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CREATE AGGREGATE

   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).

   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 36.12.1. 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 36.12.4. 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 36.12.

Compatibility

   CREATE AGGREGATE is a PostgreSQL language extension. The SQL standard
   does not provide for user-defined aggregate functions.

See Also

   ALTER AGGREGATE, DROP AGGREGATE