Postgres (date) range coverage function

daterange_covered.sql

CREATE IF NOT EXISTS EXTENSION btree_gist;

CREATE TABLE split_vat (
    cif varchar,
    interval daterange NOT NULL,
    applies_split_vat boolean NOT NULL,
  EXCLUDE USING 
    gist (interval WITH &&, cif WITH =)
);

insert into split_vat values('1', daterange('2017-12-15', '2017-12-30', '[]'), true);
insert into split_vat values('1', daterange('2018-02-15', '2018-03-15', '[]'), true);

CREATE OR REPLACE FUNCTION uncovered_split_vat(daterange default daterange('2017-09-01', current_date), varchar default null)
RETURNS TABLE(cif varchar, uncovered_ranges daterange)
AS $$
    WITH RECURSIVE calendar AS (
        SELECT
            $1 AS left,
            $1 AS center,
            $1 AS right,
            $2 AS cif
        UNION
        SELECT
            CASE split_vat.interval && calendar.left
                WHEN TRUE THEN daterange(lower(calendar.left), lower(split_vat.interval * calendar.left))
                ELSE daterange(lower(calendar.right), lower(split_vat.interval * calendar.right))
            END AS left,
            CASE split_vat.interval && calendar.left
                WHEN TRUE THEN split_vat.interval * calendar.left
                ELSE split_vat.interval * calendar.right
            END AS center,
            CASE split_vat.interval && calendar.right
                WHEN TRUE THEN daterange(upper(split_vat.interval * calendar.right), upper(calendar.right))
                ELSE daterange(upper(split_vat.interval * calendar.left), upper(calendar.left))
            END AS right,
            split_vat.cif as cif
        FROM calendar
        JOIN split_vat ON
            split_vat.interval && $1 AND
            split_vat.interval <> calendar.center AND (
                split_vat.interval && calendar.left OR
                split_vat.interval && calendar.right
            ) AND ($2 IS NULL OR split_vat.cif = $2)
)
SELECT *
FROM (
    SELECT
        a.cif AS cif,
        a.left AS uncovered_ranges
    FROM calendar a
    LEFT OUTER JOIN calendar b ON
        a.left <> b.left AND
        a.left @> b.left
    GROUP BY a.left, a.cif
    HAVING NOT bool_or(COALESCE(a.left @> b.left, FALSE))
    UNION
    SELECT
        a.cif AS cif,
        a.right AS uncovered_ranges
    FROM calendar a
    LEFT OUTER JOIN calendar b ON
        a.right <> b.right AND
        a.right @> b.right
    GROUP BY a.right, a.cif
    HAVING NOT bool_or(COALESCE(a.right @> b.right, FALSE))
) a
$$ LANGUAGE SQL STABLE;