path: root/doc/src/sgml/xfunc.sgml

<Chapter 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.
     <ProductName>Postgres</ProductName>  <Acronym>SQL</Acronym>  
     provides  two  types of functions: query language functions 
     (functions written in <Acronym>SQL</Acronym>  and  programming  
     language  functions  (functions  written in a compiled 
     programming language such as <Acronym>C</Acronym>.)  Either  kind
     of  function  can take a base type, a composite type or
     some combination as arguments (parameters).   In  addition, 
     both kinds of functions can return a base type or
     a composite type.  It's easier 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>

<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 <Acronym>int4</Acronym>:
     
<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 target list for  the  function
     (with  the  name  RESULT),  but  the target list of the
     query that invoked the function overrode the function's
     target  list.   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
     notation 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 (or when you execute a .*  query).
</Para>
      </ListItem>
      <ListItem>
<Para>
You must typecast the expressions (using ::) very carefully 
or you will see  the  following error:
        
<ProgramListing>
   WARN::function declared to return type EMP does not retrieve (EMP.*)
</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;
            WARN: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., <Acronym>insert</Acronym>,  
     <Acronym>update</Acronym> and <Acronym>delete</Acronym>) as well 
     as <Acronym>select</Acronym> queries.  However, the final command 
     must be a <Acronym>select</Acronym> 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>Programming Language Functions</Title>

<Sect2>
<Title>Programming Language Functions on Base Types</Title>

<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 
     <Acronym>long</Acronym> type is dangerous because  it  
     is 4 bytes on some machines and 8 bytes on others, whereas 
     <Acronym>int</Acronym>  type  is  4  bytes  on  most  
     <Acronym>UNIX</Acronym> machines  (though  not  on most 
     personal computers).  A reasonable implementation of  
     the  <Acronym>int4</Acronym>  type  on  <Acronym>UNIX</Acronym>
     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.
     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:
</Para>

<Para>
<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. 
     Suppose <FileName>funcs.c</FileName> look like:
<ProgramListing>
         #include &lt;string.h&gt;
         #include "postgres.h"

         /* By Value */
         
         int
         add_one(int arg)
         {
             return(arg + 1);
         }
         
         /* By Reference, Fixed Length */
         
         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));
             memset(new_t, 0, VARSIZE(t));
             VARSIZE(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);
             VARSIZE(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>
     On <Acronym>OSF/1</Acronym> we would type:
     
<ProgramListing>
         CREATE FUNCTION add_one(int4) RETURNS int4
              AS 'PGROOT/tutorial/funcs.so' LANGUAGE 'c';

         CREATE FUNCTION makepoint(point, point) RETURNS point
              AS 'PGROOT/tutorial/funcs.so' LANGUAGE 'c';
    
         CREATE FUNCTION concat_text(text, text) RETURNS text
              AS 'PGROOT/tutorial/funcs.so' LANGUAGE 'c';
                                  
         CREATE FUNCTION copytext(text) RETURNS text
              AS 'PGROOT/tutorial/funcs.so' LANGUAGE 'c';
</ProgramListing>
</Para>

<Para>
     On  other  systems,  we might have to make the filename
     end in .sl (to indicate that it's a shared library).
</Para>
</Sect2>

<Sect2>
<Title>Programming Language Functions on Composite Types</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 <Acronym>TUPLE</Acronym>.
     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 */
                    int4 limit)
         {
             bool isnull = false;
             int4 salary;
             salary = (int4) GetAttributeByName(t, "salary", &amp;isnull);
             if (isnull)
                 return (false);
             return(salary &gt; limit);
         }
</ProgramListing>
</Para>

<Para>
     <Acronym>GetAttributeByName</Acronym> is the 
     <ProductName>Postgres</ProductName> system function that
     returns attributes out of the current instance.  It has
     three arguments: the argument of type TUPLE passed into
     the  function, the name of the desired attribute, and a
     return parameter that describes whether  the  attribute
     is  null.   <Acronym>GetAttributeByName</Acronym> will 
     align data properly so you can cast its return value to 
     the  desired  type. For  example, if you have an attribute 
     name which is of the type name, the <Acronym>GetAttributeByName</Acronym> 
     call would look like:
<ProgramListing>
         char *str;
         ...
         str = (char *) GetAttributeByName(t, "name", &amp;isnull)
</ProgramListing>
</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 'PGROOT/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>Caveats</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 
     "calling convention" 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>.
     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>PGROOT/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  postgres.h,  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  Appendix  A
            for  a  detailed explanation of how to do it for
            your particular operating system.
</Para>
</ListItem>
</ItemizedList>
</Para>
</Sect2>
</sect1>
</chapter>