CREATE AGGREGATE
CREATE AGGREGATE
CREATE AGGREGATE  define a new aggregate function
Synopsis
CREATE AGGREGATEname
( [argmode
] [argname
]arg_data_type
[ , ... ] ) ( SFUNC =sfunc
, STYPE =state_data_type
[ , SSPACE =state_data_size
] [ , FINALFUNC =ffunc
] [ , FINALFUNC_EXTRA ] [ , FINALFUNC_MODIFY = { READ_ONLY  SHAREABLE  READ_WRITE } ] [ , COMBINEFUNC =combinefunc
] [ , SERIALFUNC =serialfunc
] [ , DESERIALFUNC =deserialfunc
] [ , INITCOND =initial_condition
] [ , MSFUNC =msfunc
] [ , MINVFUNC =minvfunc
] [ , MSTYPE =mstate_data_type
] [ , MSSPACE =mstate_data_size
] [ , MFINALFUNC =mffunc
] [ , MFINALFUNC_EXTRA ] [ , MFINALFUNC_MODIFY = { READ_ONLY  SHAREABLE  READ_WRITE } ] [ , MINITCOND =minitial_condition
] [ , SORTOP =sort_operator
] [ , PARALLEL = { SAFE  RESTRICTED  UNSAFE } ] ) 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 ] [ , FINALFUNC_MODIFY = { READ_ONLY  SHAREABLE  READ_WRITE } ] [ , INITCOND =initial_condition
] [ , PARALLEL = { SAFE  RESTRICTED  UNSAFE } ] [ , HYPOTHETICAL ] ) or the old syntax CREATE AGGREGATEname
( BASETYPE =base_type
, SFUNC =sfunc
, STYPE =state_data_type
[ , SSPACE =state_data_size
] [ , FINALFUNC =ffunc
] [ , FINALFUNC_EXTRA ] [ , FINALFUNC_MODIFY = { READ_ONLY  SHAREABLE  READ_WRITE } ] [ , COMBINEFUNC =combinefunc
] [ , SERIALFUNC =serialfunc
] [ , DESERIALFUNC =deserialfunc
] [ , INITCOND =initial_condition
] [ , MSFUNC =msfunc
] [ , MINVFUNC =minvfunc
] [ , MSTYPE =mstate_data_type
] [ , MSSPACE =mstate_data_size
] [ , MFINALFUNC =mffunc
] [ , MFINALFUNC_EXTRA ] [ , MFINALFUNC_MODIFY = { READ_ONLY  SHAREABLE  READ_WRITE } ] [ , MINITCOND =minitial_condition
] [ , SORTOP =sort_operator
] )
Description
CREATE AGGREGATE
defines a new aggregate
function. Some basic and commonlyused 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
( internalstate, nextdatavalues ) > nextinternalstateffunc
( internalstate ) > aggregatevalue
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 asis.
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 allnonnull input values, the first argument value replaces the state
value, and the transition function is invoked at each subsequent row with
allnonnull 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
movingaggregate 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 simpleaggregate 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
orderedset
aggregate
; or if
HYPOTHETICAL
is specified, then
a
hypotheticalset aggregate
is created. These
aggregates operate over groups of sorted values in orderdependent 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. Hypotheticalset aggregates are a subclass of orderedset
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
aggregateinput 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 nonnull 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 Btree
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 schemaqualified) of the aggregate function to create.

argmode

The mode of an argument:
IN
orVARIADIC
. (Aggregate functions do not supportOUT
arguments.) If omitted, the default isIN
. Only the last argument can be markedVARIADIC
. 
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 zeroargument aggregate function, write
*
in place of the list of argument specifications. (An example of such an aggregate iscount(*)
.) 
base_type

In the old syntax for
CREATE AGGREGATE
, the input data type is specified by abasetype
parameter rather than being written next to the aggregate name. Note that this syntax allows only one input parameter. To define a zeroargument aggregate function with this syntax, specify thebasetype
as"ANY"
(not*
). Orderedset 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, thesfunc
must takeN
+1 arguments, the first being of typestate_data_type
and the rest matching the declared input data type(s) of the aggregate. The function must return a value of typestate_data_type
. This function takes the current state value and the current input data value(s), and returns the next state value.For orderedset (including hypotheticalset) 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. Ifffunc
is not specified, then the ending state value is used as the aggregate's result, and the return type isstate_data_type
.For orderedset (including hypotheticalset) 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 isREAD_ONLY
, except for orderedset aggregates, for which the default isREAD_WRITE
. 
combinefunc

The
combinefunc
function may optionally be specified to allow the aggregate function to support partial aggregation. If provided, thecombinefunc
must combine twostate_data_type
values, each containing the result of aggregation over some subset of the input values, to produce a newstate_data_type
that represents the result of aggregating over both sets of inputs. This function can be thought of as ansfunc
, 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.The
combinefunc
must be declared as taking two arguments of thestate_data_type
and returning a value of thestate_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.For aggregate functions whose
state_data_type
isinternal
, thecombinefunc
must not be strict. In this case thecombinefunc
must ensure that null states are handled correctly and that the state being returned is properly stored in the aggregate memory context. 
serialfunc

An aggregate function whose
state_data_type
isinternal
can participate in parallel aggregation only if it has aserialfunc
function, which must serialize the aggregate state into abytea
value for transmission to another process. This function must take a single argument of typeinternal
and return typebytea
. A correspondingdeserialfunc
is also required. 
deserialfunc

Deserialize a previously serialized aggregate state back into
state_data_type
. This function must take two arguments of typesbytea
andinternal
, and produce a result of typeinternal
. (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
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 movingaggregate 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 fromstate_data_type
. 
minvfunc

The name of the inverse state transition function to be used in movingaggregate 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 movingaggregate mode.

mstate_data_size

The approximate average size (in bytes) of the aggregate's state value, when using movingaggregate 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 movingaggregate mode. This works the same as
ffunc
, except that its first argument's type ismstate_data_type
and extra dummy arguments are specified by writingMFINALFUNC_EXTRA
. The aggregate result type determined bymffunc
ormstate_data_type
must match that determined by the aggregate's regular implementation. 
MFINALFUNC_MODIFY
= {READ_ONLY
SHAREABLE
READ_WRITE
} 
This option is like
FINALFUNC_MODIFY
, but it describes the behavior of the movingaggregate final function. 
minitial_condition

The initial setting for the state value, when using movingaggregate mode. This works the same as
initial_condition
. 
sort_operator

The associated sort operator for a
MIN
 orMAX
like aggregate. This is just an operator name (possibly schemaqualified). The operator is assumed to have the same input data types as the aggregate (which must be a singleargument normal aggregate). 
PARALLEL =
{SAFE
RESTRICTED
UNSAFE
} 
The meanings of
PARALLEL SAFE
,PARALLEL RESTRICTED
, andPARALLEL UNSAFE
are the same as in CREATE FUNCTION . An aggregate will not be considered for parallelization if it is markedPARALLEL UNSAFE
(which is the default!) orPARALLEL RESTRICTED
. Note that the parallelsafety markings of the aggregate's support functions are not consulted by the planner, only the marking of the aggregate itself. 
HYPOTHETICAL

For orderedset aggregates only, this flag specifies that the aggregate arguments are to be processed according to the requirements for hypotheticalset aggregates: that is, the last few direct arguments must match the data types of the aggregated (
WITHIN GROUP
) arguments. TheHYPOTHETICAL
flag has no effect on runtime behavior, only on parsetime 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 transitionstate 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 transitionstate 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 finalfunction
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 movingaggregate 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 movingaggregate 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 movingaggregate 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 movingaggregate 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 movingaggregate 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 readonly: 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 orderedset 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, orderedset 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 hypotheticalset 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, orderedset aggregates do not need to support movingaggregate mode, since they cannot be used as window functions.
Partial (including parallel) aggregation is currently not supported for
orderedset 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 userdefined aggregate functions.