create_aggregate.sgml

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$Header: /cvsroot/pgsql/doc/src/sgml/ref/create_aggregate.sgml,v 1.20 2002/05/18 15:44:47 petere Exp $
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<refentry id="SQL-CREATEAGGREGATE">
 <refmeta>
  <refentrytitle id="sql-createaggregate-title">CREATE AGGREGATE</refentrytitle>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

 <refnamediv>
  <refname>
   CREATE AGGREGATE
  </refname>
  <refpurpose>
   define a new aggregate function
  </refpurpose>
 </refnamediv>
 <refsynopsisdiv>
  <refsynopsisdivinfo>
   <date>2000-07-16</date>
  </refsynopsisdivinfo>
  <synopsis>
CREATE AGGREGATE <replaceable class="PARAMETER">name</replaceable> ( BASETYPE = <replaceable class="PARAMETER">input_data_type</replaceable>,
    SFUNC = <replaceable class="PARAMETER">sfunc</replaceable>, STYPE = <replaceable class="PARAMETER">state_type</replaceable>
    [ , FINALFUNC = <replaceable class="PARAMETER">ffunc</replaceable> ]
    [ , INITCOND = <replaceable class="PARAMETER">initial_condition</replaceable> ] )
  </synopsis>

  <refsect2 id="R2-SQL-CREATEAGGREGATE-1">
   <refsect2info>
    <date>2000-07-16</date>
   </refsect2info>
   <title>
    Inputs
   </title>
   <para>
    <variablelist>
     <varlistentry>
      <term><replaceable class="PARAMETER">name</replaceable></term>
      <listitem>
       <para>
	The name (optionally schema-qualified) of an aggregate function to
	create.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="PARAMETER">input_data_type</replaceable></term>
      <listitem>
       <para>
	The input data type on which this aggregate function operates.
	This can be specified as ANY for an aggregate that does not
	examine its input values
	(an example is <function>count(*)</function>).
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="PARAMETER">sfunc</replaceable></term>
      <listitem>
       <para>
	The name of the state transition function
	to be called for each input data value.
	This is normally a function of two arguments, the first being of
	type <replaceable class="PARAMETER">state_type</replaceable>
	and the second of
	type <replaceable class="PARAMETER">input_data_type</replaceable>.
	Alternatively, for an aggregate that does not examine its input
	values, the function takes just one argument of
	type <replaceable class="PARAMETER">state_type</replaceable>.
	In either case the function must return a value of
	type <replaceable class="PARAMETER">state_type</replaceable>.
	This function takes the current state value and the current
	input data item, and returns the next state value.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="PARAMETER">state_type</replaceable></term>
      <listitem>
       <para>
	The data type for the aggregate's state value.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="PARAMETER">ffunc</replaceable></term>
      <listitem>
       <para>
	The name of the final function called to compute the aggregate's
	result after all input data has been traversed.  The function
	must take a single argument of type
	<replaceable class="PARAMETER">state_type</replaceable>.
	The output data type of the aggregate is defined as the return
	type of this function.
	If <replaceable class="PARAMETER">ffunc</replaceable>
	is not specified, then the ending state value is used as the
	aggregate's result, and the output type is
	<replaceable class="PARAMETER">state_type</replaceable>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="PARAMETER">initial_condition</replaceable></term>
      <listitem>
       <para>
	The initial setting for the state value.  This must be a literal
	constant in the form accepted for the data type
	<replaceable class="PARAMETER">state_type</replaceable>.
	If not specified, the state value starts out NULL.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>
  </refsect2>

  <refsect2 id="R2-SQL-CREATEAGGREGATE-2">
   <refsect2info>
    <date>1998-09-09</date>
   </refsect2info>
   <title>
    Outputs
   </title>
   <para>

    <variablelist>
     <varlistentry>
      <term><computeroutput>
CREATE AGGREGATE
       </computeroutput></term>
      <listitem>
       <para>
	Message returned if the command completes successfully.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>
  </refsect2>
 </refsynopsisdiv>

 <refsect1 id="R1-SQL-CREATEAGGREGATE-1">
  <refsect1info>
   <date>2000-07-16</date>
  </refsect1info>
  <title>
   Description
  </title>
  <para>
   <command>CREATE AGGREGATE</command> 
   allows a user or programmer to extend <productname>PostgreSQL</productname>
   functionality by defining new aggregate functions. Some aggregate functions
   for base types such as <function>min(integer)</function>
   and <function>avg(double precision)</function> are already provided in the base
   distribution. If one defines new types or needs an aggregate function not
   already provided, then <command>CREATE AGGREGATE</command>
   can be used to provide the desired features.
  </para>
  <para>
   If a schema name is given (for example, <literal>CREATE AGGREGATE
   myschema.myagg ...</>) then the aggregate function is created in the
   specified schema.  Otherwise it is created in the current schema (the one
   at the front of the search path; see <literal>CURRENT_SCHEMA()</>).
  </para>
  <para>
   An  aggregate  function is identified by its name and input data type.
   Two aggregates in the same schema can have the same name if they operate on
   different input types.  The
   name and input data type of an aggregate must also be distinct from
   the name and input data type(s) of every ordinary function in the same
   schema.
  </para>
  <para>
   An  aggregate function is made from one or two ordinary
   functions:
   a state transition function
   <replaceable class="PARAMETER">sfunc</replaceable>,
   and an optional final calculation function
   <replaceable class="PARAMETER">ffunc</replaceable>.
   These are used as follows:
   <programlisting>
<replaceable class="PARAMETER">sfunc</replaceable>( internal-state, next-data-item ) ---> next-internal-state
<replaceable class="PARAMETER">ffunc</replaceable>( internal-state ) ---> aggregate-value
   </programlisting>
  </para>
  <para>
   <productname>PostgreSQL</productname> creates a temporary variable
   of data type <replaceable class="PARAMETER">stype</replaceable>
   to hold the current internal state of the aggregate.  At each input
   data item,
   the state transition function is invoked to calculate a new
   internal state value.  After all the data has been processed,
   the final function is invoked once to calculate the aggregate's output
   value.  If there is no final function then the ending state value
   is returned as-is.
  </para>
  
  <para>
   An aggregate function may provide an initial condition,
   that is, an initial value for the internal state value.
   This is specified and stored in the database as a field of type
   <type>text</type>, but it must be a valid external representation
   of a constant of the state value data type.  If it is not supplied
   then the state value starts out NULL.
  </para>
  
  <para>
   If the state transition function is declared <quote>strict</quote>,
   then it cannot be called with NULL inputs.  With such a transition
   function, aggregate execution behaves as follows.  NULL input values
   are ignored (the function is not called and the previous state value
   is retained).  If the initial state value is NULL, then the first
   non-NULL input value replaces the state value, and the transition
   function is invoked beginning with the second non-NULL input value.
   This is handy for implementing aggregates like <function>max</function>.
   Note that this behavior is only available when
   <replaceable class="PARAMETER">state_type</replaceable>
   is the same as
   <replaceable class="PARAMETER">input_data_type</replaceable>.
   When these types are different, you must supply a non-NULL initial
   condition or use a non-strict transition function.
  </para>
  
  <para>
   If the state transition function is not strict, then it will be called
   unconditionally at each input value, and must deal with NULL inputs
   and NULL transition values for itself.  This allows the aggregate
   author to have full control over the aggregate's handling of NULLs.
  </para>
  
  <para>
   If the final function is declared <quote>strict</quote>, then it will not
   be called when the ending state value is NULL; instead a NULL result
   will be output automatically.  (Of course this is just the normal
   behavior of strict functions.)  In any case the final function has
   the option of returning NULL.  For example, the final function for
   <function>avg</function> returns NULL when it sees there were zero
   input tuples.
  </para>
  
  <refsect2 id="R2-SQL-CREATEAGGREGATE-3">
   <refsect2info>
    <date>2000-07-16</date>
   </refsect2info>
   <title>
    Notes
   </title>
   <para>
    Use <command>DROP AGGREGATE</command>
    to drop aggregate functions.
   </para>

   <para>
    The parameters of <command>CREATE AGGREGATE</command> can be written
    in any order, not just the order illustrated above.
   </para>

  </refsect2>
 </refsect1>

 <refsect1 id="R1-SQL-CREATEAGGREGATE-2">
  <title>
   Usage
  </title>
  <para>
   Refer to the chapter on aggregate functions
   in the <citetitle>PostgreSQL Programmer's Guide</citetitle> for
   complete examples of usage.
  </para>
 </refsect1>

 <refsect1 id="R1-SQL-CREATEAGGREGATE-3">
  <title>
   Compatibility
  </title>

  <refsect2 id="R2-SQL-CREATEAGGREGATE-4">
   <refsect2info>
    <date>1998-09-09</date>
   </refsect2info>
   <title>
    SQL92
   </title>
   <para>
    <command>CREATE AGGREGATE</command> 
    is a <productname>PostgreSQL</productname> language extension.
    There is no <command>CREATE AGGREGATE</command> in SQL92.
   </para>
  </refsect2>
 </refsect1>
</refentry>

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