Description
CREATE AGGREGATE allows a user or programmer to extend PostgreSQL functionality by defining 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 current 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 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 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 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 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.
Notes
Use DROP AGGREGATE to drop aggregate functions.
The parameters of CREATE AGGREGATE can be written in any order, not just the order illustrated above.
