ST_GeometryN
ST_GeometryN returns a geometry pointed to by the input index of the input geometry, as follows:
If the input is a point, linestring, or polygon, then a geometry is returned as is if the index is equal to one (1), and null if the index is other than one (1).
If the input is a multipoint, multilinestring, multipolygon, or geometry collection, then a point, linestring, polygon, or geometry collection is returned as pointed to by an input index.
The index is onebased. The spatial reference system identifier (SRID) of the result is the same as that of the input geometry. The dimension of the returned geometry is the same as that of the input geometry.
Syntax
ST_GeometryN(geom, index)
Arguments
 geom

A value of data type
GEOMETRY
or an expression that evaluates to aGEOMETRY
type.  index

A value of data type
INTEGER
that represents the position of a onebased index.
Return type
GEOMETRY
If geom or index is null, then null is returned.
If index is out of range, then an error is returned.
Examples
The following SQL returns the geometries in a geometry collection.
WITH tmp1(idx) AS (SELECT 1 UNION SELECT 2), tmp2(g) AS (SELECT ST_GeomFromText('GEOMETRYCOLLECTION(POLYGON((0 0,10 0,0 10,0 0)),LINESTRING(20 10,20 0,10 0))')) SELECT idx, ST_AsEWKT(ST_GeometryN(g, idx)) FROM tmp1, tmp2 ORDER BY idx;
idx  st_asewkt
+
1  POLYGON((0 0,10 0,0 10,0 0))
2  LINESTRING(20 10,20 0,10 0)