## 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 9.15. 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.