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 ]
    [ , CFUNC = cfunc ]
    [ , 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 to maximum three ordinary functions: a state transition function sfunc, an optional collection function cfunc, 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

In Postgres-XC the aggregation works in two different modes.

    • Two phased aggregation - is used when the entire aggregation takes place on the Coordinator node. In first phase called transition phase, Postgres-XC 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, in the second phase or finalization phase 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.

    • Three phased aggregation - is used when the process of aggregation is divided between Coordinator and Datanodes. In this mode, each Postgres-XC Datanode involved in the query carries out the first phase named transition phase. This phase is similar to the first phase in the two phased aggregation mode discussed above, except that, every Datanode applies this phase on the rows available at the Datanode. The result of transition phase is then transferred to the Coordinator node. Second phase called collection phase takes place on the Coordinator. Postgres-XC Coordinator node creates a temporary variable of data type stype to hold the current internal state of the collection phase. For every input from the Datanode (result of transition phase on that node), the collection function is invoked with the current collection state value and the new transition value (obtained from the Datanode) to calculate a new internal collection state value. After all the transition values from data nodes have been processed, in the third or finalization phase the final function is invoked once to calculate the aggregate's return value. If there is no final function then the ending collection state value is returned as-is.

Irrespective of the mode used for aggregation, the result of aggregation should be same if the same set of data rows is participating in the aggregate. Postgres-XC planner chooses the cheapest feasible mode of the above two, during planning.

    Note

    The following description applies only to Postgres-XC

An aggregate function can provide an initial condition, that is, an initial value for the internal transition or collection 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.

    Note

    The following description applies only to Postgres-XC

If the collection function is declared “strict”, then it cannot be called with null inputs. With such a collection function, aggregate execution behaves as follows. Null state transition results are ignored (the function is not called and the previous collection state value is retained). If the initial state value is null, then at the first non-null state transition result replaces the collection state value, and the collection function is invoked at subsequent rows with all-nonnull transition values. This is handy for implementing aggregates like max.

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.

    Note

    The following description applies only to Postgres-XC

If the state transition and/or collection function is not strict, then it will be called unconditionally at each input row, and must deal with null inputs and null transition/collection 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.

cfunc

        Note
        The following description applies only to Postgres-XC

    The name of the state collection function to be called for each input row. The sfunc must take 2 arguments, both of them being of type state_data_type. The function must return a value of type state_data_type. This function takes the current collection state value and the current transition value, and returns the next collection state value. If cfunc is omitted for an aggregate, the two phase aggregation mode is used for that aggregate. All the aggregates involed in a query use the same aggregation mode.

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.

initial_collection_condition

        Note
        The following description applies only to Postgres-XC

    The initial setting for the state collection value. This must be a string constant in the form accepted for the data type state_data_type. If not specified, the collection state value starts out null. If the collection function cfunc is not supplied, this parameter is stored but not used.

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 36.10, “User-defined Aggregates”, in the documentation.

COMPATIBILITY

CREATE AGGREGATE is a Postgres-XC 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))