GEOMETRY ST_Extent(GEOMETRY geom);
GEOMETRY ST_Extent(GEOMETRYCOLLECTION geom);

SELECT ST_Extent('MULTIPOINT((5 6), (1 2), (3 4), (10 3))'::Geometry);
-- Answer: POLYGON((1 2, 1 6, 10 6, 10 2, 1 2))

SELECT ST_Extent('POINT(5 6)'::Geometry);
-- Answer: POINT(5 6)

CREATE TABLE input_table(geom GEOMETRY);
INSERT INTO input_table VALUES
     ('POLYGON((0 0, 3 -1, 1.5 2, 0 0))'),
     ('POLYGON((2 0, 3 3, 4 2, 2 0))'),
     ('POINT(5 6)'),
     ('LINESTRING(1 1, 1 6)');

-- ST_Envelope is a scalar function.
SELECT ST_Envelope(geom) ENV FROM input_table;
-- Answer:
--     |                  ENV                   |
--     |----------------------------------------|
--     | POLYGON((0 -1, 0 6, 5 6, 5 -1, 0 -1))  |
--     | POLYGON((2 0, 2 3, 4 3, 4 0, 2 0))     |
--     | POINT(5 6)                             |
--     | LINESTRING(1 1, 1 6)                   |

-- ST_Extent is an aggregate function.
SELECT ST_Extent(geom) EXT FROM input_table;
-- Answer: POLYGON((0 -1, 0 2, 3 2, 3 -1, 0 -1))

-- However, we can make ST_Envelope behave like ST_Extent as follows:
SELECT ST_Equals(ST_Envelope(ST_Accum(geom)),
                 ST_Extent(geom)) FROM input_table;
-- Answer: TRUE