ST_Extent
Signatures
GEOMETRY ST_Extent(GEOMETRY geom);
GEOMETRY ST_Extent(GEOMETRYCOLLECTION geom);
Description
Returns the minimum bounding box that encloses geom as a Geometry.
Examples
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)
Comparison with ST_Envelope
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
