path: root/doc/src/sgml/xaggr.sgml

<!-- doc/src/sgml/xaggr.sgml -->

 <sect1 id="xaggr">
  <title>User-defined Aggregates</title>

  <indexterm zone="xaggr">
   <primary>aggregate function</primary>
   <secondary>user-defined</secondary>
  </indexterm>

  <para>
   Aggregate functions in <productname>PostgreSQL</productname>
   are defined in terms of <firstterm>state values</firstterm>
   and <firstterm>state transition functions</firstterm>.
   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 takes the previous state
   value and the aggregate's input value(s) for the current row, and
   returns a new state value.
   A <firstterm>final function</firstterm>
   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.  The final function takes the last state value
   and returns whatever is wanted as the aggregate result.
   In principle, the transition and final functions are just ordinary
   functions that could also be used outside the context of the
   aggregate.  (In practice, it's often helpful for performance reasons
   to create specialized transition functions that can only work when
   called as part of an aggregate.)
  </para>

  <para>
   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.
  </para>

  <para>
   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.  <function>sum</>  is  an
   example  of  this  kind  of aggregate.  <function>sum</> starts at
   zero and always adds the current  row's  value  to
   its  running  total.  For example, if we want to make a <function>sum</>
   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:

<programlisting>
CREATE AGGREGATE sum (complex)
(
    sfunc = complex_add,
    stype = complex,
    initcond = '(0,0)'
);
</programlisting>

   which we might use like this:

<programlisting>
SELECT sum(a) FROM test_complex;

   sum
-----------
 (34,53.9)
</programlisting>

   (Notice that we are relying on function overloading: there is more than
    one aggregate named <function>sum</>, but
   <productname>PostgreSQL</productname> can figure out which kind
   of sum applies to a column of type <type>complex</type>.)
  </para>

  <para>
   The above definition of <function>sum</function> will return zero
   (the initial state value) if there are no nonnull input values.
   Perhaps we want to return null in that case instead &mdash; the SQL standard
   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
   value is null.  Ordinarily this would mean that the <literal>sfunc</literal>
   would need to check for a null state-value 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
   the state variable and then start applying the transition function
   at the second nonnull input value.  <productname>PostgreSQL</productname>
   will do that automatically if the initial state value is null and
   the transition function is marked <quote>strict</> (i.e., not to be called
   for null inputs).
  </para>

  <para>
   Another bit of default behavior for a <quote>strict</> 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.
  </para>

  <para>
   <function>avg</> (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 <function>avg(float8)</function>
   looks like:

<programlisting>
CREATE AGGREGATE avg (float8)
(
    sfunc = float8_accum,
    stype = float8[],
    finalfunc = float8_avg,
    initcond = '{0,0,0}'
);
</programlisting>
  </para>

  <note>
   <para>
   <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>
  </note>

  <para>
   Aggregate functions can use polymorphic
   state transition functions or final functions, so that the same functions
   can be used to implement multiple aggregates.
   See <xref linkend="extend-types-polymorphic">
   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:

<programlisting>
CREATE AGGREGATE array_accum (anyelement)
(
    sfunc = array_append,
    stype = anyarray,
    initcond = '{}'
);
</programlisting>

   Here, the actual state type for any given 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 <function>array_agg</> provides similar
   functionality, with better performance than this definition would have.)
  </para>

  <para>
   Here's the output using two different actual data types as arguments:

<programlisting>
SELECT attrelid::regclass, array_accum(attname)
    FROM pg_attribute
    WHERE attnum &gt; 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 &gt; 0 AND attrelid = 'pg_tablespace'::regclass
    GROUP BY attrelid;

   attrelid    |        array_accum        
---------------+---------------------------
 pg_tablespace | {name,oid,aclitem[],text[]}
(1 row)
</programlisting>
  </para>

  <para>
   An aggregate function can be made to accept a varying number of arguments
   by declaring its last argument as a <literal>VARIADIC</> array, in much
   the same fashion as for regular functions; see
   <xref linkend="xfunc-sql-variadic-functions">.  The aggregate's transition
   function must have the same array type as its last argument.  The
   transition function typically would also be marked <literal>VARIADIC</>,
   but this is not strictly required.
  </para>

  <note>
   <para>
    Variadic aggregates are easily misused in connection with
    the <literal>ORDER BY</> option (see <xref linkend="syntax-aggregates">),
    since the parser cannot tell whether the wrong number of actual arguments
    have been given in such a combination.  Keep in mind that everything to
    the right of <literal>ORDER BY</> is a sort key, not an argument to the
    aggregate.  For example, in
<programlisting>
SELECT myaggregate(a ORDER BY a, b, c) FROM ...
</programlisting>
    the parser will see this as a single aggregate function argument and
    three sort keys.  However, the user might have intended
<programlisting>
SELECT myaggregate(a, b, c ORDER BY a) FROM ...
</programlisting>
    If <literal>myaggregate</> is variadic, both these calls could be
    perfectly valid.
   </para>

   <para>
    For the same reason, it's wise to think twice before creating aggregate
    functions with the same names and different numbers of regular arguments.
   </para>
  </note>

  <para>
   Aggregate function calls in SQL allow <literal>DISTINCT</>
   and <literal>ORDER BY</> options that control which rows are fed
   to the aggregate's transition function and in what order.  These
   options are implemented behind the scenes and are not the concern
   of the aggregate's support functions.
  </para>

  <para>
   The aggregates we have been describing so far are <quote>normal</>
   aggregates.  <productname>PostgreSQL</> also
   supports <firstterm>ordered-set aggregates</>, which differ from
   normal aggregates in two key ways.  First, in addition to ordinary
   aggregated arguments that are evaluated once per input row, an
   ordered-set aggregate can have <quote>direct</> arguments that are
   evaluated only once per aggregation operation.  Second, the syntax
   for the ordinary aggregated arguments specifies a sort ordering
   for them explicitly.  An ordered-set aggregate is usually
   used to implement a computation that depends on a specific row
   ordering, for instance rank or percentile, so that the sort ordering
   is a required aspect of any call.  For example, the built-in
   definition of <function>percentile_disc</> is equivalent to:

<programlisting>
CREATE AGGREGATE percentile_disc (float8 ORDER BY anyelement)
(
    sfunc = ordered_set_transition,
    stype = internal,
    finalfunc = percentile_disc_final
);
</programlisting>

   which could be used to obtain a median household income like this:

<programlisting>
SELECT percentile_disc(0.5) WITHIN GROUP (ORDER BY income) FROM households;
 percentile_disc
-----------------
           50489
</programlisting>

   Here, <literal>0.5</> is a direct argument; it would make no sense
   for the percentile fraction to be a value varying across rows.
  </para>

  <para>
   Unlike the case for normal aggregates, the sorting of input rows for
   an ordered-set aggregate is <emphasis>not</> done behind the scenes,
   but is the responsibility of the aggregate's support functions.
   The typical implementation approach is to keep a reference to
   a <quote>tuplesort</> object in the aggregate's state value, feed the
   incoming rows into that object, and then complete the sorting and
   read out the data in the final function.  This design allows the
   final function to perform special operations such as injecting
   additional <quote>hypothetical</> rows into the data to be sorted.
   While normal aggregates can often be implemented with support
   functions written in <application>PL/pgSQL</application> or another
   PL language, ordered-set aggregates generally have to be written in
   C, since their state values aren't definable as any SQL datatype.
   (In the above example, notice that the state value is declared as
   type <type>internal</> &mdash; this is typical.)
  </para>

  <para>
   The state transition function for an ordered-set aggregate receives
   the current state value plus the aggregated input values for
   each row, and returns the updated state value.  This is the
   same definition as for normal aggregates, but note that the direct
   arguments (if any) are not provided.  The final function receives
   the last state value, the values of the direct arguments if any,
   and null values corresponding to the aggregated input(s).  While the
   null values seem useless at first sight, they are important because
   they make it possible to include the data types of the aggregated
   input(s) in the final function's signature, which may be necessary
   to resolve the output type of a polymorphic aggregate.  For example,
   the built-in <function>mode()</> ordered-set aggregate takes a
   single aggregated column of any sortable data type and returns a
   value of that same type.  This is possible because the final function
   is declared as <literal>mode_final(internal, anyelement) returns
   anyelement</>, with the <type>anyelement</> parameter corresponding
   to the dummy null argument that represents the aggregated column.
   The actual data is conveyed in the <type>internal</>-type state
   value, but type resolution needs a parse-time indication of what the
   result data type will be, and the dummy argument provides that.
   In the example of <function>percentile_disc</>, the support functions
   are respectively declared as
   <literal>ordered_set_transition(internal, "any") returns internal</>
   and <literal>percentile_disc_final(internal, float8, anyelement)
   returns anyelement</>.
  </para>

  <para>
   A function written in C can detect that it is being called as an
   aggregate transition or final function by calling
   <function>AggCheckCallContext</>, for example:
<programlisting>
if (AggCheckCallContext(fcinfo, NULL))
</programlisting>
   One reason for checking this is that when it is true for a transition
   function, the first input
   must be a temporary state value and can therefore safely be modified
   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, final functions for normal aggregates must not
   modify their inputs in any case, because in some cases they will be
   re-executed on the same final state value.)
  </para>

  <para>
   Another support routine available to aggregate functions written in C
   is <function>AggGetAggref</>, which returns the <literal>Aggref</>
   parse node that defines the aggregate call.  This is mainly useful
   for ordered-set aggregates, which can inspect the substructure of
   the <literal>Aggref</> node to find out what sort ordering they are
   supposed to implement.  Examples can be found
   in <filename>orderedsetaggs.c</> in the <productname>PostgreSQL</>
   source code.
  </para>

  <para>
   For further details see the
   <xref linkend="sql-createaggregate">
   command.
  </para>
 </sect1>