CREATE_AGGREGATE (7)  Linux Manuals
CREATE_AGGREGATE: define a new aggregate function
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 commonlyused 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( internalstate, nextdatavalues ) > 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 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 asis.
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 allnonnull input values, the first argument value replaces the state value, and the transition function is invoked at subsequent rows with allnonnull 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 nonnull 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 Btree 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 schemaqualified) of the aggregate function to create.
input_data_type
 An input data type on which this aggregate function operates. To create a zeroargument 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 zeroargument 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 Nargument 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 MAXlike aggregate. This is just an operator name (possibly schemaqualified). The operator is assumed to have the same input data types as the aggregate (which must be a singleargument aggregate).
The parameters of CREATE AGGREGATE can be written in any order, not just the order illustrated above.
EXAMPLES
See Section 35.10, "Userdefined Aggregates", in the documentation.
COMPATIBILITY
CREATE AGGREGATE is a PostgreSQL language extension. The SQL standard does not provide for userdefined aggregate functions.