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CREATE AGGREGATE(7)              SQL Commands              CREATE AGGREGATE(7)

       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 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 represen-
       tation 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 transi-
       tion  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 non-
       strict 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

       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 func-
              tion, write * in place of the list of input data types. (An example of such an aggregate 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 match-
              ing  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.

              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 state_data_type.

              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 aggre-
       gate functions.

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

SQL - Language Statements         2014-02-17               CREATE AGGREGATE(7)