create_aggregate
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 ]
)
DESCRIPTION
CREATE AGGREGATE defines a new aggregate function. Some aggregate
functions for base types such as min(integer) and avg(double preci-
sion) are already provided in the standard distribution. 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
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 can-
not 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 val-
ues.
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.
PARAMETERS
name The name (optionally schema-qualified) of the aggregate func-
tion 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 aggre-
gate’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 end-
ing 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.
The parameters of CREATE AGGREGATE can be written in any order, not
just the order illustrated above.
EXAMPLES
See [XRef to XAGGR].
COMPATIBILITY
CREATE AGGREGATE is a PostgreSQL language extension. The SQL standard
does not provide for user-defined aggregate function.
SEE ALSO
ALTER AGGREGATE [alter_aggregate(7)], DROP AGGREGATE [drop_aggre-
gate(l)]
SQL - Language Statements 2008-01-03 CREATE AGGREGATE()
