Index for Section AGGREGATE

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CREATE
NAME
  CREATE AGGREGATE - define a new aggregate function

SYNOPSIS
  CREATE AGGREGATE name ( BASETYPE = input_data_type,
      SFUNC = sfunc, STYPE = state_type
      [ , FINALFUNC = ffunc ]
      [ , INITCOND = initial_condition ] )

  INPUTS

  name The name of an aggregate function to create.

  input_data_type
       The input data type on which this aggregate function operates.  This
       can be specified as ANY for an aggregate that does not examine its
       input values (an example is count(*)).

  sfunc
       The name of the state transition function to be called for each input
       data value.  This is normally a function of two arguments, the first
       being of type state_type and the second of type input_data_type.
       Alternatively, for an aggregate that does not examine its input
       values, the function takes just one argument of type state_type.	 In
       either case the function must return a value of type state_type.	 This
       function takes the current state value and the current input data
       item, and returns the next state value.

  state_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 data has been traversed. The function must take
       a single argument of type state_type.  The output 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 output type is state_type.

  initial_condition
       The initial setting for the state value. This must be a literal
       constant in the form accepted for the data type state_type.  If not
       specified, the state value starts out NULL.

  OUTPUTS

  CREATE
       Message returned if the command completes successfully.

DESCRIPTION
  CREATE AGGREGATE allows a user or programmer to extend PostgreSQL
  functionality by defining new aggregate functions. Some aggregate functions
  for base types such as min(integer) and avg(double precision) are already
  provided in the base distribution. If one defines new types or needs an
  aggregate function not already provided, then CREATE AGGREGATE can be used
  to provide the desired features.

  An aggregate function is identified by its name and input data type.	Two
  aggregates can have the same name if they operate on different input types.
  To avoid confusion, do not make an ordinary function of the same name and
  input data type as an aggregate.

  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-item ) ---> 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 data item, the state
  transition function is invoked to calculate a new internal state value.
  After all the data has been processed, the final function is invoked once
  to calculate the aggregate's output 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 field 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. 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 the first non-NULL input value replaces the state
  value, and the transition function is invoked beginning with the second
  non-NULL input value.	 This is handy for implementing aggregates like max.
  Note that this behavior is only available when state_type is the same as
  input_data_type.  When these types are different, you must supply a non-
  NULL initial condition or use a non-strict transition function.

  If the state transition function is not strict, then it will be called
  unconditionally at each input value, 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 NULLs.

  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 output
  automatically. (Of course this is just the normal behavior of strict
  functions.) In any case the final function has the option of returning
  NULL. For example, the final function for avg returns NULL when it sees
  there were zero input tuples.

  NOTES

  Use DROP AGGREGATE to drop aggregate functions.

  The parameters of CREATE AGGREGATE can be written in any order, not just
  the order illustrated above.

USAGE
  Refer to the chapter on aggregate functions in the PostgreSQL Programmer's
  Guide for complete examples of usage.

COMPATIBILITY
  SQL92

  CREATE AGGREGATE is a PostgreSQL language extension.	There is no CREATE
  AGGREGATE in SQL92.

Index for Section AGGREGATE

Alphabetical listing for C

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