expects <function>sum</function> to behave that way. We can do this simply by

omitting the <literal>initcond</literal> phrase, so that the initial state

condition is null. Ordinarily this would mean that the <literal>sfunc</literal>

would need to check for a null state-condition input, but for

would need to check for a null state-condition input. But for

<function>sum</function> and some other simple aggregates like

<function>max</> and <function>min</>,

it is sufficient to insert the first nonnull input value into

It requires

two pieces of running state: the sum of the inputs and the count

of the number of inputs. The final result is obtained by dividing

these quantities. Average is typically implemented by usinga

two-elementarray as the state value. For example,

these quantities. Average is typically implemented by usingan

array as the state value. For example,

the built-in implementation of <function>avg(float8)</function>

looks like:

initcond = '{0,0,0}'

);

</programlisting>

(<function>float8_accum</> requires a three-element array, not just

two elements, because it accumulates the sum of squares as well as

the sum and count of the inputs. This is so that it can be used for

some other aggregates besides <function>avg</>.)

</para>

<para>

One reason for checking this is that when it is true for a transition

function, the first input

must be a temporary transition value and can therefore safely be modified

in-place rather than allocating a new copy. (This is the <emphasis>only</>

in-place rather than allocating a new copy.

See <literal>int8inc()</> for an example.

(This is the <emphasis>only</>

case where it is safe for a function to modify a pass-by-reference input.

In particular, aggregate final functions should not modify their inputs in

any case, because in some cases they will be re-executed on the same

final transition value.)

See <literal>int8inc()</> for an example.

</para>

<para>