| PostgreSQL 8.1.23 Documentation | ||||
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CREATE AGGREGATE name (
BASETYPE = input_data_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 Section 9.15. 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.
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.
The name (optionally schema-qualified) of the aggregate function to create.
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(*)).
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.
The data type for the aggregate's state value.
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.
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.
The parameters of CREATE AGGREGATE can be written in any order, not just the order illustrated above.