CREATE AGGREGATEname( [argmode] [argname]arg_data_type[ , ... ] ) ( SFUNC =sfunc, STYPE =state_data_type[ , SSPACE =state_data_size] [ , FINALFUNC =ffunc] [ , FINALFUNC_EXTRA ] [ , INITCOND =initial_condition] [ , MSFUNC =msfunc] [ , MINVFUNC =minvfunc] [ , MSTYPE =mstate_data_type] [ , MSSPACE =mstate_data_size] [ , MFINALFUNC =mffunc] [ , MFINALFUNC_EXTRA ] [ , MINITCOND =minitial_condition] [ , SORTOP =sort_operator] ) CREATE AGGREGATEname( [ [argmode] [argname]arg_data_type[ , ... ] ] ORDER BY [argmode] [argname]arg_data_type[ , ... ] ) ( SFUNC =sfunc, STYPE =state_data_type[ , SSPACE =state_data_size] [ , FINALFUNC =ffunc] [ , FINALFUNC_EXTRA ] [ , INITCOND =initial_condition] [ , HYPOTHETICAL ] ) or the old syntax CREATE AGGREGATEname( BASETYPE =base_type, SFUNC =sfunc, STYPE =state_data_type[ , SSPACE =state_data_size] [ , FINALFUNC =ffunc] [ , FINALFUNC_EXTRA ] [ , INITCOND =initial_condition] [ , MSFUNC =sfunc] [ , MINVFUNC =invfunc] [ , MSTYPE =state_data_type] [ , MSSPACE =state_data_size] [ , MFINALFUNC =ffunc] [ , MFINALFUNC_EXTRA ] [ , MINITCOND =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 9.20. 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 operate 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. This behavior is identical to overloading of ordinary function names (see CREATE FUNCTION).

A simple 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-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 row, the aggregate 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 aggregate'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 each subsequent row with all-nonnull input values. This is
handy for implementing aggregates like `max`

. Note that this behavior is only available
when `state_data_type` is the same
as the first `arg_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 state 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.

Sometimes it is useful to declare the final function as taking
not just the state value, but extra parameters corresponding to
the aggregate's input values. The main reason for doing this is
if the final function is polymorphic and the state value's data
type would be inadequate to pin down the result type. These extra
parameters are always passed as NULL (and so the final function
must not be strict when the `FINALFUNC_EXTRA` option is used), but nonetheless
they are valid parameters. The final function could for example
make use of `get_fn_expr_argtype`

to
identify the actual argument type in the current call.

An aggregate can optionally support *moving-aggregate mode*, as described in Section 35.10.1. This
requires specifying the `MSFUNC`,
`MINVFUNC`, and `MSTYPE` parameters, and optionally the `MSPACE`, `MFINALFUNC`,
`MFINALFUNC_EXTRA`, and `MINITCOND` parameters. Except for `MINVFUNC`, these parameters work like the
corresponding simple-aggregate parameters without `M`; they define a separate implementation of the
aggregate that includes an inverse transition function.

The syntax with `ORDER BY` in the
parameter list creates a special type of aggregate called an
*ordered-set aggregate*; or if `HYPOTHETICAL` is specified, then a *hypothetical-set aggregate* is created. These
aggregates operate over groups of sorted values in
order-dependent ways, so that specification of an input sort
order is an essential part of a call. Also, they can have
*direct* arguments, which are arguments
that are evaluated only once per aggregation rather than once per
input row. Hypothetical-set aggregates are a subclass of
ordered-set aggregates in which some of the direct arguments are
required to match, in number and data types, the aggregated
argument columns. This allows the values of those direct
arguments to be added to the collection of aggregate-input rows
as an additional "hypothetical"
row.

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.

To be able to create an aggregate function, you must have
`USAGE` privilege on the argument types,
the state type(s), and the return type, as well as `EXECUTE` privilege on the transition and final
functions.

`name`-
The name (optionally schema-qualified) of the aggregate function to create.

`argmode`-
The mode of an argument:

`IN`or`VARIADIC`. (Aggregate functions do not support`OUT`arguments.) If omitted, the default is`IN`. Only the last argument can be marked`VARIADIC`. `argname`-
The name of an argument. This is currently only useful for documentation purposes. If omitted, the argument has no name.

`arg_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 argument specifications. (An example of such an aggregate 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 with this syntax, specify the`basetype`as`"ANY"`(not`*`). Ordered-set aggregates cannot be defined with the old syntax. `sfunc`-
The name of the state transition function to be called for each input row. For a normal

`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 aggregate. 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.For ordered-set (including hypothetical-set) aggregates, the state transition function receives only the current state value and the aggregated arguments, not the direct arguments. Otherwise it is the same.

`state_data_type`-
The data type for the aggregate's state value.

`state_data_size`-
The approximate average size (in bytes) of the aggregate's state value. If this parameter is omitted or is zero, a default estimate is used based on the

`state_data_type`. The planner uses this value to estimate the memory required for a grouped aggregate query. The planner will consider using hash aggregation for such a query only if the hash table is estimated to fit in work_mem; therefore, large values of this parameter discourage use of hash aggregation. `ffunc`-
The name of the final function called to compute the aggregate's result after all input rows have been traversed. For a normal aggregate, this 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`.For ordered-set (including hypothetical-set) aggregates, the final function receives not only the final state value, but also the values of all the direct arguments.

If

`FINALFUNC_EXTRA`is specified, then in addition to the final state value and any direct arguments, the final function receives extra NULL values corresponding to the aggregate's regular (aggregated) arguments. This is mainly useful to allow correct resolution of the aggregate result type when a polymorphic aggregate is being defined. `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. `msfunc`-
The name of the forward state transition function to be called for each input row in moving-aggregate mode. This is exactly like the regular transition function, except that its first argument and result are of type

`mstate_data_type`, which might be different from`state_data_type`. `minvfunc`-
The name of the inverse state transition function to be used in moving-aggregate mode. This function has the same argument and result types as

`msfunc`, but it is used to remove a value from the current aggregate state, rather than add a value to it. The inverse transition function must have the same strictness attribute as the forward state transition function. `mstate_data_type`-
The data type for the aggregate's state value, when using moving-aggregate mode.

`mstate_data_size`-
The approximate average size (in bytes) of the aggregate's state value, when using moving-aggregate mode. This works the same as

`state_data_size`. `mffunc`-
The name of the final function called to compute the aggregate's result after all input rows have been traversed, when using moving-aggregate mode. This works the same as

`ffunc`, except that its first argument's type is`mstate_data_type`and extra dummy arguments are specified by writing`MFINALFUNC_EXTRA`. The aggregate result type determined by`mffunc`or`mstate_data_type`must match that determined by the aggregate's regular implementation. `minitial_condition`-
The initial setting for the state value, when using moving-aggregate mode. This works the same as

`initial_condition`. `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 normal aggregate). `HYPOTHETICAL`-
For ordered-set aggregates only, this flag specifies that the aggregate arguments are to be processed according to the requirements for hypothetical-set aggregates: that is, the last few direct arguments must match the data types of the aggregated (

`WITHIN GROUP`) arguments. The`HYPOTHETICAL`flag has no effect on run-time behavior, only on parse-time resolution of the data types and collations of the aggregate's arguments.

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

In parameters that specify support function names, you can
write a schema name if needed, for example `SFUNC = public.sum`. Do not write argument types
there, however — the argument types of the support functions are
determined from other parameters.

If an aggregate supports moving-aggregate mode, it will
improve calculation efficiency when the aggregate is used as a
window function for a window with moving frame start (that is, a
frame start mode other than `UNBOUNDED
PRECEDING`). Conceptually, the forward transition function
adds input values to the aggregate's state when they enter the
window frame from the bottom, and the inverse transition function
removes them again when they leave the frame at the top. So, when
values are removed, they are always removed in the same order
they were added. Whenever the inverse transition function is
invoked, it will thus receive the earliest added but not yet
removed argument value(s). The inverse transition function can
assume that at least one row will remain in the current state
after it removes the oldest row. (When this would not be the
case, the window function mechanism simply starts a fresh
aggregation, rather than using the inverse transition
function.)

The forward transition function for moving-aggregate mode is not allowed to return NULL as the new state value. If the inverse transition function returns NULL, this is taken as an indication that the inverse function cannot reverse the state calculation for this particular input, and so the aggregate calculation will be redone from scratch for the current frame starting position. This convention allows moving-aggregate mode to be used in situations where there are some infrequent cases that are impractical to reverse out of the running state value.

If no moving-aggregate implementation is supplied, the aggregate can still be used with moving frames, but PostgreSQL will recompute the whole aggregation whenever the start of the frame moves. Note that whether or not the aggregate supports moving-aggregate mode, PostgreSQL can handle a moving frame end without recalculation; this is done by continuing to add new values to the aggregate's state. It is assumed that the final function does not damage the aggregate's state value, so that the aggregation can be continued even after an aggregate result value has been obtained for one set of frame boundaries.

The syntax for ordered-set aggregates allows `VARIADIC` to be specified for both the last direct
parameter and the last aggregated (`WITHIN
GROUP`) parameter. However, the current implementation
restricts use of `VARIADIC` in two ways.
First, ordered-set aggregates can only use `VARIADIC "any"`, not other variadic array types.
Second, if the last direct parameter is `VARIADIC "any"`, then there can be only one
aggregated parameter and it must also be `VARIADIC "any"`. (In the representation used in the
system catalogs, these two parameters are merged into a single
`VARIADIC "any"` item, since `pg_proc` cannot represent functions with more
than one `VARIADIC` parameter.) If the
aggregate is a hypothetical-set aggregate, the direct arguments
that match the `VARIADIC "any"` parameter
are the hypothetical ones; any preceding parameters represent
additional direct arguments that are not constrained to match the
aggregated arguments.

Currently, ordered-set aggregates do not need to support moving-aggregate mode, since they cannot be used as window functions.