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CREATE AGGREGATE(7) PostgreSQL 9.2.24 Documentation CREATE AGGREGATE(7)
NAME
CREATE_AGGREGATE - define a new aggregate function
SYNOPSIS
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 ]
)
DESCRIPTION
CREATE AGGREGATE defines a new aggregate function. Some basic and commonly-used aggregate
functions are included with the distribution; they are documented in Section 9.20,
"Aggregate Functions", 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 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 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 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 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 class.
To be able to create an aggregate function, you must have USAGE privilege on the argument
types, the state type, and the return type, as well as EXECUTE privilege on the transition
and final functions.
PARAMETERS
name
The name (optionally schema-qualified) of the aggregate function to create.
input_data_type
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 aggregate is count(*).)
base_type
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
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.
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 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.
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.
sort_operator
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.
EXAMPLES
See Section 35.10, "User-defined Aggregates", in the documentation.
COMPATIBILITY
CREATE AGGREGATE is a PostgreSQL language extension. The SQL standard does not provide for
user-defined aggregate functions.
SEE ALSO
ALTER AGGREGATE (ALTER_AGGREGATE(7)), DROP AGGREGATE (DROP_AGGREGATE(7))
PostgreSQL 9.2.24 2017-11-06 CREATE AGGREGATE(7)
Generated by $Id: phpMan.php,v 4.55 2007/09/05 04:42:51 chedong Exp $ Author: Che Dong
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