Postgresql function with values (from another table) as arguments - postgresql

Question

I can't figure out how to call a function with inputs specified from another table.
Let us assume the following function is being used to create a time interval:
create or replace function interval_generator(dt_start timestamp with TIME ZONE,
dt_end timestamp with TIME ZONE,
round_interval INTERVAL)
returns TABLE(time_start timestamp with TIME ZONE,
time_end timestamp with TIME ZONE) as $$
BEGIN
return query
SELECT
(n) time_start,
(n + round_interval) time_end
FROM generate_series(date_trunc('minute', dt_start), dt_end, round_interval) n;
END
$$
LANGUAGE 'plpgsql';
Let us create a dummy table for the minimal example:
DROP TABLE IF EXISTS lookup;
CREATE TEMP TABLE lookup
as
select *
from (
VALUES
('2017-08-17 04:00:00.000'::timestamp),
('2017-08-17 05:00:00.000'::timestamp),
('2017-08-18 06:00:00.000'::timestamp)
) as t (datetime);
Now my attempt is as follows:
select interval_generator(
SELECT datetime FROM lookup Order By datetime limit 1,
SELECT datetime FROM lookup Order By datetime Desc limit 1,
'1 hours'::interval
);
and it just yields the generic error ERROR: syntax error at or near "SELECT"

Answer 1

Enclose the SELECT statements in parentheses to make them expressions like this:
select * from interval_generator(
(SELECT datetime FROM lookup Order By datetime limit 1),
(SELECT datetime FROM lookup Order By datetime Desc limit 1),
'1 hours'::interval
);
Please note that
SELECT datetime FROM lookup Order By datetime limit 1
is exactly
SELECT min(datetime) FROM lookup
which seems to me better readable. As the function body of interval_generator comprises of a single SQL query why don't you make it a plain SQL function instead of pl/pgsql?
<your-function-declaration> as $$
SELECT
(n) time_start,
(n + round_interval) time_end
FROM generate_series(date_trunc('minute', dt_start), dt_end, round_interval) n;
$$
LANGUAGE 'sql';