phpman > man > CREATE_AGGREGATE(7)

CREATE AGGREGATE(7)                PostgreSQL 14.23 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 14.23                                2026                             CREATE AGGREGATE(7)