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 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, aggre-
       gate  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  func-
	      tion.  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 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)]

SQL - Language Statements					    2010-05-14						       CREATE AGGREGATE(7)