On Apache
Under GNU General Public License
2024-12-12 16:54 @52.14.118.57 CrawledBy Mozilla/5.0 AppleWebKit/537.36 (KHTML, like Gecko; compatible; ClaudeBot/1.0; +claudebot@anthropic.com)
CREATE AGGREGATE(7) PostgreSQL 14.15 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 14.15 2024 CREATE AGGREGATE(7)
Generated by $Id: phpMan.php,v 4.55 2007/09/05 04:42:51 chedong Exp $ Author: Che Dong
On Apache
Under GNU General Public License
2024-12-12 16:54 @52.14.118.57 CrawledBy Mozilla/5.0 AppleWebKit/537.36 (KHTML, like Gecko; compatible; ClaudeBot/1.0; +claudebot@anthropic.com)