<entry><literal><link linkend="catalog-pg-proc"><structname>pg_proc</structname></link>.oid</literal></entry>

<entry>Final function (zero if none)</entry>

</row>

<row>

<entry><structfield>aggcombinefn</structfield></entry>

<entry><type>regproc</type></entry>

<entry><literal><link linkend="catalog-pg-proc"><structname>pg_proc</structname></link>.oid</literal></entry>

<entry>Combine function (zero if none)</entry>

</row>

<row>

<entry><structfield>aggmtransfn</structfield></entry>

<entry><type>regproc</type></entry>

functions:

a state transition function

<replaceable class="PARAMETER">sfunc</replaceable>,

an optional final calculation function

<replaceable class="PARAMETER">ffunc</replaceable>,

and an optional combine function

<replaceable class="PARAMETER">combinefunc</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-values ) ---> next-internal-state

<replaceable class="PARAMETER">ffunc</replaceable>( internal-state ) ---> aggregate-value

<replaceable class="PARAMETER">combinefunc</replaceable>( internal-state, internal-state ) ---> next-internal-state

</programlisting>

</para>

is returned as-is.

</para>

<para>

An aggregate function may also supply a combining function, which allows

the aggregation process to be broken down into multiple steps. This

facilitates query optimization techniques such as parallel query.

</para>

<para>

An aggregate function can provide an initial condition,

that is, an initial value for the internal state value.

</listitem>

</varlistentry>

<varlistentry>

<term><replaceable class="PARAMETER">combinefunc</replaceable></term>

<listitem>

<para>

The <replaceable class="PARAMETER">combinefunc</replaceable> may

optionally be specified in order to allow the aggregate function to

support partial aggregation. This is a prerequisite to allow the

aggregate to participate in certain optimizations such as parallel

aggregation.

</para>

<para>

This function can be thought of as an <replaceable class="PARAMETER">

sfunc</replaceable>, where instead of acting upon individual input rows

and adding these to the aggregate state, it adds other aggregate states

to the aggregate state.

</para>

<para>

The <replaceable class="PARAMETER">combinefunc</replaceable> must accept

two arguments of <replaceable class="PARAMETER">state_data_type

</replaceable> and return <replaceable class="PARAMETER">state_data_type

</replaceable>. Optionally this function may be <quote>strict</quote>. In

this case the function will not be called when either of the input states

are null.

</para>

<para>

For aggregate functions with an <literal>INTERNAL</literal>

<replaceable class="PARAMETER">state_data_type</replaceable> the

<replaceable class="PARAMETER">combinefunc</replaceable> must not be

<quote>strict</quote>. In this scenario the

<replaceable class="PARAMETER">combinefunc</replaceable> must take charge

and ensure that the null states are handled correctly and that the state

being returned is a pointer to memory which belongs in the aggregate

memory context.

</para>

</listitem>

</varlistentry>

<varlistentry>

<term><replaceable class="PARAMETER">initial_condition</replaceable></term>

<listitem>