<!--
$Header: /cvsroot/pgsql/doc/src/sgml/ref/create_aggregate.sgml,v 1.20 2002/05/18 15:44:47 petere Exp $
PostgreSQL documentation
-->
<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>
<!-- Keep this comment at the end of the file
Local variables:
mode: sgml
sgml-omittag:nil
sgml-shorttag:t
sgml-minimize-attributes:nil
sgml-always-quote-attributes:t
sgml-indent-step:1
sgml-indent-data:t
sgml-parent-document:nil
sgml-default-dtd-file:"../reference.ced"
sgml-exposed-tags:nil
sgml-local-catalogs:"/usr/lib/sgml/catalog"
sgml-local-ecat-files:nil
End:
-->