WITH
left_table AS (
  SELECT *
  FROM (VALUES
    ('A', 1),
    ('A', 2),
    ('A', 3),
    ('A', 4),
    ('B', 5),
    ('B', 6),
    ('B', 7),
    ('B', 8)
  ) AS t(grp_id_left, record_id)
),
right_table AS (
  SELECT *
  FROM (VALUES
    ('X', 1),
    ('X', 2),
    ('Y', 3),
    ('Y', 4),
    ('M', 1),
    ('N', 5),
    ('Q', 5),
    ('Q', 6),
    ('Q', 7)
  ) AS t(grp_id_rgt, record_id)
),
aggregated AS (
  SELECT
    grp_id_left,
    grp_id_rgt,
    array_agg(record_id ORDER BY record_id) AS records,
    cardinality(array_agg(record_id)) AS records_size
  FROM left_table l
  JOIN right_table r USING (record_id)
  GROUP BY 1,2
),
deduped AS (
  SELECT a.*
  FROM aggregated a
  WHERE NOT EXISTS (
    SELECT 1
    FROM aggregated b
    WHERE
      b.grp_id_left = a.grp_id_left
      AND a.records <@ b.records        -- a is subset of b
      AND a.grp_id_rgt <> b.grp_id_rgt  -- but a and b are different groups
      AND cardinality(a.records) < cardinality(b.records)
  )
)
SELECT grp_id_left, records AS list, grp_id_rgt
FROM deduped
ORDER BY grp_id_left, grp_id_rgt;