PostgreSQL 8.1.23 Documentation | ||||
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CREATE AGGREGATEname( 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-stateffunc( 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.

`name`-
The name (optionally schema-qualified) of the 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_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. `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 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`. `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.

The parameters of `CREATE AGGREGATE`
can be written in any order, not just the order illustrated
above.

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