The ST_Dimension() function

A geometry can have one of three dimensions:

0

The geometry has neither length nor area.

1

The geometry has length.

2

The geometry has area.

The dimension_test table is created with the columns geotype and g1. The geotype column stores the name of the subclass stored in the g1 ST_Geometry column:

CREATE TABLE dimension_test (geotype varchar(20),
                             g1      ST_Geometry);

The following INSERT statements insert a sample subclass into the dimension_test table:

INSERT INTO dimension_test VALUES( 
   'Point',
   ST_PointFromText('point (10.02 20.01)',1000)
);

INSERT INTO dimension_test VALUES( 
   'Linestring', 
   ST_LineFromText('linestring (10.02 20.01,10.32 23.98,11.92 25.64)',1000)
);

INSERT INTO dimension_test VALUES(
   'Polygon',
   ST_PolyFromText('polygon ((10.02 20.01,11.92 35.64,25.02 34.15,
19.15 33.94,10.02 20.01))',1000)
);

INSERT INTO dimension_test VALUES( 
   'Multipoint',
   ST_MPointFromText('multipoint (10.02 20.01,10.32 23.98,11.92 25.64)',1000)
);

INSERT INTO dimension_test VALUES( 
   'Multilinestring',
   ST_MLineFromText('multilinestring ((10.02 20.01,10.32
23.98,11.92 25.64),(9.55 23.75,15.36 30.11))',1000) 
);

INSERT INTO dimension_test VALUES( 
   'Multipolygon', 
   ST_MPolyFromText('multipolygon (((10.02 20.01,11.92 35.64,25.02
34.15,19.15 33.94,10.02 20.01)),((51.71 21.73,73.36 27.04,71.52
32.87,52.43 31.90,51.71 21.73)))',1000) 
);

The SELECT statement lists the subclass name stored in the geotype column with the dimension of that geotype:

SELECT geotype, ST_Dimension(g1) Dimension 
   FROM dimension_test;

geotype                dimension 

Point                          0
Linestring                     1
Polygon                        2
Multipoint                     0
Multilinestring                1
Multipolygon                   2