CREATE AGGREGATE

Defines a new aggregate function.

Synopsis

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

Description

CREATE AGGREGATE defines a new aggregate function. Some basic and commonly-used aggregate functions such as count, min, max, sum, avg and so on are already provided in HAWQ. If one defines new types or needs an aggregate function not already provided, then CREATE AGGREGATE can be used to provide the desired features.

An aggregate function is identified by its name and input data types. Two aggregate functions in the same schema can have the same name if they operate on different input types. The name and input data types of an aggregate function must also be distinct from the name and input data types of every ordinary function in the same schema.

An aggregate function is made from one, two or three ordinary functions (all of which must be IMMUTABLE functions):

• A state transition function
• An optional preliminary segment-level calculation function
• An optional final calculation function

These functions are used as follows:

sfunc( internal-state, next-data-values ) ---> next-internal-state
prefunc( internal-state, internal-state ) ---> next-internal-state
ffunc( internal-state ) ---> aggregate-value

You can specify PREFUNC as method for optimizing aggregate execution. By specifying PREFUNC, the aggregate can be executed in parallel on segments first and then on the master. When a two-level execution is performed, SFUNC is executed on the segments to generate partial aggregate results, and PREFUNC is executed on the master to aggregate the partial results from segments. If single-level aggregation is performed, all the rows are sent to the master and is applied to the rows.

Single-level aggregation and two-level aggregation are equivalent execution strategies. Either type of aggregation can be implemented in a query plan. When you implement the functions and , you must ensure that the invocation of on the segment instances followed by on the master produce the same result as single-level aggregation that sends all the rows to the master and then applies only the to the rows.

HAWQ creates a temporary variable of data type to hold the current internal state of the aggregate function. At each input row, the aggregate argument values are calculated and the state transition function is invoked with the current state value and the new argument values to calculate a new internal state value. After all the rows have been processed, the final function is invoked once to calculate the aggregate return value. If there is no final function then the ending state value is returned as-is.

An aggregate function can provide an optional initial condition, 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 non-null input values, the first argument value replaces the state value, and the transition function is invoked at subsequent rows with all non-null input values. This is useful for implementing aggregates like max. Note that this behavior is only available when is the same as the first . When these types are different, you must supply a non-null initial condition or use a nonstrict transition function.

If the state transition function is not declared 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 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. (This is 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.

Single argument aggregate functions, such as 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 function 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.

Ordered Aggregates

If the optional qualification ORDERED appears, the created aggregate function is an ordered aggregate. In this case, the preliminary aggregation function, prefunc cannot be specified.

An ordered aggregate is called with the following syntax.

<name> ( <arg> [ , ... ] [ORDER BY <sortspec> [ , ...]] )

If the optional ORDER BY is omitted, a system-defined ordering is used. The transition function of an ordered aggregate function is called on its input arguments in the specified order and on a single segment. There is a new column aggordered in the pg_aggregate table to indicate the aggregate function is defined as an ordered aggregate.

Parameters

CREATE FUNCTION scube_accum(numeric, numeric) RETURNS numeric 
    AS 'select $1 + $2 * $2 * $2' 
    LANGUAGE SQL 
    IMMUTABLE 
    RETURNS NULL ON NULL INPUT;
CREATE AGGREGATE scube(numeric) ( 
    SFUNC = scube_accum, 
    STYPE = numeric, 
    INITCOND = 0 );

CREATE TABLE x(a INT);
INSERT INTO x VALUES (1),(2),(3);
SELECT scube(a) FROM x;

SELECT sum(a*a*a) FROM x;