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## CREATE AGGREGATE

CREATE AGGREGATE — define a new aggregate function

## Synopsis

CREATE AGGREGATE( [`name`

] [`argmode`

]`argname`

[ , ... ] ) ( SFUNC =`arg_data_type`

, STYPE =`sfunc`

[ , SSPACE =`state_data_type`

] [ , FINALFUNC =`state_data_size`

] [ , FINALFUNC_EXTRA ] [ , FINALFUNC_MODIFY = { READ_ONLY | SHAREABLE | READ_WRITE } ] [ , COMBINEFUNC =`ffunc`

] [ , SERIALFUNC =`combinefunc`

] [ , DESERIALFUNC =`serialfunc`

] [ , INITCOND =`deserialfunc`

] [ , MSFUNC =`initial_condition`

] [ , MINVFUNC =`msfunc`

] [ , MSTYPE =`minvfunc`

] [ , MSSPACE =`mstate_data_type`

] [ , MFINALFUNC =`mstate_data_size`

] [ , MFINALFUNC_EXTRA ] [ , MFINALFUNC_MODIFY = { READ_ONLY | SHAREABLE | READ_WRITE } ] [ , MINITCOND =`mffunc`

] [ , SORTOP =`minitial_condition`

] [ , PARALLEL = { SAFE | RESTRICTED | UNSAFE } ] ) CREATE AGGREGATE`sort_operator`

( [ [`name`

] [`argmode`

]`argname`

[ , ... ] ] ORDER BY [`arg_data_type`

] [`argmode`

]`argname`

[ , ... ] ) ( SFUNC =`arg_data_type`

, STYPE =`sfunc`

[ , SSPACE =`state_data_type`

] [ , FINALFUNC =`state_data_size`

] [ , FINALFUNC_EXTRA ] [ , FINALFUNC_MODIFY = { READ_ONLY | SHAREABLE | READ_WRITE } ] [ , INITCOND =`ffunc`

] [ , PARALLEL = { SAFE | RESTRICTED | UNSAFE } ] [ , HYPOTHETICAL ] ) or the old syntax CREATE AGGREGATE`initial_condition`

( BASETYPE =`name`

, SFUNC =`base_type`

, STYPE =`sfunc`

[ , SSPACE =`state_data_type`

] [ , FINALFUNC =`state_data_size`

] [ , FINALFUNC_EXTRA ] [ , FINALFUNC_MODIFY = { READ_ONLY | SHAREABLE | READ_WRITE } ] [ , COMBINEFUNC =`ffunc`

] [ , SERIALFUNC =`combinefunc`

] [ , DESERIALFUNC =`serialfunc`

] [ , INITCOND =`deserialfunc`

] [ , MSFUNC =`initial_condition`

] [ , MINVFUNC =`msfunc`

] [ , MSTYPE =`minvfunc`

] [ , MSSPACE =`mstate_data_type`

] [ , MFINALFUNC =`mstate_data_size`

] [ , MFINALFUNC_EXTRA ] [ , MFINALFUNC_MODIFY = { READ_ONLY | SHAREABLE | READ_WRITE } ] [ , MINITCOND =`mffunc`

] [ , SORTOP =`minitial_condition`

] )`sort_operator`

## 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 9.20. 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

*. These are used as follows:*

`ffunc`

( internal-state, next-data-values ) ---> next-internal-state`sfunc`

( internal-state ) ---> aggregate-value`ffunc`

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

*. When these types are different, you must supply a nonnull initial condition or use a nonstrict transition function.*

`arg_data_type`

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 38.11.1. This requires
specifying the `MSFUNC`

, `MINVFUNC`

,
and `MSTYPE`

parameters, and optionally
the `MSSPACE`

, `MFINALFUNC`

,
`MFINALFUNC_EXTRA`

, `MFINALFUNC_MODIFY`

,
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.

An aggregate can optionally support *partial aggregation*,
as described in Section 38.11.4.
This requires specifying the `COMBINEFUNC`

parameter.
If the * state_data_type*
is

`internal`

, it's usually also appropriate to provide the
`SERIALFUNC`

and `DESERIALFUNC`

parameters so that
parallel aggregation is possible. Note that the aggregate must also be
marked `PARALLEL SAFE`

to enable parallel aggregation.
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 supporting functions.

## Parameters

`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

-argument aggregate function, the`N`

must take`sfunc`

+1 arguments, the first being of type`N`

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.`state_data_type`

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

. 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.`state_data_type`

`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

. The return data type of the aggregate is defined as the return type of this function. If`state_data_type`

is not specified, then the ending state value is used as the aggregate's result, and the return type is`ffunc`

.`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.`FINALFUNC_MODIFY`

= {`READ_ONLY`

|`SHAREABLE`

|`READ_WRITE`

}This option specifies whether the final function is a pure function that does not modify its arguments.

`READ_ONLY`

indicates it does not; the other two values indicate that it may change the transition state value. See Notes below for more detail. The default is`READ_ONLY`

, except for ordered-set aggregates, for which the default is`READ_WRITE`

.`combinefunc`

The

function may optionally be specified to allow the aggregate function to support partial aggregation. If provided, the`combinefunc`

must combine two`combinefunc`

values, each containing the result of aggregation over some subset of the input values, to produce a new`state_data_type`

that represents the result of aggregating over both sets of inputs. This function can be thought of as an`state_data_type`

, where instead of acting upon an individual input row and adding it to the running aggregate state, it adds another aggregate state to the running state.`sfunc`

The

must be declared as taking two arguments of the`combinefunc`

and returning a value of the`state_data_type`

. Optionally this function may be “strict”. In this case the function will not be called when either of the input states are null; the other state will be taken as the correct result.`state_data_type`

For aggregate functions whose

is`state_data_type`

`internal`

, themust not be strict. In this case the`combinefunc`

must ensure that null states are handled correctly and that the state being returned is properly stored in the aggregate memory context.`combinefunc`

`serialfunc`

An aggregate function whose

is`state_data_type`

`internal`

can participate in parallel aggregation only if it has afunction, which must serialize the aggregate state into a`serialfunc`

`bytea`

value for transmission to another process. This function must take a single argument of type`internal`

and return type`bytea`

. A correspondingis also required.`deserialfunc`

`deserialfunc`

Deserialize a previously serialized aggregate state back into

. This function must take two arguments of types`state_data_type`

`bytea`

and`internal`

, and produce a result of type`internal`

. (Note: the second,`internal`

argument is unused, but is required for type safety reasons.)`initial_condition`

The initial setting for the state value. This must be a string constant in the form accepted for the data type

. If not specified, the state value starts out null.`state_data_type`

`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

, which might be different from`mstate_data_type`

.`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

, 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.`msfunc`

`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

, except that its first argument's type is`ffunc`

and extra dummy arguments are specified by writing`mstate_data_type`

`MFINALFUNC_EXTRA`

. The aggregate result type determined byor`mffunc`

must match that determined by the aggregate's regular implementation.`mstate_data_type`

`MFINALFUNC_MODIFY`

= {`READ_ONLY`

|`SHAREABLE`

|`READ_WRITE`

}This option is like

`FINALFUNC_MODIFY`

, but it describes the behavior of the moving-aggregate final function.`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).`PARALLEL =`

{`SAFE`

|`RESTRICTED`

|`UNSAFE`

}The meanings of

`PARALLEL SAFE`

,`PARALLEL RESTRICTED`

, and`PARALLEL UNSAFE`

are the same as in CREATE FUNCTION. An aggregate will not be considered for parallelization if it is marked`PARALLEL UNSAFE`

(which is the default!) or`PARALLEL RESTRICTED`

. Note that the parallel-safety markings of the aggregate's support functions are not consulted by the planner, only the marking of the aggregate itself.`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.

## Notes

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.

Ordinarily, PostgreSQL functions are expected to be true functions that
do not modify their input values. However, an aggregate transition
function, *when used in the context of an aggregate*,
is allowed to cheat and modify its transition-state argument in place.
This can provide substantial performance benefits compared to making
a fresh copy of the transition state each time.

Likewise, while an aggregate final function is normally expected not to
modify its input values, sometimes it is impractical to avoid modifying
the transition-state argument. Such behavior must be declared using
the `FINALFUNC_MODIFY`

parameter.
The `READ_WRITE`

value indicates that the final function modifies the transition state in
unspecified ways. This value prevents use of the aggregate as a window
function, and it also prevents merging of transition states for aggregate
calls that share the same input values and transition functions.
The `SHAREABLE`

value indicates that the transition function
cannot be applied after the final function, but multiple final-function
calls can be performed on the ending transition state value. This value
prevents use of the aggregate as a window function, but it allows merging
of transition states. (That is, the optimization of interest here is not
applying the same final function repeatedly, but applying different final
functions to the same ending transition state value. This is allowed as
long as none of the final functions are marked `READ_WRITE`

.)

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. This is why use of an aggregate as a window function requires that the final function be read-only: it must 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.

Partial (including parallel) aggregation is currently not supported for
ordered-set aggregates. Also, it will never be used for aggregate calls
that include `DISTINCT`

or `ORDER BY`

clauses, since
those semantics cannot be supported during partial aggregation.

## Examples

See Section 38.11.

## Compatibility

`CREATE AGGREGATE`

is a
PostgreSQL language extension. The SQL
standard does not provide for user-defined aggregate functions.