CREATE AGGREGATE() SQL Commands CREATE AGGREGATE()NAME
CREATE AGGREGATE - Defines 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 Postgres func‐
tionality 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
Postgres 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" in pg_proc, 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 begin‐
ning with the second non-NULL input value. This is handy for imple‐
menting 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 out‐
put 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 Program‐
mer's Guide for complete examples of usage.
COMPATIBILITY
SQL92
CREATE AGGREGATE is a Postgres language extension. There is no CREATE
AGGREGATE in SQL92.
SQL - Language Statements 29 March 2001 CREATE AGGREGATE()
