Provided by: postgresql-client-9.1_9.1.3-2_i386 bug

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
       commonly-used aggregate functions are included with the distribution;
       they are documented in Section 9.18, "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( internal-state, next-data-values ) ---> 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 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 as-is.

       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 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.

       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 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.

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.

       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 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 35.10, "User-defined Aggregates", in the documentation.

COMPATIBILITY

       CREATE AGGREGATE is a PostgreSQL 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))