CREATE AGGREGATE(7) SQL Commands CREATE AGGREGATE(7)
CREATE AGGREGATE - define a new aggregate function
CREATE AGGREGATE name ( BASETYPE = input_data_type,
SFUNC = sfunc, STYPE = state_type
[ , FINALFUNC = ffunc ]
[ , INITCOND = initial_condition ] )
name The name (optionally schema-qualified) of an aggregate function to create.
The input data type on which this aggregate function operates. This can be speci-
fied as "ANY" for an aggregate that does not examine its input values (an example
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_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_type. In either case the function must return a value of type state_type.
This function takes the current state value and the current input data item, and
returns the next state value.
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_type. The output 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 output type is state_type.
The initial setting for the state value. This must be a literal constant in the
form accepted for the data type state_type. If not specified, the state value
starts out NULL.
Message returned if the command completes successfully.
CREATE AGGREGATE allows a user or programmer to extend PostgreSQL functionality by defin-
ing new aggregate functions. Some aggregate functions for base types such as min(integer)
and avg(double precision) are already provided in the base 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 cur-
rent schema (the one at the front of the search path; see 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 func-
tion 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 output 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 field 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 non-NULL input value
replaces the state value, and the transition function is invoked beginning with the second
non-NULL input value. This is handy for implementing aggregates like max. Note that this
behavior is only available when state_type is the same as input_data_type. When these
types are different, you must supply a non-NULL initial condition or use a non-strict
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
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 output automatically. (Of course this
is just the normal behavior of strict functions.) In any case the final function has the
option of returning NULL. For example, the final function for avg returns NULL when it
sees there were zero input tuples.
Use DROP AGGREGATE to drop aggregate functions.
The parameters of CREATE AGGREGATE can be written in any order, not just the order illus-
Refer to the chapter on aggregate functions in the PostgreSQL Programmer's Guide for com-
plete examples of usage.
CREATE AGGREGATE is a PostgreSQL language extension. There is no CREATE AGGREGATE in
SQL - Language Statements 2002-11-22 CREATE AGGREGATE(7)