CREATE AGGREGATE() SQL Commands CREATE AGGREGATE()
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 com-
monly-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 oper-
ate 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 aggre-
gate 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 aggre-
gate'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 aggre-
gates 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 opera-
tor. 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 opera-
tor for MIN, and > is the proper sort operator for MAX. Note that the
optimization will never actually take effect unless the specified oper-
ator is the ``less than'' or ``greater than'' strategy member of a B-
tree index operator class.
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 aggre-
gate 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 aggre-
gate. 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 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(l)], DROP AGGREGATE [drop_aggre-
gate(l)]
SQL - Language Statements 2008-09-19 CREATE AGGREGATE()
postgresql 8.3.4 - Generated Thu Oct 2 10:05:02 CDT 2008