

 
Manual Reference Pages  CREATE_AGGREGATE (7)
.ds Aq ’
NAME
CREATE_AGGREGATE  define a new aggregate function
CONTENTS
SYNOPSIS
CREATE AGGREGATE name ( [ argmode ] [ argname ] arg_data_type [ , ... ] ) (
SFUNC = sfunc,
STYPE = state_data_type
[ , SSPACE = state_data_size ]
[ , FINALFUNC = ffunc ]
[ , FINALFUNC_EXTRA ]
[ , INITCOND = initial_condition ]
[ , MSFUNC = msfunc ]
[ , MINVFUNC = minvfunc ]
[ , MSTYPE = mstate_data_type ]
[ , MSSPACE = mstate_data_size ]
[ , MFINALFUNC = mffunc ]
[ , MFINALFUNC_EXTRA ]
[ , MINITCOND = minitial_condition ]
[ , SORTOP = sort_operator ]
)
CREATE 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 ]
[ , INITCOND = initial_condition ]
[ , HYPOTHETICAL ]
)
or the old syntax
CREATE AGGREGATE name (
BASETYPE = base_type,
SFUNC = sfunc,
STYPE = state_data_type
[ , SSPACE = state_data_size ]
[ , FINALFUNC = ffunc ]
[ , FINALFUNC_EXTRA ]
[ , INITCOND = initial_condition ]
[ , MSFUNC = msfunc ]
[ , MINVFUNC = minvfunc ]
[ , MSTYPE = mstate_data_type ]
[ , MSSPACE = mstate_data_size ]
[ , MFINALFUNC = mffunc ]
[ , MFINALFUNC_EXTRA ]
[ , MINITCOND = minitial_condition ]
[ , SORTOP = sort_operator ]
)
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, Aggregate Functions\(rq, in the documentation. 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 (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( internalstate, nextdatavalues ) > nextinternalstate
ffunc( 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 aggregates 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
\(lqstrict\(rq, 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 aggregates handling of null values.
If the final function is declared
\(lqstrict\(rq, 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 aggregates input values. The main reason for doing this is if the final function is polymorphic and the state values 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 35.10.1, \(lqMovingAggregate Mode\(rq, in the documentation. This requires specifying the
MSFUNC,
MINVFUNC, and
MSTYPE
parameters, and optionally the
MSPACE,
MFINALFUNC,
MFINALFUNC_EXTRA, and
MINITCOND
parameters. Except for
MINVFUNC, these parameters work like the corresponding 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
\(lqhypothetical\(rq
row.
Aggregates that behave like
MIN
or
MAX
can sometimes be optimized by looking into an index instead of scanning every input row. If this aggregate can be so optimized, indicate it by specifying a
sort operator. The basic requirement is that the aggregate must yield the first element in the sort ordering induced by the operator; in other words:
SELECT agg(col) FROM tab;
must be equivalent to:
SELECT col FROM tab ORDER BY col USING sortop LIMIT 1;
Further assumptions are that the aggregate ignores null inputs, and that it delivers a null result if and only if there were no nonnull inputs. Ordinarily, a data types
<
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
\(lqless than\(rq
or
\(lqgreater than\(rq
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 transition and final functions.
PARAMETERS
name
The name (optionally schemaqualified) 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 zeroargument 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 zeroargument aggregate function with this syntax, specify the
basetype
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
Nargument 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 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 aggregates state value.
state_data_size
The approximate average size (in bytes) of the aggregates 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 aggregates 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 aggregates result, and the return type is
state_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 aggregates regular (aggregated) arguments. This is mainly useful to allow correct resolution of the aggregate result type when a polymorphic aggregate is being defined.
initial_condition
The initial setting for the state value. This must be a string constant in the form accepted for the data type
state_data_type. If not specified, the state value starts out null.
msfunc
The name of the forward state transition function to be called for each input row in 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 from
state_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 aggregates state value, when using movingaggregate mode.
mstate_data_size
The approximate average size (in bytes) of the aggregates 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 aggregates result after all input rows have been traversed, when using movingaggregate mode. This works the same as
ffunc, except that its first arguments 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 aggregates regular implementation.
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 or
MAXlike 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).
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. The
HYPOTHETICAL
flag has no effect on runtime behavior, only on parsetime resolution of the data types and collations of the aggregates 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.
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 aggregates 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 aggregates state. It is assumed that the final function does not damage the aggregates 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.
EXAMPLES
See
Section 35.10, \(lqUserdefined Aggregates\(rq, in the documentation.
COMPATIBILITY
CREATE AGGREGATE
is a
PostgreSQL
language extension. The SQL standard does not provide for userdefined aggregate functions.
SEE ALSO
ALTER AGGREGATE (ALTER_AGGREGATE(7)), DROP AGGREGATE (DROP_AGGREGATE(7))
PostgreSQL 9.5.2  CREATE AGGREGATE (7)  2016 
Visit the GSP FreeBSD Man Page Interface. Output converted with manServer 1.07. 