create_aggregate man page on UnixWare

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       CREATE AGGREGATE - define a new aggregate function

       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 ]

       CREATE  AGGREGATE defines a new aggregate function. Some basic and com‐
       monly-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	 oper‐
       ate  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	aggre‐
       gate 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	aggre‐
       gate's  return  value.  If  there  is no final function then the ending
       state value is returned as-is.

       An aggregate function may 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 aggre‐
       gates 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 opera‐
       tor. 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 opera‐
       tor for MIN, and > is the proper sort operator for MAX. Note  that  the
       optimization will never actually take effect unless the specified oper‐
       ator is the ``less than'' or ``greater than'' strategy member of	 a  B-
       tree index operator class.

       name   The name (optionally schema-qualified) of the aggregate function
	      to create.

	      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	aggre‐
	      gate is count(*).)

	      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	aggre‐
	      gate.  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.

	      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

	      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.

	      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.

       See in the documentation.

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

       ALTER  AGGREGATE	 [alter_aggregate(5)],	DROP  AGGREGATE	  [drop_aggre‐

SQL - Language Statements	  2008-01-03		    CREATE AGGREGATE()
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