Description
CREATE AGGREGATE defines a new aggregate
function. Some basic and commonly-used aggregate functions are
included with the distribution; they are documented in Section 9.15. If one defines new types or needs
an aggregate function not already provided, then CREATE
AGGREGATE can be used to provide the desired features.
If a schema name is given (for example, CREATE AGGREGATE
myschema.myagg ...) then the aggregate function is created in the
specified schema. Otherwise it is created in the current schema.
An aggregate function is identified by its name and input data type.
Two aggregates in the same schema can have the same name if they operate on
different input types. The
name and input data type of an aggregate must also be distinct from
the name and input data type(s) of every ordinary function in the same
schema.
An aggregate function is made from one or two ordinary
functions:
a state transition function
sfunc,
and an optional final calculation function
ffunc.
These are used as follows:
sfunc( internal-state, next-data-item ) ---> next-internal-state
ffunc( internal-state ) ---> aggregate-value
PostgreSQL creates a temporary variable
of data type stype
to hold the current internal state of the aggregate. At each input
data item,
the state transition function is invoked to calculate a new
internal state value. After all the data has been processed,
the final function is invoked once to calculate the aggregate's return
value. If there is no final function then the ending state value
is returned as-is.
An aggregate function may provide an initial condition,
that is, an initial value for the internal state value.
This is specified and stored in the database as a column of type
text, but it must be a valid external representation
of a constant of the state value data type. If it is not supplied
then the state value starts out null.
If the state transition function is declared "strict",
then it cannot be called with null inputs. With such a transition
function, aggregate execution behaves as follows. Null input values
are ignored (the function is not called and the previous state value
is retained). If the initial state value is null, then the first
nonnull input value replaces the state value, and the transition
function is invoked beginning with the second nonnull input value.
This is handy for implementing aggregates like max.
Note that this behavior is only available when
state_data_type
is the same as
input_data_type.
When these types are different, you must supply a nonnull initial
condition or use a nonstrict transition function.
If the state transition function is not strict, then it will be called
unconditionally at each input value, and must deal with null inputs
and null transition values for itself. This allows the aggregate
author to have full control over the aggregate's handling of null values.
If the final function is declared "strict", then it will not
be called when the ending state value is null; instead a null result
will be returned automatically. (Of course this is just the normal
behavior of strict functions.) In any case the final function has
the option of returning a null value. For example, the final function for
avg returns null when it sees there were zero
input rows.
Aggregates that behave like MIN or MAX can
sometimes be optimized by looking into an index instead of scanning every
input row. If this aggregate can be so optimized, indicate it by
specifying a sort operator. The basic requirement is that
the aggregate must yield the first element in the sort ordering induced by
the operator; in other words
SELECT agg(col) FROM tab;
must be equivalent to
SELECT col FROM tab ORDER BY col USING sortop LIMIT 1;
Further assumptions are that the aggregate ignores null inputs, and that
it delivers a null result if and only if there were no non-null inputs.
Ordinarily, a data type's < operator is the proper sort
operator for MIN, and > is the proper sort
operator for MAX. Note that the optimization will never
actually take effect unless the specified operator is the "less than" or
"greater than" strategy member of a B-tree index operator class.
