Calculate closest working day in Postgres

Question

I need to schedule some items in a postgres query based on a requested delivery date for an order. So for example, the order has a requested delivery on a Monday (20120319

Answer 1

SELECT y.d AS prep_day
FROM  (
    SELECT generate_series(dday - 8, dday - 1, interval '1d')::date AS d
    FROM (SELECT '2012-03-19'::date AS dday) x
    ) y
LEFT   JOIN holiday h USING (d)
WHERE  h.d IS NULL
AND    extract(isodow from y.d) < 6
ORDER  BY y.d DESC
LIMIT  1;

• It should be faster to generate only as many days as necessary. I generate one week prior to the delivery. That should cover all possibilities.

• isodow as extract parameter is more convenient than dow to test for workdays.

• min() / max(), ORDER BY / LIMIT 1, that's a matter of taste with the few rows in my query.

• To get several candidate days in descending order, not just the top pick, change the LIMIT 1.

• I put the dday (delivery day) in a subquery so you only have to input it once. You can enter any date or timestamp literal. It is cast to date either way.