Provided by: postgres-xc-client_1.1-2ubuntu2_amd64 
NAME
CREATE_AGGREGATE - define a new aggregate function
SYNOPSIS
CREATE AGGREGATE name ( input_data_type [ , ... ] ) (
SFUNC = sfunc,
STYPE = state_data_type
[ , FINALFUNC = ffunc ]
[ , CFUNC = cfunc ]
[ , INITCOND = initial_condition ]
[ , SORTOP = sort_operator ]
)
or the old syntax
CREATE AGGREGATE name (
BASETYPE = base_type,
SFUNC = sfunc,
STYPE = state_data_type
[ , FINALFUNC = ffunc ]
[ , INITCOND = initial_condition ]
[ , SORTOP = sort_operator ]
)
DESCRIPTION
CREATE AGGREGATE defines a new aggregate function. Some basic and commonly-used aggregate functions are
included with the distribution; they are documented in Section 9.20, “Aggregate Functions”, 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.
An aggregate function is made from one or to maximum three ordinary functions: a state transition
function sfunc, an optional collection function cfunc, 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
In Postgres-XC the aggregation works in two different modes.
• Two phased aggregation - is used when the entire aggregation takes place on the Coordinator node. In
first phase called transition phase, Postgres-XC 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, in
the second phase or finalization phase 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.
• Three phased aggregation - is used when the process of aggregation is divided between Coordinator and
Datanodes. In this mode, each Postgres-XC Datanode involved in the query carries out the first phase
named transition phase. This phase is similar to the first phase in the two phased aggregation mode
discussed above, except that, every Datanode applies this phase on the rows available at the
Datanode. The result of transition phase is then transferred to the Coordinator node. Second phase
called collection phase takes place on the Coordinator. Postgres-XC Coordinator node creates a
temporary variable of data type stype to hold the current internal state of the collection phase. For
every input from the Datanode (result of transition phase on that node), the collection function is
invoked with the current collection state value and the new transition value (obtained from the
Datanode) to calculate a new internal collection state value. After all the transition values from
data nodes have been processed, in the third or finalization phase the final function is invoked once
to calculate the aggregate's return value. If there is no final function then the ending collection
state value is returned as-is.
Irrespective of the mode used for aggregation, the result of aggregation should be same if the same set
of data rows is participating in the aggregate. Postgres-XC planner chooses the cheapest feasible mode
of the above two, during planning.
Note
The following description applies only to Postgres-XC
An aggregate function can provide an initial condition, that is, an initial value for the internal
transition or collection 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.
Note
The following description applies only to Postgres-XC
If the collection function is declared “strict”, then it cannot be called with null inputs. With such a
collection function, aggregate execution behaves as follows. Null state transition results are ignored
(the function is not called and the previous collection state value is retained). If the initial state
value is null, then at the first non-null state transition result replaces the collection state value,
and the collection function is invoked at subsequent rows with all-nonnull transition values. This is
handy for implementing aggregates like max.
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 input_data_type. When these types are different, you must supply
a nonnull initial condition or use a nonstrict transition function.
Note
The following description applies only to Postgres-XC
If the state transition and/or collection function is not strict, then it will be called unconditionally
at each input row, and must deal with null inputs and null transition/collection 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.
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.
PARAMETERS
name
The name (optionally schema-qualified) of the aggregate function to create.
input_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 input data types. (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, specify the basetype as "ANY" (not *).
sfunc
The name of the state transition function to be called for each input row. For an 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.
cfunc
Note
The following description applies only to Postgres-XC
The name of the state collection function to be called for each input row. The sfunc must take 2
arguments, both of them being of type state_data_type. The function must return a value of type
state_data_type. This function takes the current collection state value and the current transition
value, and returns the next collection state value. If cfunc is omitted for an aggregate, the two
phase aggregation mode is used for that aggregate. All the aggregates involed in a query use the same
aggregation mode.
state_data_type
The data type for the aggregate's state value.
ffunc
The name of the final function called to compute the aggregate's result after all input rows have
been traversed. The 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.
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.
initial_collection_condition
Note
The following description applies only to Postgres-XC
The initial setting for the state collection value. This must be a string constant in the form
accepted for the data type state_data_type. If not specified, the collection state value starts out
null. If the collection function cfunc is not supplied, this parameter is stored but not used.
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 aggregate).
The parameters of CREATE AGGREGATE can be written in any order, not just the order illustrated above.
EXAMPLES
See Section 36.10, “User-defined Aggregates”, in the documentation.
COMPATIBILITY
CREATE AGGREGATE is a Postgres-XC 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))
Postgres-XC 1.1 2014-04-07 CREATE AGGREGATE(7)