CREATE AGGREGATE 7 SQL - Language Statements CREATE AGGREGATE define a new aggregate function CREATE AGGREGATE CREATE AGGREGATE name ( [ argmode ] [ argname ] arg_data_type [ , ... ] ) ( SFUNC = sfunc, STYPE = state_data_type [ , SSPACE = state_data_size ] [ , FINALFUNC = ffunc ] [ , INITCOND = initial_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 ] [ , 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 ] [ , INITCOND = initial_condition ] [ , SORTOP = sort_operator ] ) CREATE AGGREGATE defines a new aggregate function. Some basic and commonly-used aggregate functions are included with the distribution; they are documented in . 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 ). An 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 subsequent rows 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. 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 datatypes, 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. 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, and the return type, as well as EXECUTE privilege on the transition and final functions. 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. The planner will consider using hash aggregation for such a query only if the hash table is estimated to fit in ; therefore, large values of this parameter discourage use of hash aggregation. 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, followed by null values corresponding to each aggregated argument. (The reason for including the aggregated arguments in the function signature is that this may be necessary 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. 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). 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. 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. See . CREATE AGGREGATE is a PostgreSQL language extension. The SQL standard does not provide for user-defined aggregate functions.