How to create aggregate function, like sum, with an implicit DISTINCT?

Question

We have an ERP system which allows aggregates to be used (eg SUM(foo)) but not with DISTINCT (eg SUM(DISTINCT foo).

Is it possible to create an aggregate function (SUM_DISTINCT), that returns the same result as as SUM(DISTINCT foo), so SUM_DISTINCT(foo) = SUM(DISTINCT foo)?

Answer 1

Is it possible to create an aggregate function (SUM_DISTINCT), that returns the same result as as SUM(DISTINCT foo), so SUM_DISTINCT(foo) = SUM(DISTINCT foo)?

Yes, it is possible — you need a User-defined Aggregate, such as this:

create or replace function f_sum_distinct (numeric[], numeric) returns numeric[]
    language sql as $$
select $1||$2;
$$;

create or replace function f_sum_distinct_final (numeric[]) returns numeric 
    language sql as $$
select sum(v) from (select distinct unnest($1) v) z;
$$;

create aggregate sum_distinct(numeric)
( sfunc     = f_sum_distinct
 ,stype     = numeric[]
 ,finalfunc = f_sum_distinct_final
);

with w(v) as (select 2 union all select 2 union all select 3)
select sum(v) "Plain SUM", sum(distinct v) "SUM(DISTINCT)", sum_distinct(v) "SUM_DISTINCT" from w;

/*
|Plain SUM|SUM(DISTINCT)|SUM_DISTINCT|
|--------:|------------:|-----------:|
|        7|            5|           5|
*/

dbfiddle here

Note though (thanks @Erwin), that performance is going to be very substantially worse than the built-in aggregates. If this matters you will have to consider writing the helper functions in C, which is much more of an undertaking.

Answer 2

I'm not sure I understand the problem. Just to rehash...

>>> 1+2+3+4+5
15

Now sample data.

CREATE TABLE foo
AS
  SELECT trunc(random()*5+1) AS x
  FROM generate_series(1,100);

Then we SUM(DISTINCT..)

SELECT SUM(DISTINCT x)
FROM foo;
 sum 
-----
  15

If as @JackDouglas suggests you want your own aggregate with distinct, just create the aggregate, from the docs

Aggregate function calls in SQL allow DISTINCT and ORDER BY options that control which rows are fed to the aggregate's transition function and in what order. These options are implemented behind the scenes and are not the concern of the aggregate's support functions.

I just don't see what this has to do with DISTINCT

If you want to know how to impliment your own sum() it's almost in the docs exactly (over the complex type)

CREATE AGGREGATE mysum (numeric)
(
    sfunc = numeric_add,
    stype = numeric,
    initcond = '0'
);

SELECT mysum(x)
FROM ( VALUES (1),(5),(8),(8),(8) )
  AS t(x);

SELECT mysum(DISTINCT x)
FROM ( VALUES (1),(5),(8),(8),(8) )
  AS t(x);

As a special note numeric_add(numeric,numeric) is an undocumented internal function. It's used by the + operator, but you can put anything you want there that takes two numeric.