Chapter 34. Extending SQL: Aggregates

Aggregates in Postgres are expressed in terms of state transition functions. That is, an aggregate can be defined in terms of state that is modified whenever an instance is processed. Some state functions look at a particular value in the instance when computing the new state (sfunc1 in the create aggregate syntax) while others only keep track of their own internal state (sfunc2). If we define an aggregate that uses only sfunc1, we define an aggregate that computes a running function of the attribute values from each instance. "Sum" is an example of this kind of aggregate. "Sum" starts at zero and always adds the current instance's value to its running total. We will use the int4pl that is built into Postgres to perform this addition.

         CREATE AGGREGATE complex_sum (
              sfunc1 = complex_add,
              basetype = complex,
              stype1 = complex,
              initcond1 = '(0,0)'
           );

         SELECT complex_sum(a) FROM test_complex;

         +------------+
         |complex_sum |
         +------------+
         |(34,53.9)   |
         +------------+

If we define only sfunc2, we are specifying an aggregate that computes a running function that is independent of the attribute values from each instance. "Count" is the most common example of this kind of aggregate. "Count" starts at zero and adds one to its running total for each instance, ignoring the instance value. Here, we use the built-in int4inc routine to do the work for us. This routine increments (adds one to) its argument.

         CREATE AGGREGATE my_count (sfunc2 = int4inc, -- add one
                                      basetype = int4, stype2 = int4,
                                      initcond2 = '0')

         SELECT my_count(*) as emp_count from EMP;

         +----------+
         |emp_count |
         +----------+
         |5         |
         +----------+

"Average" is an example of an aggregate that requires both a function to compute the running sum and a function to compute the running count. When all of the instances have been processed, the final answer for the aggregate is the running sum divided by the running count. We use the int4pl and int4inc routines we used before as well as the Postgres integer division routine, int4div, to compute the division of the sum by the count.

         CREATE AGGREGATE my_average (sfunc1 = int4pl, --  sum
                                        basetype = int4,
                                        stype1 = int4,
                                        sfunc2 = int4inc, -- count
                                        stype2 = int4,
                                        finalfunc = int4div, -- division
                                        initcond1 = '0',
                                        initcond2 = '0')

         SELECT my_average(salary) as emp_average FROM EMP;

         +------------+
         |emp_average |
         +------------+
         |1640        |
         +------------+