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><A
NAME="XAGGR"
>35.10. User-defined Aggregates</A
></H1
><P
> Aggregate functions in <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
are expressed in terms of <I
CLASS="FIRSTTERM"
>state values</I
>
and <I
CLASS="FIRSTTERM"
>state transition functions</I
>.
That is, an aggregate operates using a state value that is updated
as each successive input row is processed.
To define a new aggregate
function, one selects a data type for the state value,
an initial value for the state, and a state transition
function. The state transition function is just an
ordinary function that could also be used outside the
context of the aggregate. A <I
CLASS="FIRSTTERM"
>final function</I
>
can also be specified, in case the desired result of the aggregate
is different from the data that needs to be kept in the running
state value.
</P
><P
> Thus, in addition to the argument and result data types seen by a user
of the aggregate, there is an internal state-value data type that
might be different from both the argument and result types.
</P
><P
> If we define an aggregate that does not use a final function,
we have an aggregate that computes a running function of
the column values from each row. <CODE
CLASS="FUNCTION"
>sum</CODE
> is an
example of this kind of aggregate. <CODE
CLASS="FUNCTION"
>sum</CODE
> starts at
zero and always adds the current row's value to
its running total. For example, if we want to make a <CODE
CLASS="FUNCTION"
>sum</CODE
>
aggregate to work on a data type for complex numbers,
we only need the addition function for that data type.
The aggregate definition would be:
</P><PRE
CLASS="SCREEN"
>CREATE AGGREGATE sum (complex)
(
sfunc = complex_add,
stype = complex,
initcond = '(0,0)'
);
SELECT sum(a) FROM test_complex;
sum
-----------
(34,53.9)</PRE
><P>
(Notice that we are relying on function overloading: there is more than
one aggregate named <CODE
CLASS="FUNCTION"
>sum</CODE
>, but
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> can figure out which kind
of sum applies to a column of type <TT
CLASS="TYPE"
>complex</TT
>.)
</P
><P
> The above definition of <CODE
CLASS="FUNCTION"
>sum</CODE
> will return zero (the initial
state condition) if there are no nonnull input values.
Perhaps we want to return null in that case instead — the SQL standard
expects <CODE
CLASS="FUNCTION"
>sum</CODE
> to behave that way. We can do this simply by
omitting the <TT
CLASS="LITERAL"
>initcond</TT
> phrase, so that the initial state
condition is null. Ordinarily this would mean that the <TT
CLASS="LITERAL"
>sfunc</TT
>
would need to check for a null state-condition input. But for
<CODE
CLASS="FUNCTION"
>sum</CODE
> and some other simple aggregates like
<CODE
CLASS="FUNCTION"
>max</CODE
> and <CODE
CLASS="FUNCTION"
>min</CODE
>,
it is sufficient to insert the first nonnull input value into
the state variable and then start applying the transition function
at the second nonnull input value. <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
will do that automatically if the initial condition is null and
the transition function is marked <SPAN
CLASS="QUOTE"
>"strict"</SPAN
> (i.e., not to be called
for null inputs).
</P
><P
> Another bit of default behavior for a <SPAN
CLASS="QUOTE"
>"strict"</SPAN
> transition function
is that the previous state value is retained unchanged whenever a
null input value is encountered. Thus, null values are ignored. If you
need some other behavior for null inputs, do not declare your
transition function as strict; instead code it to test for null inputs and
do whatever is needed.
</P
><P
> <CODE
CLASS="FUNCTION"
>avg</CODE
> (average) is a more complex example of an aggregate.
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 using an
array as the state value. For example,
the built-in implementation of <CODE
CLASS="FUNCTION"
>avg(float8)</CODE
>
looks like:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE AGGREGATE avg (float8)
(
sfunc = float8_accum,
stype = float8[],
finalfunc = float8_avg,
initcond = '{0,0,0}'
);</PRE
><P>
(<CODE
CLASS="FUNCTION"
>float8_accum</CODE
> 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 <CODE
CLASS="FUNCTION"
>avg</CODE
>.)
</P
><P
> Aggregate functions can use polymorphic
state transition functions or final functions, so that the same functions
can be used to implement multiple aggregates.
See <A
HREF="extend-type-system.html#EXTEND-TYPES-POLYMORPHIC"
>Section 35.2.5</A
>
for an explanation of polymorphic functions.
Going a step further, the aggregate function itself can be specified
with polymorphic input type(s) and state type, allowing a single
aggregate definition to serve for multiple input data types.
Here is an example of a polymorphic aggregate:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE AGGREGATE array_accum (anyelement)
(
sfunc = array_append,
stype = anyarray,
initcond = '{}'
);</PRE
><P>
Here, the actual state type for any aggregate call is the array type
having the actual input type as elements. The behavior of the aggregate
is to concatenate all the inputs into an array of that type.
(Note: the built-in aggregate <CODE
CLASS="FUNCTION"
>array_agg</CODE
> provides similar
functionality, with better performance than this definition would have.)
</P
><P
> Here's the output using two different actual data types as arguments:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT attrelid::regclass, array_accum(attname)
FROM pg_attribute
WHERE attnum > 0 AND attrelid = 'pg_tablespace'::regclass
GROUP BY attrelid;
attrelid | array_accum
---------------+---------------------------------------
pg_tablespace | {spcname,spcowner,spcacl,spcoptions}
(1 row)
SELECT attrelid::regclass, array_accum(atttypid::regtype)
FROM pg_attribute
WHERE attnum > 0 AND attrelid = 'pg_tablespace'::regclass
GROUP BY attrelid;
attrelid | array_accum
---------------+---------------------------
pg_tablespace | {name,oid,aclitem[],text[]}
(1 row)</PRE
><P>
</P
><P
> A function written in C can detect that it is being called as an
aggregate transition or final function by calling
<CODE
CLASS="FUNCTION"
>AggCheckCallContext</CODE
>, for example:
</P><PRE
CLASS="PROGRAMLISTING"
>if (AggCheckCallContext(fcinfo, NULL))</PRE
><P>
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.
See <TT
CLASS="LITERAL"
>int8inc()</TT
> for an example.
(This is the <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>only</I
></SPAN
>
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.)
</P
><P
> For further details see the
<A
HREF="sql-createaggregate.html"
>CREATE AGGREGATE</A
>
command.
</P
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