/nabstime.h</entry>
	</row>
	<row>
	 <entry>bool</entry>
	 <entry>bool</entry>
	 <entry>include/c.h</entry>
	</row>
	<row>
	 <entry>box</entry>
	 <entry>(BOX *)</entry>
	 <entry>utils/geo-decls.h</entry>
	</row>
	<row>
	 <entry>bytea</entry>
	 <entry>(bytea *)</entry>
	 <entry>include/postgres.h</entry>
	</row>
	<row>
	 <entry>char</entry>
	 <entry>char</entry>
	 <entry>N/A</entry>
	</row>
	<row>
	 <entry>cid</entry>
	 <entry>CID</entry>
	 <entry>include/postgres.h</entry>
	</row>
	<row>
	 <entry>datetime</entry>
	 <entry>(DateTime *)</entry>
	 <entry>include/c.h or include/postgres.h</entry>
	</row>
	<row>
	 <entry>int2</entry>
	 <entry>int2 or int16</entry>
	 <entry>include/postgres.h</entry>
	</row>
	<row>
	 <entry>int2vector</entry>
	 <entry>(int2vector *)</entry>
	 <entry>include/postgres.h</entry>
	</row>
	<row>
	 <entry>int4</entry>
	 <entry>int4 or int32</entry>
	 <entry>include/postgres.h</entry>
	</row>
	<row>
	 <entry>float4</entry>
	 <entry>(float4 *)</entry>
	<entry>include/c.h or include/postgres.h</entry>
	</row>
	<row>
	 <entry>float8</entry>
	 <entry>(float8 *)</entry>
	 <entry>include/c.h or include/postgres.h</entry>
	</row>
	<row>
	 <entry>lseg</entry>
	 <entry>(LSEG *)</entry>
	 <entry>include/geo-decls.h</entry>
	</row>
	<row>
	 <entry>name</entry>
	 <entry>(Name)</entry>
	 <entry>include/postgres.h</entry>
	</row>
	<row>
	 <entry>oid</entry>
	 <entry>oid</entry>
	 <entry>include/postgres.h</entry>
	</row>
	<row>
	 <entry>oidvector</entry>
	 <entry>(oidvector *)</entry>
	 <entry>include/postgres.h</entry>
	</row>
	<row>
	 <entry>path</entry>
	 <entry>(PATH *)</entry>
	 <entry>utils/geo-decls.h</entry>
	</row>
	<row>
	 <entry>point</entry>
	 <entry>(POINT *)</entry>
	 <entry>utils/geo-decls.h</entry>
	</row>
	<row>
	 <entry>regproc</entry>
	 <entry>regproc or REGPROC</entry>
	 <entry>include/postgres.h</entry>
	</row>
	<row>
	 <entry>reltime</entry>
	 <entry>RelativeTime</entry>
	 <entry>utils/nabstime.h</entry>
	</row>
	<row>
	 <entry>text</entry>
	 <entry>(text *)</entry>
	 <entry>include/postgres.h</entry>
	</row>
	<row>
	 <entry>tid</entry>
	 <entry>ItemPointer</entry>
	 <entry>storage/itemptr.h</entry>
	</row>
	<row>
	 <entry>timespan</entry>
	 <entry>(TimeSpan *)</entry>
	 <entry>include/c.h or include/postgres.h</entry>
	</row>
	<row>
	 <entry>tinterval</entry>
	 <entry>TimeInterval</entry>
	 <entry>utils/nabstime.h</entry>
	</row>
	<row>
	 <entry>uint2</entry>
	 <entry>uint16</entry>
	 <entry>include/c.h</entry>
	</row>
	<row>
	 <entry>uint4</entry>
	 <entry>uint32</entry>
	 <entry>include/c.h</entry>
	</row>
	<row>
	 <entry>xid</entry>
	 <entry>(XID *)</entry>
	 <entry>include/postgres.h</entry>
	</row>
       </tbody>
      </tgroup>
     </table>
    </para>

    <para>
     Internally, <productname>Postgres</productname> regards a
     base type as a "blob  of memory."   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
     (even if your computer supports by-value types of other
     sizes).   <productname>Postgres</productname>  itself 
     only passes integer types by value.  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 
     <literal>int</literal>  type  is  4  bytes  on  most  
     Unix machines  (though  not  on most 
     personal computers).  A reasonable implementation of  
     the  <literal>int4</literal>  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>Postgres</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.
    </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 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>Old-style Calling Conventions for C-Language Functions</title>

    <para>
     We present the "old style" calling convention first --- although
     this approach is now deprecated, it's easier to get a handle on
     initially.  In the "old style" 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 &lt;string.h&gt;
#include "postgres.h"

/* 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);

    memset((void *) new_text, 0, new_text_size);
    VARATT_SIZEP(new_text) = new_text_size;
    strncpy(VARDATA(new_text), VARDATA(arg1), VARSIZE(arg1)-VARHDRSZ);
    strncat(VARDATA(new_text), 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.so' 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.so',
        'add_one_float8'
     LANGUAGE 'c' WITH (isStrict);

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

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

    <para>
     Here <replaceable>PGROOT</replaceable> stands for the full path to
     the <productname>Postgres</productname> source tree.  Note that
     depending on your system, the filename for a shared object might
     not end in <literal>.so</literal>, but in <literal>.sl</literal>
     or something else; adapt accordingly.
    </para>

    <para>
     Notice that we have specified the functions as "strict", 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 old-style 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 new-style
     convention, presented next, overcomes these objections.
    </para>
   </sect2>

   <sect2>
    <title>New-style Calling Conventions for C-Language Functions</title>

    <para>
     The new-style calling convention relies on macros to suppress most
     of the complexity of passing arguments and results.  The C declaration
     of a new-style function is always
     <programlisting>
                Datum funcname(PG_FUNCTION_ARGS)
     </programlisting>
     Each actual argument is fetched using a PG_GETARG_xxx() macro that
     corresponds to the argument's datatype, and the result is returned
     using a PG_RETURN_xxx() macro for the return type.
    </para>

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

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

/* By Value */
         
Datum
add_one(PG_FUNCTION_ARGS)
{
    int32   arg = PG_GETARG_INT32(0);

    PG_RETURN_INT32(arg + 1);
}

/* By Reference, Fixed Length */

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);
}

Datum
makepoint(PG_FUNCTION_ARGS)
{
    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 */

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);
}

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);

    memset((void *) new_text, 0, new_text_size);
    VARATT_SIZEP(new_text) = new_text_size;
    strncpy(VARDATA(new_text), VARDATA(arg1), VARSIZE(arg1)-VARHDRSZ);
    strncat(VARDATA(new_text), 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 old-style equivalents, except that the language is specified
     as '<literal>newC</literal>' not '<literal>C</literal>'.
    </para>

    <para>
     At first glance, the new-style 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 "toasted" (compressed or out-of-line) values.
     The old-style copytext and concat_text functions shown above are
     actually wrong in the presence of toasted values, because they don't
     call pg_detoast_datum() on their inputs.
    </para>

    <para>
     The new-style function call conventions also make it possible to
     test for NULL inputs to a non-strict function, return a NULL result
     (from either strict or non-strict functions), return "set" results,
     and implement trigger functions and procedural-language call handlers.
     For more details see <filename>src/backend/utils/fmgr/README</filename>.
    </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 instances, each instance 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 instance of EMP */
           int32 limit)
{
    bool isnull;
    int32 salary;

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

/* In new-style coding, the above would look like this: */

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 instance.  It has
     three arguments: the argument of type TupleTableSlot* 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 DatumGetXXX() macro.
    </para>

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

     <programlisting>
CREATE FUNCTION c_overpaid(EMP, int4) 
RETURNS bool
AS '<replaceable>PGROOT</replaceable>/tutorial/obj/funcs.so' 
LANGUAGE 'c';
     </programlisting>
    </para>

    <para>
     While there are ways to construct new instances or modify  
     existing instances from within a C function, these
     are far too complex to discuss in this manual.
    </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>
     C functions with base type arguments can be written in a
     straightforward fashion.  The C equivalents of built-in Postgres types
     are accessible in a C file if 
     <filename><replaceable>PGROOT</replaceable>/src/backend/utils/builtins.h</filename>
     is included as a header file.  This can be achieved by having

     <programlisting>
#include &lt;utils/builtins.h&gt;
     </programlisting>

     at the top of the C source file.
    </para>

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

     <itemizedlist>
      <listitem>
       <para>
	Most of the header (include) files for 
	<productname>Postgres</productname>
	should      already      be     installed     in
	<filename><replaceable>PGROOT</replaceable>/include</filename>  (see  Figure  2).
	You should always include

	<programlisting>
-I$PGROOT/include
	</programlisting>

	on  your  cc  command lines.  Sometimes, you may
	find that you require header files that  are  in
	the  server source itself (i.e., you need a file
	we neglected to install in include).   In  those
	cases you may need to add one or more of

	<programlisting>
-I$PGROOT/src/backend
-I$PGROOT/src/backend/include
-I$PGROOT/src/backend/port/&lt;PORTNAME&gt;
-I$PGROOT/src/backend/obj
	</programlisting>

	(where &lt;PORTNAME&gt; is the name of the port, e.g.,
	alpha or sparc).
       </para>
      </listitem>
      <listitem>
       <para>
	When allocating memory, use  the
	<productname>Postgres</productname>
	routines  palloc  and  pfree  instead of the 
	corresponding <acronym>C</acronym> library  routines  
	malloc  and  free.
	The  memory  allocated  by  palloc 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
	memset or bzero.  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>,
	    so  it's a good 
	idea to always include that file as well.  Including 
	postgres.h will also include elog.h and palloc.h for you.
       </para>
      </listitem>
      <listitem>
       <para>
	Compiling and loading your object code  so  that
	it  can  be  dynamically  loaded  into  
	<productname>Postgres</productname>
	always requires special flags.
	See <xref linkend="dfunc-title" endterm="dfunc-title">
	for  a  detailed explanation of how to do it for
	your particular operating system.
       </para>
      </listitem>
     </itemizedlist>
    </para>
   </sect2>
  </sect1>

  <sect1>
   <title>Function Overloading</title>

   <para>
    More than one function may be defined with the same name, as long as
    the arguments they take are different.  In other words, function names
    can be <firstterm>overloaded</firstterm>.
    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>

   <sect2>
    <title>Name Space Conflicts</title>

    <para>
     As of <productname>Postgres</productname> v7.0, the alternative
     form of the AS clause for the SQL
     <command>CREATE FUNCTION</command> command
     decouples the SQL function name from the function name in the C
     source code. This is now the preferred technique to accomplish
     function overloading.
    </para>

    <sect3>
     <title>Pre-v7.0</title>

     <para>
      For functions written in C, the SQL name declared in
      <command>CREATE FUNCTION</command>
      must be exactly the same as the actual name of the function in the
      C code (hence it must be a legal C function name).
     </para>

     <para>
      There is a subtle implication of this restriction: while the
      dynamic loading routines in most operating systems are more than 
      happy to allow you to load any number of shared libraries that 
      contain conflicting (identically-named) function names, they may 
      in fact botch the load in interesting ways.  For example, if you
      define a dynamically-loaded function that happens to have the
      same name as a function built into Postgres, the DEC OSF/1 dynamic 
      loader causes Postgres to call the function within itself rather than 
      allowing Postgres to call your function.  Hence, if you want your
      function to be used on different architectures, we recommend that 
      you do not overload C function names.
     </para>

     <para>
      There is a clever trick to get around the problem just described.
      Since there is no problem overloading SQL functions, you can 
      define 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>

     <para>
      Another solution is not to use dynamic loading, but to link your
      functions into the backend statically and declare them as INTERNAL
      functions.  Then, the functions must all have distinct C names but
      they can be declared with the same SQL names (as long as their
      argument types differ, of course).  This way avoids the overhead of
      an SQL wrapper function, at the cost of more effort to prepare a
      custom backend executable.  (This option is only available in version
      6.5 and later, since prior versions required internal functions to
      have the same name in SQL as in the C code.)
     </para>
    </sect3>
   </sect2>
  </sect1>
 </chapter>

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