CREATE AGGREGATE(7) PostgreSQL 15.7 Documentation CREATE AGGREGATE(7)
NAME
CREATE_AGGREGATE - define a new aggregate function
SYNOPSIS
CREATE [ OR REPLACE ] AGGREGATE name ( [ 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 [ OR REPLACE ] AGGREGATE name ( [ [ 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 [ OR REPLACE ] AGGREGATE name (
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. CREATE OR REPLACE
AGGREGATE will either define a new aggregate function or replace an
existing definition. Some basic and commonly-used aggregate functions
are included with the distribution; they are documented in
Section 9.21. If one defines new types or needs an aggregate function
not already provided, then CREATE AGGREGATE can be used to provide the
desired features.
When replacing an existing definition, the argument types, result type,
and number of direct arguments may not be changed. Also, the new
definition must be of the same kind (ordinary aggregate, ordered-set
aggregate, or hypothetical-set aggregate) as the old one.
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
(CREATE_FUNCTION(7))).
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-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 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 38.12.1. 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 38.12.4. 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 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.
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.
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 combinefunc function may optionally be specified to allow the
aggregate function to support partial aggregation. If provided, the
combinefunc must combine two state_data_type 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 sfunc, 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 the
state_data_type 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.
For aggregate functions whose state_data_type is internal, the
combinefunc must 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.
serialfunc
An aggregate function whose state_data_type is internal can
participate in parallel aggregation only if it has a serialfunc
function, which must serialize the aggregate state into a bytea
value for transmission to another process. This function must take
a single argument of type internal and return type bytea. A
corresponding deserialfunc is also required.
deserialfunc
Deserialize a previously serialized aggregate state back into
state_data_type. This function must take two arguments of types
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 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.
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 38.12.
COMPATIBILITY
CREATE AGGREGATE is a PostgreSQL language extension. The SQL standard
does not provide for user-defined aggregate functions.
SEE ALSO
ALTER AGGREGATE (ALTER_AGGREGATE(7)), DROP AGGREGATE
(DROP_AGGREGATE(7))
PostgreSQL 15.7 2024 CREATE AGGREGATE(7)
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