xfunc.sgml

关系型数据库 Postgresql 6.5.2

<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