How can wrap-on-overflow math be done in Postgres? - sql

Question

Given the following example function:
CREATE OR REPLACE FUNCTION add_max_value(_x BIGINT)
RETURNS BIGINT
LANGUAGE sql
AS $$
SELECT 9223372036854775807 + _x;
$$;
If this function is called with any positive value, the following error is returned:
SELECT add_max_value(1); -- Expecting -9223372036854775808 if math wrapped
-- SQL Error [22003]: ERROR: bigint out of range
How can I do wrap-on-overflow integer math in Postgres?
Please note:
I want to do this in the database, not in the application
I don't want it to promote to an arbitrary precision integer (NUMERIC)
Although the example only does addition, in practice I'm interested in other operations as well

Answer 1

As a SQL function there isn't a way. SQL functions cannot process exceptions. But a plpgsql function can:
CREATE OR REPLACE FUNCTION add_max_value(_x BIGINT)
RETURNS BIGINT
LANGUAGE plpgsql
AS $$
declare
bigx bigint;
begin
bigx = 9223372036854775807 + _x;
return bigx;
exception
when sqlstate '22003' then
return (9223372036854775807::numeric + _x - 2^64)::bigint;
end;
$$;

Answer 2

This is easily in my top five of the most wasteful SQL I have ever written:
create or replace function add_max_value(_x bigint)
returns bigint
language sql
as $$
with recursive inputs as (
select s.rn, r.a::int, s.b::int, (r.a::int + s.b::int) % 2 as sumbit,
(r.a::bit & s.b::bit)::int as carry
from regexp_split_to_table((9223372036854775807::bit(64))::text, '') with ordinality as r(a, rn)
join regexp_split_to_table((_x::bit(64))::text, '') with ordinality as s(b, rn)
on s.rn = r.rn
), addition as (
select rn, sumbit, sumbit as s2, carry, carry as upcarry
from inputs
where rn = 64
union
select i.rn, i.sumbit, (i.sumbit + a.upcarry) % 2, i.carry,
(i.carry::bit | a.upcarry::bit)::int
from addition a
join inputs i on i.rn = a.rn - 1
)
select (string_agg(s2::text, '' order by rn)::bit(64))::bigint
from addition
$$;