CREATE AGGREGATE - define a new aggregate function

Synopsis

CREATE AGGREGATEname( BASETYPE =input_data_type, SFUNC =sfunc, STYPE =state_data_type[ , FINALFUNC =ffunc] [ , INITCOND =initial_condition] [ , SORTOP =sort_operator] )

CREATE AGGREGATEdefines a new aggregate function. Some basic and commonly-used 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, thenCREATE AGGREGATEcan 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 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 function

sfunc, and an optional final calculation functionffunc. These are used as follows:

sfunc( internal-state, next-data-item ) ---> next-internal-stateffunc( internal-state ) ---> aggregate-value

PostgreSQL creates a temporary variable of data type

stypeto 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 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 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 whenstate_data_typeis the same asinput_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

avgreturns null when it sees there were zero input rows.Aggregates that behave like

MINorMAXcan 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 asort 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 forMAX. 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.

The parameters of

nameThe name (optionally schema-qualified) of the aggregate function to create. input_data_typeThe 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(*)).sfuncThe 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_typeand the second of typeinput_data_type. Alternatively, for an aggregate that does not examine its input values, the function takes just one argument of typestate_data_type. In either case the function must return a value of typestate_data_type. This function takes the current state value and the current input data item, and returns the next state value.state_data_typeThe data type for the aggregate’s state value. ffuncThe 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. Ifffuncis not specified, then the ending state value is used as the aggregate’s result, and the return type isstate_data_type.initial_conditionThe 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_operatorThe associated sort operator for a MIN- orMAX-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.CREATE AGGREGATEcan be written in any order, not just the order illustrated above.

See the documentation.

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

ALTER AGGREGATE [alter_aggregate(7)], DROP AGGREGATE [drop_aggregate(l)]

SQL - Language Statements |
CREATE AGGREGATE () | 2010-12-14 |