CREATE 
AGGREGATE(7)					  PostgreSQL 9.2.7 Documentation				       CREATE AGGREGATE(7)

NAME

       CREATE_AGGREGATE - define a new aggregate function

SYNOPSIS

       CREATE AGGREGATE name ( input_data_type [ , ... ] ) (
	   SFUNC = sfunc,
	   STYPE = state_data_type
	   [ , FINALFUNC = ffunc ]
	   [ , INITCOND = initial_condition ]
	   [ , SORTOP = sort_operator ]
       )

       or the old syntax

       CREATE AGGREGATE name (
	   BASETYPE = base_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 commonly-used aggregate functions are included with the distribution;
       they are documented in Section 9.20, "Aggregate Functions", 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(s). Two aggregates in the same schema can have the same name if they
       operate on different input types. The name and input data type(s) 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-values ) ---> 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 row, the
       aggregate argument value(s) are calculated and the state transition function is invoked with the current state value and the new argument
       value(s) to calculate a new internal state value. After all the rows have 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 can provide an initial condition, that is, an initial value for the internal state value. This is specified and
       stored in the database as a value 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. Rows with any 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 at the first row with all-nonnull input values, the first argument value
       replaces the state value, and the transition function is invoked at subsequent rows with all-nonnull input values. This is handy for
       implementing aggregates like max. Note that this behavior is only available when state_data_type is the same as the first 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 row, 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.

       To be able to create an aggregate function, you must have USAGE privilege on the argument types, the state type, and the return type, as
       well as EXECUTE privilege on the transition and final functions.

PARAMETERS

       name
	   The name (optionally schema-qualified) of the aggregate function to create.

       input_data_type
	   An input data type on which this aggregate function operates. To create a zero-argument aggregate function, write * in place of the
	   list of input data types. (An example of such an aggregate is count(*).)

       base_type
	   In the old syntax for CREATE AGGREGATE, the input data type is specified by a basetype parameter rather than being written next to the
	   aggregate name. Note that this syntax allows only one input parameter. To define a zero-argument aggregate function, specify the
	   basetype as "ANY" (not *).

       sfunc
	   The name of the state transition function to be called for each input row. For an N-argument aggregate function, the sfunc must take
	   N+1 arguments, the first being of type state_data_type and the rest matching the declared input data type(s) of the aggregate. The
	   function must return a value of type state_data_type. This function takes the current state value and the current input data value(s),
	   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 rows have 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 MAX-like aggregate. This is just an operator name (possibly schema-qualified). The operator
	   is assumed to have the same input data types as the aggregate (which must be a single-argument aggregate).

       The parameters of CREATE AGGREGATE can be written in any order, not just the order illustrated above.

EXAMPLES

       See Section 35.10, "User-defined Aggregates", in the documentation.

COMPATIBILITY

       CREATE AGGREGATE is a PostgreSQL language extension. The SQL standard does not provide for user-defined aggregate functions.

SEE ALSO

       ALTER AGGREGATE (ALTER_AGGREGATE(7)), DROP AGGREGATE (DROP_AGGREGATE(7))

PostgreSQL 9.2.7						    2014-02-17						       CREATE AGGREGATE(7)