After looking at func.c and changing the structure SumCtx to use LONGDOUBLE_TYPE as used in printf.c the result indeed improve,but it goes a bit over the result of decimal_sum (after fixing my mistakes on the numbers, and the alternation on the big/small numbers are intentional for the testing):
=====
create table S(
id integer primary key not null,
value real
);
WITH RECURSIVE
cnt(x) AS (VALUES(1) UNION ALL SELECT x+1 FROM cnt WHERE x<6000008)
insert into S select x as id, iif((x % 2) == 0, 0.0000000123, 12345.0000000001) as value
from cnt;
--select * from S limit 5;
--select printf("count %d, sum: %35.15f", count(*), decimal_sum(value)) from S;
select printf("count %d, sum: %35.15f", count(*), sum(value)) from S;
=====
/*
** An instance of the following structure holds the context of a
** sum() or avg() aggregate computation.
*/
typedef struct SumCtx SumCtx;
struct SumCtx {
LONGDOUBLE_TYPE rSum; /* Floating point sum */
i64 iSum; /* Integer sum */
i64 cnt; /* Number of elements summed */
u8 overflow; /* True if integer overflow seen */
u8 approx; /* True if non-integer value was input to the sum */
};
=====
=====
/usr/bin/time sqlite3 < sum-long-long-test.sql
count 6000008, sum: 37035049380.037510000000000
3.27user 0.01system 0:03.29elapsed 100%CPU (0avgtext+0avgdata 115292maxresident)k
0inputs+0outputs (0major+28059minor)pagefaults 0swaps
=====
/usr/bin/time sqlite3 < sum-decimal-test.sql
count 6000008, sum: 37035049380.037200000000000
5.91user 0.02system 0:05.94elapsed 99%CPU (0avgtext+0avgdata 115428maxresident)k
0inputs+0outputs (0major+28063minor)pagefaults 0swaps
=====
/usr/bin/time sqlite3 < sum-default-test.sql
count 6000008, sum: 37035049380.000160000000000
3.22user 0.02system 0:03.25elapsed 100%CPU (0avgtext+0avgdata 115432maxresident)k
0inputs+0outputs (0major+28065minor)pagefaults 0swaps
=====