Bounding box extent

extent(box): box

WITH boxes(b) AS (
  SELECT tbox 'TBOX((1, 2000-01-01),(3, 2000-01-03))' UNION
  SELECT tbox 'TBOX((5, 2000-01-05),(7, 2000-01-07))' UNION
  SELECT tbox 'TBOX((6, 2000-01-06),(8, 2000-01-08))'
)
SELECT extent(b) FROM boxes;
-- TBOX((1,2000-01-01 00:00:00+01),(8,2000-01-08 00:00:00+01))
WITH boxes(b) AS (
  SELECT stbox 'STBOX Z((1, 1, 1),(3, 3, 3))' UNION
  SELECT stbox 'STBOX Z((5, 5, 5),(7, 7, 7))' UNION
  SELECT stbox 'STBOX Z((6, 6, 6),(8, 8, 8))'
)
SELECT extent(b) FROM boxes;
-- STBOX Z((1,1,1),(8,8,8))