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_data_type
      [ , FINALFUNC = ffunc ]
      [ , INITCOND = initial_condition ]
  )

DESCRIPTION
  CREATE AGGREGATE defines a new aggregate function. Some aggregate functions
  for base types such as min(integer) and avg(double precision) are already
  provided in the standard 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.

  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.	Two
  aggregates in the same schema can have the same name if they operate on
  different input types. The name and input data type 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-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 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 column 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 nonnull input value replaces the state
  value, and the transition function is invoked beginning with the second
  nonnull input value.	This is handy for implementing aggregates like max.
  Note that this behavior is only available when state_data_type is the same
  as 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 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 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.

PARAMETERS
  name The name (optionally schema-qualified) of the 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_data_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_data_type.
       In either case the function must return a value of type
       state_data_type. This function takes the current state value and the
       current input data item, 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 data has 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.

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

EXAMPLES
  See the section called ``User-defined Aggregates'' in the documentation.

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

SEE ALSO
  ALTER AGGREGATE [alter_aggregate(5)], DROP AGGREGATE [drop_aggregate(l)]

Index for Section AGGREGATE

Alphabetical listing for C

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