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<refentry id="SQL-CREATEAGGREGATE">
<refmeta>
<refentrytitle>
CREATE AGGREGATE
</refentrytitle>
<refmiscinfo>SQL - Language Statements</refmiscinfo>
</refmeta>
<refnamediv>
<refname>
CREATE AGGREGATE
</refname>
<refpurpose>
Defines a new aggregate function
</refpurpose>
</refnamediv>
<refsynopsisdiv>
<refsynopsisdivinfo>
<date>1998-09-09</date>
</refsynopsisdivinfo>
<synopsis>
CREATE AGGREGATE <replaceable class="PARAMETER">name</replaceable> [ AS ]
( BASETYPE = <replaceable class="PARAMETER">data_type</replaceable>
[ , SFUNC1 = <replaceable class="PARAMETER">sfunc1</replaceable>
, STYPE1 = <replaceable class="PARAMETER">sfunc1_return_type</replaceable> ]
[ , SFUNC2 = <replaceable class="PARAMETER">sfunc2</replaceable>
, STYPE2 = <replaceable class="PARAMETER">sfunc2_return_type</replaceable> ]
[ , FINALFUNC = <replaceable class="PARAMETER">ffunc</replaceable> ]
[ , INITCOND1 = <replaceable class="PARAMETER">initial_condition1</replaceable> ]
[ , INITCOND2 = <replaceable class="PARAMETER">initial_condition2</replaceable> ]
)
</synopsis>
<refsect2 id="R2-SQL-CREATEAGGREGATE-1">
<refsect2info>
<date>1998-09-09</date>
</refsect2info>
<title>
Inputs
</title>
<para>
<variablelist>
<varlistentry>
<term><replaceable class="PARAMETER">name</replaceable></term>
<listitem>
<para>
The name of an aggregate function to create.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="PARAMETER">data_type</replaceable></term>
<listitem>
<para>
The fundamental data type on which this aggregate function operates.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="PARAMETER">sfunc1</replaceable></term>
<listitem>
<para>
The state transition function
to be called for every non-NULL field from the source column.
It takes a variable of
type <replaceable class="PARAMETER">sfunc1_return_type</replaceable> as
the first argument and that field as the
second argument.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="PARAMETER">sfunc1_return_type</replaceable></term>
<listitem>
<para>
The return type of the first transition function.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="PARAMETER">sfunc2</replaceable></term>
<listitem>
<para>
The state transition function
to be called for every non-NULL field from the source column.
It takes a variable
of type <replaceable class="PARAMETER">sfunc2_return_type</replaceable>
as the only argument and returns a variable of the same type.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="PARAMETER">sfunc2_return_type</replaceable></term>
<listitem>
<para>
The return type of the second transition function.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="PARAMETER">ffunc</replaceable></term>
<listitem>
<para>
The final function
called after traversing all input fields. This function must
take two arguments of types
<replaceable class="PARAMETER">sfunc1_return_type</replaceable>
and
<replaceable class="PARAMETER">sfunc2_return_type</replaceable>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="PARAMETER">initial_condition1</replaceable></term>
<listitem>
<para>
The initial value for the first transition function argument.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="PARAMETER">initial_condition2</replaceable></term>
<listitem>
<para>
The initial value for the second transition function argument.
</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
</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>1998-09-09</date>
</refsect1info>
<title>
Description
</title>
<para>
<command>CREATE AGGREGATE</command>
allows a user or programmer to extend <productname>Postgres</productname>
functionality by defining new aggregate functions. Some aggregate functions
for base types such as <function>min(int4)</function>
and <function>avg(float8)</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>
An aggregate function can require up to three functions, two
state transition functions,
<replaceable class="PARAMETER">sfunc1</replaceable>
and <replaceable class="PARAMETER">sfunc2</replaceable>:
<programlisting>
<replaceable class="PARAMETER">sfunc1</replaceable>( internal-state1, next-data_item ) ---> next-internal-state1 <replaceable class="PARAMETER">sfunc2</replaceable>( internal-state2 ) ---> next-internal-state2
</programlisting>
and a final calculation function,
<replaceable class="PARAMETER">ffunc</replaceable>:
<programlisting>
<replaceable class="PARAMETER">ffunc</replaceable>(internal-state1, internal-state2) ---> aggregate-value
</programlisting>
</para>
<para>
<productname>Postgres</productname> creates up to two temporary variables
(referred to here as <replaceable class="PARAMETER">temp1</replaceable>
and <replaceable class="PARAMETER">temp2</replaceable>)
to hold intermediate results used as arguments to the transition functions.
</para>
<para>
These transition functions are required to have the following properties:
<itemizedlist>
<listitem>
<para>
The arguments to
<replaceable class="PARAMETER">sfunc1</replaceable>
must be
<replaceable class="PARAMETER">temp1</replaceable>
of type
<replaceable class="PARAMETER">sfunc1_return_type</replaceable>
and
<replaceable class="PARAMETER">column_value</replaceable>
of type <replaceable class="PARAMETER">data_type</replaceable>.
The return value must be of type
<replaceable class="PARAMETER">sfunc1_return_type</replaceable>
and will be used as the first argument in the next call to
<replaceable class="PARAMETER">sfunc1</replaceable>.
</para>
</listitem>
<listitem>
<para>
The argument and return value of
<replaceable class="PARAMETER">sfunc2</replaceable>
must be
<replaceable class="PARAMETER">temp2</replaceable>
of type
<replaceable class="PARAMETER">sfunc2_return_type</replaceable>.
</para>
</listitem>
<listitem>
<para>
The arguments to the final-calculation-function
must be
<replaceable class="PARAMETER">temp1</replaceable>
and
<replaceable class="PARAMETER">temp2</replaceable>
and its return value must
be a <productname>Postgres</productname>
base type (not necessarily
<replaceable class="PARAMETER">data_type</replaceable>
which had been specified for BASETYPE).
</para>
</listitem>
<listitem>
<para>
FINALFUNC should be specified
if and only if both state-transition functions are
specified.
</para></listitem>
</itemizedlist>
</para>
<para>
An aggregate function may also require one or two initial conditions,
one for
each transition function. These are specified and stored
in the database as fields of type <type>text</type>.
</para>
<refsect2 id="R2-SQL-CREATEAGGREGATE-3">
<refsect2info>
<date>1998-09-09</date>
</refsect2info>
<title>
Notes
</title>
<para>
Use <command>DROP AGGREGATE</command>
to drop aggregate functions.
</para>
<para>
It is possible to specify aggregate functions
that have varying combinations of state and final functions.
For example, the <function>count</function> aggregate requires SFUNC2
(an incrementing function) but not SFUNC1 or FINALFUNC,
whereas the <function>sum</function> aggregate requires SFUNC1 (an addition
function) but not SFUNC2 or FINALFUNC and the <function>avg</function>
aggregate requires
both of the above state functions as
well as a FINALFUNC (a division function) to produce its
answer. In any case, at least one state function must be
defined, and any SFUNC2 must have a corresponding INITCOND2.
</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>
on aggregate functions 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>Postgres</productname> language extension.
There is no <command>CREATE AGGREGATE</command> in SQL92.
</para>
</refsect2>
</refsect1>
</refentry>
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