CREATE AGGREGATE() SQL Commands CREATE AGGREGATE()
NAME
CREATE AGGREGATE  define a new aggregate function
SYNOPSIS
CREATE AGGREGATE name (
BASETYPE = input_data_type,
SFUNC = sfunc,
STYPE = state_data_type
[ , FINALFUNC = ffunc ]
[ , INITCOND = initial_condition ]
[ , SORTOP = sort_operator ]
)
DESCRIPTION
CREATE AGGREGATE defines a new aggregate function. Some basic and com
monlyused aggregate functions are included with the distribution; they
are documented in the documentation. 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 oper
ate on different input types. The name and input data type of an aggre
gate 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( internalstate, nextdataitem ) > nextinternalstate
ffunc( internalstate ) > aggregatevalue
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 asis.
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 func
tion 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 begin
ning with the second nonnull input value. This is handy for implement
ing 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 opera
tor. 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 nonnull
inputs. Ordinarily, a data type's < operator is the proper sort opera
tor for MIN, and > is the proper sort operator for MAX. Note that the
optimization will never actually take effect unless the specified oper
ator is the ''less than'' or ''greater than'' strategy member of a B
tree index operator class.
PARAMETERS
name The name (optionally schemaqualified) of the aggregate function
to create.
input_data_type
The input data type on which this aggregate function operates.
This can be specified as "ANY" for an aggregate that does not
examine its input values (an example is count(*)).
sfunc The name of the state transition function to be called for each
input data value. This is normally a function of two arguments,
the first being of type state_data_type and the second of type
input_data_type. Alternatively, for an aggregate that does not
examine its input values, the function takes just one argument
of type state_data_type. In either case the function must return
a value of type state_data_type. This function takes the current
state value and the current input data item, and returns the
next state value.
state_data_type
The data type for the aggregate's state value.
ffunc The name of the final function called to compute the aggregate's
result after all input data has been traversed. The function
must take a single argument of type state_data_type. The return
data type of the aggregate is defined as the return type of this
function. If ffunc is not specified, then the ending state value
is used as the aggregate's result, and the return type is
state_data_type.
initial_condition
The initial setting for the state value. This must be a string
constant in the form accepted for the data type state_data_type.
If not specified, the state value starts out null.
sort_operator
The associated sort operator for a MIN or MAXlike aggregate.
This is just an operator name (possibly schemaqualified). The
operator is assumed to have the same input data types as the
aggregate.
The parameters of CREATE AGGREGATE can be written in any order, not
just the order illustrated above.
EXAMPLES
See the documentation.
COMPATIBILITY
CREATE AGGREGATE is a PostgreSQL language extension. The SQL standard
does not provide for userdefined aggregate functions.
SEE ALSO
ALTER AGGREGATE [alter_aggregate(7)], DROP AGGREGATE [drop_aggre
gate(l)]
SQL  Language Statements 20101214 CREATE AGGREGATE()
