Postgres 9.6

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

Help with an interesting Postgres question: Why isn't an Index Only Scan used on a partition accessed via the parent table?.

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 $$
select sum(distinct v) from (select distinct unnest($1) v) z;
$$;
create aggregate sum_distinct(numeric)
( sfunc     = f_sum_distinct
 ,stype     = numeric[]
 ,finalfunc = f_sum_distinct_final
);
select sum(distinct g) "SUM(DISTINCT)", clock_timestamp()-statement_timestamp()
from generate_series(1, 10000) g;


SUM(DISTINCT)
?column?


50005000
00:00:00.00305


select sum_distinct(g) "SUM_DISTINCT", clock_timestamp()-statement_timestamp()
from generate_series(1, 10000) g;


SUM_DISTINCT
?column?


50005000
00:00:01.336863