## Description

`CREATE AGGREGATE`
allows a user or programmer to extend PostgreSQL
functionality 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.

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 (the one
at the front of the search path; see `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 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",
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 beginning with the second non-NULL input value.
This is handy for implementing 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 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 output 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.