CREATE AGGREGATE(7)                                       SQL Commands                                       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 ]
           [ , 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 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 pro-
       cessed, 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  con-
       stant 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 transi-
       tion 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  subse-
       quent  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-argu-
              ment 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 aggre-
              gate'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 in the documentation.

COMPATIBILITY
       CREATE  AGGREGATE  is a PostgreSQL language extension. The SQL standard does not provide for user-defined aggregate func-
       tions.

SEE ALSO
       ALTER AGGREGATE [alter_aggregate(7)], DROP AGGREGATE [drop_aggregate(7)]



SQL - Language Statements                                  2011-09-22                                        CREATE AGGREGATE(7)