- Aggregate functions
in
PostgreSQL- are expressed in terms of state values
+ Aggregate functions in
PostgreSQL+ are defined in terms of state values
and state transition functions.
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 final function
+ 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 final function
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.
+ 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.)
we only need the addition function for that data type.
The aggregate definition would be:
-<screen>
+<programlisting>
CREATE AGGREGATE sum (complex)
(
sfunc = complex_add,
stype = complex,
initcond = '(0,0)'
);
+
+
+ which we might use like this:
SELECT sum(a) FROM test_complex;
sum
-----------
(34,53.9)
-screen>
+programlisting>
(Notice that we are relying on function overloading: there is more than
one aggregate named sum, but
- The above definition of sum will return zero (the initial
- state condition) if there are no nonnull input values.
+ The above definition of sum will return zero
+ (the initial state value) if there are no nonnull input values.
Perhaps we want to return null in that case instead — the SQL standard
expects sum to behave that way. We can do this simply by
omitting the initcond phrase, so that the initial state
- condition is null. Ordinarily this would mean that the sfunc
- would need to check for a null state-condition input. But for
+ value is null. Ordinarily this would mean that the sfunc
+ would need to check for a null state-value input. But for
sum and some other simple aggregates like
max and 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.
PostgreSQL- will do that automatically if the initial condition is null and
+ will do that automatically if the initial state value is null and
the transition function is marked strict (i.e., not to be called
for null inputs).
initcond = '{0,0,0}'
);
+
- (float8_accum requires a three-element array, not just
+
+ 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 avg.)
-
+ some other aggregates besides avg.
+
+
Aggregate functions can use polymorphic
);
- Here, the actual state type for any aggregate call is the array type
+ 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 array_agg provides similar
+ Aggregate function calls in SQL allow DISTINCT
+ and 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.
+
+
+ The aggregates we have been describing so far are normal
+ aggregates.
PostgreSQL also+ supports 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 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 percentile_disc is equivalent to:
+
+CREATE AGGREGATE percentile_disc (float8 ORDER BY anyelement)
+(
+ sfunc = ordered_set_transition,
+ stype = internal,
+ finalfunc = percentile_disc_final
+);
+
+
+ which could be used to obtain a median household income like this:
+
+SELECT percentile_disc(0.5) WITHIN GROUP (ORDER BY income) FROM households;
+ percentile_disc
+-----------------
+ 50489
+
+
+ Here, 0.5 is a direct argument; it would make no sense
+ for the percentile fraction to be a value varying across rows.
+
+
+ Unlike the case for normal aggregates, the sorting of input rows for
+ an ordered-set aggregate is 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 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 hypothetical rows into the data to be sorted.
+ While normal aggregates can often be implemented with support
+ functions written in
PL/pgSQL 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 internal — this is typical.)
+
+
+ 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 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 mode_final(internal, anyelement) returns
+ anyelement, with the anyelement parameter corresponding
+ to the dummy null argument that represents the aggregated column.
+ The actual data is conveyed in the internal-type state
+ value, but type resolution needs a parse-time indication of what the
+
projects / postgresql.git / commitdiff